ELECTRICAL ENGINEERING KNOWLEDGE
By
HASAN TARIQ
TABLE OF CONTENT
TOPIC 1: SYNCHRONOUS VS INDUCTION MACHINE ...................................................................................................... 4
TOPIC 2: CIRCUIT BREAKERS AND ISOLATORS .............................................................................................................. 8
TOPIC 3: SMART GRID .......................................................................................................................................................... 11
TOPIC 4: VARIABLE FREQUENCY DRIVE ........................................................................................................................ 13
TOPIC 5: NET METERING ..................................................................................................................................................... 16
TOPIC 6: BLACKOUT AND TRIPPING OF TRANSMISSION LINES ............................................................................... 17
TOPIC 7: BASICS OF POWER SYSTEM 1............................................................................................................................ 18
TOPIC 8 INTERESTING INFORMATION RELATED TO ELECTRICAL ENGINEERING .............................................. 21
TOPIC 9: REACTIVE POWER COMPENSATION ............................................................................................................... 24
TOPIC 10: CURRENT AND POTENTIAL TRANSFORMER ............................................................................................... 25
TOPIC 11: INTRODUCTION TO POWER ELECTRONICS ................................................................................................. 28
TOPIC 12: VOLTAGE FLUCTUATIONS .............................................................................................................................. 30
TOPIC 13: GENERAL ELECTRICAL INFORMATION ....................................................................................................... 32
TOPIC 14: COOLING METHODS OF TRANSFORMERS ................................................................................................... 33
TOPIC 15: GRID AND SUBSTATIONS ................................................................................................................................. 35
TOPIC 16: NEUTRAL WIRE AND UNBALANCE CURRENT ............................................................................................ 37
TOPIC 17: POWER FACTOR IMPORTANCE ....................................................................................................................... 39
TOPIC 18: POWER CABLES .................................................................................................................................................. 42
TOPIC 19: HARMONICS ........................................................................................................................................................ 44
TOPIC 20: PV SYSTEMS ........................................................................................................................................................ 48
TOPIC 21: BILL READING..................................................................................................................................................... 52
TOPIC 22: TRANSIENTS IN POWER SYSTEM ................................................................................................................... 57
TOPIC 23: AUTO TRANSFORMER ....................................................................................................................................... 60
TOPIC 24: TYPES OF INSULATORS .................................................................................................................................... 63
TOPIC 25: PARALLEL OPERATION OF TRANSFORMER ................................................................................................ 65
TOPIC 26: PARALLEL OPERATION OF GENERATORS ................................................................................................... 66
TOPIC 27: BUCHOLZ RELAY ............................................................................................................................................... 68
TOPIC 28: SKIN AND PROXIMITY EFFECT ....................................................................................................................... 69
TOPIC 29: CAPACITOR BANK ............................................................................................................................................. 70
TOPIC 30: HOW TO CALCULATE CAPACITANCE OF A CAPACITOR BANK? ........................................................... 72
TOPIC 31: FACT DEVICES .................................................................................................................................................... 73
TOPIC 32: CIRCULATING CURRENT .................................................................................................................................. 76
TOPIC 33: STAR DELTA STARTER ..................................................................................................................................... 77
TOPIC 34: PARTS OF TRANSFORMER ............................................................................................................................... 79
TOPIC 35: DC MACHINES ..................................................................................................................................................... 81
TOPIC 36: RCCB AND MCB .................................................................................................................................................. 82
TOPIC 37: LOSSES IN TRANSFORMER .............................................................................................................................. 83
TOPIC 38: OPEN AND SHORT CIRCUIT TEST IN TRANSFORMER ............................................................................... 86
TOPIC 39: ZONES OF PROTECTION .................................................................................................................................... 87
TOPIC 40: FEEDER AND TRANSMISSION LINE ............................................................................................................... 89
TOPIC 41: DISTANCE RELAY ZONES................................................................................................................................. 89
TOPIC 42: DIRECTIONAL OVER CURRENT RELAY ........................................................................................................ 92
TOPIC 43: WHY INDUCTION MOTOR RUNS ON LAGGING POWER FACTOR ........................................................... 94
TOPIC 44: SYNCHRONOUS VS INDUCTION PART 2 ....................................................................................................... 96
TOPIC 45: LIMIT SWITCH ................................................................................................................................................... 100
TOPIC 46: DCS....................................................................................................................................................................... 103
TOPIC 47: PID CONTROLLER............................................................................................................................................. 108
TOPIC 48: SOLENOID VALVE WORKING ........................................................................................................................ 112
TOPIC 49: DIRECT VS INDIRECT FIRED BURNER......................................................................................................... 114
TOPIC 50: MOTOR TROUBLESHOOTING GUIDE ........................................................................................................... 116
TOPIC 51: TYPES OF SENSORS USED IN INDUSTRY .................................................................................................... 119
TOPIC 52: ENERGY CRISES IN PAKISTAN ...................................................................................................................... 119
TOPIC 53: ELECTRICAL STANDARDS ............................................................................................................................. 123
TOPIC 54: PURPOSE OF LOAD FLOW, SHORT CIRCUIT AND TRANSIENT ANALYSIS ......................................... 124
TOPIC 55: SOLAR PV SYSTEM SIZING ............................................................................................................................ 127
TOPIC 56: COGENERATION POWER PLANT .................................................................................................................. 130
TOPIC 57: T & D LOSSES..................................................................................................................................................... 130
TOPIC 58: REDUCE YOUR ELECTRICITY BILL.............................................................................................................. 137
TOPIC 59: DIFFERENT TYPE OF MAINTENANCE.......................................................................................................... 138
TOPIC 60: DELTA VS STAR ................................................................................................................................................ 138
TOPIC 61: AC VS DC ............................................................................................................................................................ 140
TOPIC 62: POWER SYSTEM PROTECTION ...................................................................................................................... 141
TOPIC 63: MACHINE DESIGN ............................................................................................................................................ 145
TOPIC 64: INVERTER........................................................................................................................................................... 146
TOPIC 65: VOLTAGE FLUCTUATION............................................................................................................................... 148
TOPIC 66: PROTECTION VS MEASURING CT ................................................................................................................. 149
TOPIC 67: LOAD FACTOR/ DEMAND FACTOR/ DIVERSITY FACTOR ...................................................................... 150
TOPIC 68: INTRODUCTION TO POWER SYSTEM PROTECTION................................................................................. 150
TOPIC 68: LOADSHEDDING IN PAKISTAN ..................................................................................................................... 154
TOPIC 69: MAGNETIC CONTACTOR ................................................................................................................................ 156
TOPIC 70: RLCB VS ELCB................................................................................................................................................... 157
TOPIC 71: LOAD CELL, CHARGE CONTROLLER, THERMAL OVERLOAD RELAY ................................................ 157
TOPIC 72: ELECTRICITY SECTOR OF PAKISTAN.......................................................................................................... 158
TOPIC 1: SYNCHRONOUS VS INDUCTION MACHINE
Q1) what is the difference between synchronous and induction machine? What does slip means in terms of induction
machine?
Ans) SYNCHRONOUS MACHINE: In synchronous machines, rotor (rotating part) moves in synchronism with the stator
(stationary part) i.e. speed of the rotor is equal to the speed of the stator. The speed at which it runs is called as synchronous
speed. Synchronous machine always rotates at synchronous speed irrespective of the amount of load it is carrying.
Hence the difference becomes Zero and hence the slip speed of synchronous machine is Zero
INDUCTION MACHINE: In induction machines, rotor (rotating part) moves slightly less than stator (stationary part) i.e.
speed of the rotor is less than the speed of the stator. The difference between stator and rotor speed is called SLIP.
Q2) what is the advantage of synchronous over induction machine?
Ans) the biggest advantage of synchronous over induction machine is that synchronous machine can operate at any power
factor either leading or lagging, whereas induction machine can only operate at lagging power factor.
Q3) what does self-starting means in terms of synchronous and induction machine?
Ans) SELF STARTING: Self-starting means, the rotor in an IM doesn’t requires any external push or external source to
rotate. For self-starting we need a 3 phase magnetic field
Q4) Why 3 phase induction motor are self-starting?
Ans) Three phase induction motor works on the principles of electromagnetic induction. Three phase supply is given to
stator. It produces a rotating magnetic field. These magnetic field lines interact with the rotor and induces an emf in it.
Current flows in the rotor due to Lenz’s law. As we all know current carrying conductor placed in a magnetic field
experience a force. Force produced generates torque in the rotor and rotor starts rotating.
When current flows in the rotor, its produces its own magnetic field. The rotor’s magnetic field tries to align with the stator
magnetic field. But since the stator field rotates, the rotor starts to rotate with the stator field, as the stator field keeps on rotating
the rotor keeps on rotating trying to catch up the stator field.
Considering three phases ABC, when phase A is magnetized than rotor will move towards phase a winding. Then phase B will
magnetized then rotor will move towards phase B winding. Then phase C will be magnetized and rotor will move towards
phase C winding. So rotor will continue to rotate.
It is called as induction motor because electricity is induced in it by principles of electromagnetic induction rather than
providing direct supply.
Q5) Why 3 phase synchronous machine are not self-starting?
ANS) in synchronous motor we have two excitation. One is AC excitation which is applied to the stator of the machine and
another is Dc excitation which is applied to the rotor of the machine. Stator produces three phase rotating magnetic field where
as due to dc excitation, alternate poles forms on the rotor. Remember that stator is not physically rotating, its magnetic field is
rotating whereas rotor is physically rotating. Stator can change its poles rapidly whereas rotor cannot change its poles rapidly.
DURING POSITIVE CYCLE: Consider during positive cycle, North Pole of the stator and South Pole of the rotor are in
front of each other and they will experience force of attraction. Rotor will move in either anticlockwise or clock wise direction.
Let’s say it moves in anti-clock wise direction. So during positive cycle torque is generated.
DURING NEGATIVE CYCLE: During negative cycle. Pole of the stator will change to South Pole. Now the south pole of
the stator and rotor will experience force of repulsion. Due to continuous attraction and repulsion, the net torque generated
during positive and negative cycle will be zero and rotor cannot rotate.
Q6) in which direction will the rotor of induction motor will rotate?
ANS) this can be easily understood, if you know about Lenz law. Lenz’s law states that “EFFECT OPPOSES THE
CAUSE”
Here Cause Is rotating magnetic field (RMF) which will cut the rotor and will induce emf in it and the end effect is, it
produces torque
So for effect (torque) to oppose cause (RMF), torque produced makes the rotor rotate in the same direction as that of the
RMF.
Actually magnetic field of the stator exerts a pull on the rotor so to pull it in “backward direction”. Rotor responds to it by
moving in a direction of the magnetic pull. So when the rotor rotates the magnetic field in the rotor moves back, so that there
is always a "pull from ahead".
Q7) How we can control speed of roter in induction motor?
ANS) Speed of the rotor in Induction motor depends on how fast the stator magnetic field rotates, which means it depends on
the frequency of the 3-phase currents fed to the stator. By controlling the frequency, speed can be varied.
Q8) what does excitation mean in terms on synchronous and induction machine?
Ans) In case of synchronous motor, we give DC supply (excitation) to its field winding placed on rotor. To set a required
flux, we have three conditions1- Normal excitation- In this case our machine gets required flux through the DC voltage supplied. In this case our motor
works on unity p.f.
2- Over excitation- In this case, our DC voltage provides more excitation than required so our machine counterbalance that
extra flux by setting flux in opposite direction with the help of AC supply and this is called demagnetization. In this case our
machine works on leading power factor and works as a capacitive load.
3- Under excitation- In this case, DC supply is not enough to produce required flux so AC supply comes into picture to aid
DC to produce required flux. So, our machine works on lagging power factor and works as an inductive load. This is also
called magnetization.
Q9) why do we give DC excitation to roter in synchronous machine? Why not AC? What happens if we give AC?
Ans) Excitation is a process of producing magnetic field in a machine. The reason of applying dc excitation is to get a constant
magnetic field. If we apply AC excitation than it will also produce magnetic field but it will not be constant. It will be
fluctuating magnetic field.
In a synchronous generator (alternator), stator and rotor field must lock each other. If both these fields are varying (alternating
supply), there is no possibility of locking of these 2 fields. On the other hand, if one field is stationary and the other is
rotating, they can lock each other.
Q10) why single phase machines are not self-starting?
Ans) 3 PHASE IM: In 3 phase IM, we provide 3 phase supply to the stator and it produces a 3 phase magnetic field. Rotating
magnetic field is having the same phase difference of 120 degrees. These magnetic field lines interact with rotor and induces
an EMF in it.
If we consider three phases than when phase A is magnetized than rotor will move towards phase A winding. Then phase B
will magnetized then rotor will move towards phase B winding. Then phase C will be magnetized and rotor will move towards
phase C winding. So rotor will continue to rotate
SINGLE PHASE IM: Single phase induction motor produces alternating magnetic field hence they are not self-starting Now,
an alternating or pulsating flux acting on a rotor cannot produce rotation (only a revolving flux can produce rotation) and to
make them self-starting several methods are used such as:





Split Phase or Resistance Start.
Capacitor Start Motor.
Permanent Split Capacitor (PSC) Motor.
Capacitor Start Capacitor Run Motor.
Electronic Starter circuit.
Q11) examples of induction machines?
Induction motor is most widely employed motors in Electrical Industry and almost 90% industry machines are of Induction
motors.
First of all the most common example is Electric Fan at our home which uses Single Phase induction Motor. Apart from
that they are found in Mixer, Pump, Compressor, Drilling machine, bulldozers conveyors
Q12) what is damper winding or how synchronous motors are made self-starting?
Ans) Synchronous motors are not self-starting therefore to start them a special set of winding known as damper winding is
placed on rotor poles. These windings are similar to squirrel cage rotor which consist of short circuited copper bars embedded
on the rotor poles.
When Ac supply is given to stator of a 3 phase synchronous motor then it produces rotating magnetic field. Due to damper
winding present on the rotor, MACHINE STARTS AS AN INDUCTION MOTOR i.e. (Synchronous motor starts as an
induction motor and continue to accelerate).
The exciter of a synchronous motor moves along with the rotor and when the motor attains 95% of the synchronous speed then
rotor winding is connected to exciter terminals and rotor is magnetically locked by the rotating magnetic field of stator and it
runs as synchronous motor
Q13) why is the induction motor more used in industry than a synchronous motor or DC motor?
There are many reasons for selecting the induction motor .some of them are as follows:
1. Induction motor is a singly excited motor whereas synchronous motor is a doubly excited one. Synchronous motor
need an additional dc source for excitation of rotor windings.
2. Induction motor is a self-starting motor , whereas synchronous is not a self-starting motor , as rotor of synchronous
motor must be rotated near to synchronous to get the magnetic coupling of rotor and stator.
3. Induction motor is cheaper when compared to synchronous motor.
4. The main requirement in industry is to operate motor at different loads and different speeds, to drive mechanical
loads, so, in this induction motor is the best to choose because it has so many types of speed control. Whereas
synchronous motor always runs at synchronous speed.
5. Induction motor robust in construction and can work in any dusty environment .DC and sync cannot work in dusty
and needs regular maintenance.
6. When coming to the efficiency, upper hand is for synchronous motor but the starting mechanism adds up extra cost,
so even though less efficient induction motor are implemented.
7. Coming to dc motors and synchronous motor, these cannot work in harsh environments, whereas induction motor
can
8. DC motors have brushes and synchronous motor have slip rings where induction motor (squirrel cage type) doesn’t
have, so reduced losses, efficient due to no frictional losses would be present.
Q14) why synchronous generators are used for parallel operation? Why not induction generator?
There are a few reasons why synchronous generators are used to supply electric power
1. Constant Voltage.
2. Constant frequency.
3. Capability to deliver active as well as reactive power.
Appropriate synchronization between all alternators to an infinite bus bar (V=1<0 p.u constant).
Various generators from across the country synchronize with this infinite bus which can supply electric power from anywhere
in the country. This has flexibility to increase the number of generating units synchronized with the infinite bus bar.
On the other hand if we use induction generators instead of synchronous generators, there will be two aspects
1. Reactive power requirement of I.G
2. Harmonic content in output power spectrum.
The I.G requires reactive power for its operation which needs to be provided either by capacitor banks or I.G. units must be
connected to a stable grid where reactive power is available. If we use I.G’s for generation of electric power we will have
severe active and reactive power issues and hence voltage will have imbalance. If we have to install capacitor banks, there will
be huge costing in installation of these equipment s.
The output frequency of Induction Generator is variable which will create uncertainties in deciding ratings for electric
equipment and load side equipment will have problems with varying frequency. To deal with this we need to convert Variable
AC power to DC power and back to Fixed frequency AC power using power electronic equipment which induces high amount
of harmonic content in output spectrum which is undesirable and design of Harmonic filters and their implementation will
increase cost.
Induction machines are also known as Asynchronous Machines, `that is they rotate below synchronous speed when used as
a motor, and above synchronous speed when used as a generator. So when rotated faster than its normal operating or no-load
speed, an induction generator produces AC electricity. Induction generator only produce active power and not reactive
Q15) Does synchronous generator generates real and reactive power?
Yes, synchronous generator can deliver real and reactive power both. When synchronous generator is in over excited
condition, it will deliver reactive power. In general synchronous generator works at 0.9 to 0.97 lagging power factor.
If we consider synchronous motor, so it consumes real power but can deliver reactive power in over excited condition. A
synchronous motor operating at no load and at over excited condition is called as synchronous condenser which is used for
power factor correction. In this condition, motor works at leading power factor and can supply reactive power.
Q16) what is torque and how an induction motor develops torque?
Induction motor is usually 3 multi-phase motor. Consider a 3 phase motor is supplied by 3 phase AC. Due to arrangement of
the stator coils and 3 phase supply applied to them, resultant magnetic field inside the motor is a vector constantly rotating
with fixed rpm which depends on frequency of AC and no. of poles. On the rotor of the motor, there are conducting rods
insulated from each other along the length, but are connected together at the ends. Consider motor is just switched ON, and
rotor is not moving but magnetic field is rotating at fixed rpm. This rotating magnetic field is cut by the rotor rods, hence EMF
is induced in them. This emf produce current along the length of the rods. And this current produces its own magnetic field.
This field interact with rotating magnetic field produced by coils to develop torque. Remember rotor cannot achieve rpm equal
to rotating magnetic field. Because if it does so, magnetic rotor rods will not be able to cut rotating magnetic field and no
torque will be generated.
Q18) How synchronous generator works?
Synchronous generator are called as synchronous because here rotor and magnetic field rotate with the same speed, because
the magnetic field is generated through a shaft mounted permanent magnet mechanism and current is induced into the stationary
armature
Synchronous generator works on the principle of electro-magnetic induction i.e. whenever a current carrying conductor is
placed in a magnetic field than emf is induced in it. Similarly when field magnets rotate between stationary armature
conductors, so armature cuts the in it and current flux than emf is induced starts flowing in it. Thus, for each winding the
current flows in one direction for the first cycle and then in another direction in the other cycle with a time lag of 120 degrees.
Q19) why does an induction generator need reactive power?
In the case of a stand-alone induction generator a prime mover will cause the rotor to move and maintain a constant speed but
since insufficient MMF is present (in the form of residual flux) the machine is unable to self-excite and establish a stator
voltage required to cause a stator current to flow to develop sufficient MMF.
This, as noted earlier, is due to a source or lack of reactive power. This may be overcome by connecting a capacitor to the
terminals of the induction generator such that the residual flux is able to generate a voltage to induce and store energy within
the capacitor, which serves as a source of reactive energy / power, and vice-versa and which leads to self-excitation and a
buildup of the terminal voltage of the induction generator. Once sufficient reactive energy / power is available then the
Induction Generator is able to sustain the em fields and hence the ability to convert mechanical energy from the prime mover
to electrical energy stored in the em fields which propagates this energy to the load
TOPIC 2: CIRCUIT BREAKERS AND ISOLATORS
Q1) what is arc quenching in circuit breaker?
ANS) Circuit breaker is used for interrupting large amount of current in case of fault. It has two contacts: moving contact and
fixed contact. During fault, current may increase rapidly and during interruption of this current, there would be large arcing in
between the contacts of circuit breaker. Therefore we have to reduce or quench this arcing.
Generally load contains reactive element (inductor and capacitor). Sudden change in the current due to breaking induces a
large amount of emf which ruptures the insulation of air in between the contacts of breaker and hence arcing occurs.
This arcing can be quench safely by increasing the di electric strength of the medium between the contacts of circuit breaker.
Di electric strength can be increased by cooling the arcing medium since cooling increase the resistance of arcing path or by
replacing the ionized arcing medium by fresh gasses
OR
When a short circuit current of usually 10 times greater than the normal circuit flows through the circuit breaker under fault
condition. During this if the circuit breaker opens its contacts then the contact area reduces which increases current density
(electron density) further it increases the temperature near the contacts. This temperature ionizes the medium (oil, air, gas)
between the contacts which forces the medium to acts a conductor for electron flow. This instant electron layer with high
temperature is known as arc of the circuit breaker.
NOTE:
DIELECTRIC STRENGTH: Di electric strength is the measure of maximum voltage a material can withstand without
breaking it. It is measured in kV/mm or kV/cm
Q2) why we use VCB (vacuum circuit breaker) at high transmission system? Why can’t we use ACB (air circuit
breaker)?
ANS) vacuum has high arc quenching property than air. Because di electric strength of vacuum is 8 times than that of air.
Therefore VCB’s are used at high transmission system.
Q3) what is the working principle of circuit breakers?
ANS) Circuit breaker has two contacts: Moving and fixed contacts. Under normal conditions, moving and fixed contacts are
in contact with each other forming a close path for the current to flow. Where as in abnormal or faulty conditions (when current
exceeds the rated value) an arc is produced between the fixed and moving contacts and it forms an open circuit, breaking the
circuit.
Q4) what is the difference between over loading and over current?
ANS) Over current is simply the short circuit where the current increases from its allowed value whereas over loading means
increasing the number of loads connected to the system which would increase the current and produces heating effect in the
system. Overloading can be the cause of over current but generally Overload protection is protection against overheating and
over current protection is protection against short circuit.
Q5) what is the difference between fuse and breaker?
ANS) a fuse is a short piece of wire which melts when excessive current flows through it for a sufficient time. Both fuses and
circuit breakers are used for the protection of our power system. Most important difference between fuse and CB is that, circuit
breaker provides protection against both over current (which happens normally in case of short circuit) and over loading.
Whereas fuses provides protection against over loading only and will be burnt if over current passes through them but a circuit
breaker will open if over current passes through it. Fuses does both detection and interruption of fault. Circuit breaker does
only interruptions whereas detection of fault is done by relay.
Q6) what is the difference between off load and on load device?
ANS) an on load device has the capacity to handle the current while switching whereas an off load device cannot handle the
current while switching. Hence for the switching of an off load device, first the load is disconnected then switching is done
and then again load is connected after switching.
Circuit breaker is an on load device whereas isolator is an off load device
Q7) what is the difference between isolator and circuit breaker?
ANS) Isolator is an off load device whereas circuit breaker is an on load device. Isolators cannot operate under fault condition
whereas circuit breaker is designed to operate under fault condition. Circuit breaker protects the system when current exceeds
its rated value whereas isolator have no protection capability and hence they are used to physically disconnect any circuit when
repairs etc. are being done
Q8) why isolator should not be opened before CB?
ANS) because isolators are not designed to carry or break large amount of current. Only circuit breaker is designed to interrupt
fault current that may be many times the rated current. But an isolator is only designed to carry its rated current.
Q9) why do you need isolators on both sides of CB?
ANS) before anyone is allowed to work on a high voltage circuit breaker, it must be tripped, electrically isolated.
Isolation means a physical and visible break in the between the circuit breaker and supply and this is provided by placing
isolator switches in each side of CB. Without these isolator, circuit breaker would remain energized even though the circuit
has been broken by the CB. You must know that power can flow in both directions of the electrical line. So we don’t know
which side of CB is not energized. That is why we place isolators on both sides of CB
Q10) what is SWITCHING CAPACITY in circuit breakers?
ANS) Maximum amount of current that a circuit breaker can interrupt safely without doing any damage to it is called switching
capacity of CB.
Q11) what is Sf6 circuit breaker and where it is used?
Ans) Sf6 (sulpher hexa fluoride) gas is used as an arc quenching medium in circuit breakers. It is termed as SF6 CB. SF6 gas
has superior arc quenching capacity and is 100 times more effective in arc quenching media than air circuit breaker. So they
have very short arcing time and can interrupt high currents.
Q12) what are the types of circuit breakers and their applications?
ANS) The different types of high voltage circuit breakers which includes the following
1. Air Circuit Breaker (used for a common protection of plants, machines like generators, motors, transformers, capacitors,
etc... Can be used up to 15kv)
2. Vacuum Circuit Breaker (generally used for voltage applications ranging from 22kV to 66kV)
3. SF6 Circuit Breaker (Used for both medium and high voltage electrical power system from 33KV to 800KV.)
4. Oil Circuit Breaker
Q13) what is trip characteristic of circuit breaker?
Trip Characteristic of Circuit Breaker: Basically, the type refers to the current range at which the circuit breaker trips
instantaneously. In other words, the type of circuit breaker must be selected so that it allows the inrush current without
tripping
Type B Circuit Breakers: Minimum trip current: 3 to 5 times rated current. This type of circuit breaker is used for purely
resistive loads, or loads with a very small inductive component such as Lighting circuits (non-inductive), General purpose
outlets
Type C Circuit Breakers: Minimum trip current: 5 to 10 times rated current. This type of circuit breakers is used for loads with
a moderate inductive component, typically electrical equipment which uses low-HP motors or specific types of lighting: Air
conditioners, Residential / Commercial Pumps, Fans, lighting which uses ballasts with an inductive component
Type D Circuit breakers: Minimum trip current: 10 to 20 times rated current. This type of circuit breaker is used for loads with
a very high inductive component, typically found in industrial settings: Large induction motors or transformers, X-ray
equipment, Welding equipment
Q14) what are poles of circuit breaker?
Poles of circuit breaker: SP (Single Pole) MCB: In Single Pole MCCB, switching & protection is affected in only one phase.
Have one hot wire and one neutral wire. Single-pole breakers are rated for 15 or 20 amps.
Application: Single Phase Supply to break the Phase only, General lighting outlets, Fans
DP (Double Pole) MCB: In Two Pole MCCB, switching & protection is affected in phases and the neutral. Have two hot
wires that share one neutral wire. Double-pole breakers are typically rated for 20 to 60 amps
Application: Single Phase Supply to break the Phase and Neutral, Central air conditioners
TP (Triple Pole) MCB: In Three Pole MCB, switching & protection is affected in only three phases and the neutral is not part
of the MCB.
3 pole MCCB signifies for the connection of three wires for three phase system (R-Y-B Phase), heavy induction motors,
Application: Three Phase Supply only (Without Neutral).
TPN (3P+N) MCB: In TPN MCB, Neutral is part of the MCB as a separate pole but without any protective given in the neutral
pole (i.e.) neutral is only switched but has no protective element incorporated. TPN for Y (or star) the connection between
ground and neutral is in many countries not allowed. Therefore, the N is also switches.
Application: Three Phase Supply with Neutral.
TOPIC 3: SMART GRID
Q1) what does smart means in smart grid?
ANS) anything that involves two way communication is called as smart such as smart phones, smart TV etc.
Q2) what is smart grid?
ANS) Most of us do keep two cellphones with us while travelling. One being an ordinary phone such as Nokia 3310 and other
is an expensive one like i-phone 6. Both the phones are used for messaging and call purpose but what does “I-phone 6” has
advantage over nokia 3310? I-phone 6 has other features such as camera, video voice recording, music, games etc.
Now apply the similar concept to the grids. Smart grid is just an evolutionary form of our traditional grid having a two way
communication between the utility company and the consumer and which provides benefits to both the consumer and the utility
such as smart meters, smart appliances, renewable energy resources, energy efficient resources etc.
The basic concept of Smart Grid is to add monitoring, analysis, control, and communication capabilities (MACC) to the
national electrical delivery system to maximize the output of the system while reducing the energy consumption.
Q3) what does two way communication means in terms of smart grid?
ANS) Here the two way communication will be between the consumer and utility company. That is, the utility company can
monitor the consumer’s electricity consumption round the clock and if they find that a particular consumer’s device is causing
the overloading on the system and increasing the burden on the system than utility company can turn off that device from their
ends or they can inform the consumer to switch off that devices.
Q4) how two way communication is done in smart grid?
ANS) Communication is done through PLC (power line communication). Power-line communication (PLC) is
a communication method that uses electrical wiring to simultaneously carry both data and electric power.
Other methods such as IEEE 802.15.4 (Zig Bee), IEEE 802.11 (wireless LAN (WLAN) or Wi-Fi) and IEEE-802.16 (WiMAX),
GSM, GPRS
Q5) is smart grid all about smart meters?
ANS) There is a misconception that smart grid is all about smart metering. Instead there are many other things involves in
smart grid such as:
1) Employment of smart devices.
2) Tracing the location of faults.
3) Self-healing of grid.
4)
5)
6)
7)
8)
9)
Preventing blackouts.
Real time dynamic pricing
Reducing the gap between demand and supply.
Monitoring of grid health.
Reducing electricity theft
Remote controlling of different appliances from a mobile app.
Q6) what benefits it provides to the utility company?
ANS) Smart grid provides many advantages to utility. The key benefits are:
1) MONITORING GRID HEALH:
On the transmission and distribution side, sensors and digital relays will be installed on power lines which will enable utility
to operate system with greater efficiency. Devices such as SYNCHROPHASERS, can sample voltage and current 30 times a
second or faster giving utility and system operator, a far more accurate view of grid’s health. A broad deployment of synchro
phasors could be used as an early warning sign to help or prevent faults before they turn into blackouts.
2) TRACKING THE FAULT:
Another application of smart grid that would be more visible to user would be the introduction of smart meters which could
track electricity use in real time and can transmit data to the utility company.
Once smart meters are installed, Power Company can determine the location of outages (area where there is no light). Utility
company won’t have to send staff to read meters.
In many places, a power company will only know that service is out if a customer calls. In a smart grid scenario, if service is
interrupted the company will know right away because certain components of the grid (smart meters in the affected area, for
instance) stop sending sensor data.
3) PREVENTING BLACKOUT
If there is unbalancing of power, like there is more demand of electricity as compared to the supply. Then this unbalancing can
combine with other grid events to trigger blackout. Since there are smart sensors deployed on the power lines that provides
data to the utility, they will detect the unbalancing. And hence utility company can act on it and prevent the blackout.
4) PREVENTING ELECTRICITY THEFT:
Problems like electricity theft can be sent by meters in real time and utility can act upon it instantly.
Q7) what benefits it provides to the consumers?
5) MONITOR THEIR ELECTRICITY USAGE:
Smart meters will communicate with smart devices in homes such as smart thermostat and other electrical appliances that
will monitor electricity usage. The consumer will be able to see to access information via web based portal, through which
they will be able to see the electricity consumption of device and as well as control that device.
6) REDUCE IN THE ELECTRICITY BILL:
Utility will able to predict the peak demand time and if demand charges are implemented in Pakistan than consumer will
know the peak demand time and they can reduce their bill by consuming less electricity at peak demand times.
Q8) what is self-healing in smart grid?
Self-healing is the ability of the grid to automatically isolate the faulty section from the rest of the grid. If a fault occurs in a
system than traditionally it used to be removed manually but with the advancement of smart grid, it will itself eliminate the
fault part from the grid without manual intervention.
In self-healing grid we have a ring main grid network having RMU’s with at least two isolator and one breaker arrangement.
The communication would be done between two FTU’S via gprs. At the time of fault on a particular section, the respective
isolator automatically opens up and eliminate the faulty part.
One of the main goals of a Self-Healing Grid is to improve system reliability. When a fault occurs, reclosers automatically
locate the fault, disconnect power at that point to isolate it, then report it and re-establish service to as many customers as
possible from alternate sources/feeders.
The smart distribution system uses algorithm based control commands to source electricity from an alternative power
generation source and adjust loads at substations and capacitor banks, while the switchgear reroutes the power to areas around
the fault location.
Smart distribution systems not only allow end-users to maintain their productivity but they also allow utilities to save millions
of lost revenue dollars.
This is done within 15-30 seconds depending upon the bandwidth of communication between two FTU’S
FTU’s = - Feeder terminal units (FTU) play a crucial role in delivering electricity to consumers they monitor I/O statuses
by collecting and processing digital and analog data.
RTU’s= Ring Main Unit (RMU) is a HT panel having 3 nos. of switches (Circuit Breakers or Isolators or LBS) that are 2
for incoming one for outgoing.
Q9) what are issues related to smart grid?
1) CONVERTING A GRID TO A SMART GRID:
Converting a grid into a smart grid is not an easy task at all. It requires lots of investment and proper planning. Deployment of
sensors on transmission line, integrating renewable energy into smart grid, using advance communication technologies,
installing new energy meters etc. These things requires huge investments.
2) RISK OF HACKING OF METERS:
Using internet for real time data transfer can be risky. One simulation found that malware experts can easily infiltrate (hack)
one smart meter and spread the virus to 15000 meters within a day, enabling hackers to remotely shut off power within a click
of button.
3) KILL SWITCH:
Utility can switch off any device of a consumer if that devices would be consuming more electricity and will be a burden to
the system. Hence it provides no control of consumer over his appliances. Utility can switch off any device instantly by what
is called a KILL SWITCH.
Q10) Renewable energy integration in smart grid?
ANS) Renewable energy will be integrated in smart grid by different sources such as wind, solar etc. by using power
electronic convertors. This will reduce the burden on the utility company
Q11) is smart grid suitable for Pakistan?
ANS) This concept is still new in Pakistan. Although different utility companies are working towards this idea by implanting
smart meters and deploying various sensors in the system. But still this requires, huge investment, planning and most
importantly educating people about the advantages of smart grid.
TOPIC 4: VARIABLE FREQUENCY DRIVE
1) What are electrical drives?
In very simple words, the systems which control the motion of the electrical machines, are known as electrical drives. This
drive system is widely used in large number of industrial and domestic applications like factories, transportation systems,
textile mills, fans, pumps, motors, robots etc. Drives are employed as prime movers for diesel or petrol engines, gas or steam
turbines, hydraulic motors and electric motors.
2) What is Variable frequency drive?
ANS) Motors speed can be changed by changing its frequency. Therefore VFD’s are actually motor controllers that controls
its speed by changing its frequency and voltage. Other names for VFD’s are adjustable speed drive, micro drive, AC drive etc.
3) How VFD’s works?
VFD’s consist of four important components:
1)
2)
3)
4)
CONVERTER: Converter circuits converts the incoming AC supply to DC.
SMOOTHING CIRCUIT: This circuit smooth’s the pulsating included in dc
INVERTER: Inverter converts DC to AC
CONTROL CIRCUIT: This circuit is used to control inverter part.
4) In VFD first we convert AC to dc by converter and then we again convert Dc to Ac by an inverter? What’s the logic
behind it?
ANS) First we convert the incoming 220v Ac supply which is at 50 hertz frequency. Then DC to AC is done again because
we want to produce the waveform at our desired frequency. Whatever frequency is required to control the speed of our motor
we can produce waveform of that particular frequency.
Secondly, it’s difficult to do frequency conversion directly (from incoming 50 Hz supply from k electric to our required
frequency) and more easy to convert it to dc and then produce waveform of our desired frequency
5. How VFD’s can save energy?
 Majority of the devices such motors used cannot change their speed by themselves and during majority of the time
they do not need to be run at full speed. During those times if the speed of that motor can be reduce than energy it
consumes can be reduce drastically. So VFD when connected to these motors can control its speed by changing its
frequency.
 During startup machines draw huge current (inrush current) which is much higher than normal rated current. So to
reduce that current we use VFD in combination with a soft starter to reduce the current and protect the equipment from
being damaged.
6. Can I put a VFD to control any motor?
The speed of any motor is dependent on its applied frequency. However some AC Motors are just not suitable for variable
speed operation such as single phase motor because generally speed single phase application are fixed speed.
To control the speed of 3 phase IM, Synchronous motors VFD’s are generally used
7. Difference between soft starter and VFD?
Soft starters and variable frequency drives are two different purpose products. VFD is for AC motor speed control, it's not only
change the output voltage but also change the frequency; Soft starter is a regulator actually for motor starting, just changing
the output voltage. Variable frequency drive has all the features of soft starters, but the price is much more expensive than the
soft starter and the structure is much more complex.
Variable frequency drive is converting power supply (50Hz or 60Hz) into a variety of frequency AC power, to achieve electric
motor variable speed operation control. Whereas Soft starters are basically used for reducing starting current of motors which
is normally 5 times of full load current and they stay in circuit for less than 30 sec and then are bypassed.
Whereas a VFD is used where your motor runs at variable speed and torque
8. Drawbacks of VDF’S?



The high-frequency PWM switching that VFDs use to generate the output voltage waveform often carries high-frequency
transients out onto the wires going to the motor. Often it is recommend of using special shielded cable between the VFD
and the motor, which can be more expensive and harder to work with (in conduit, etc.). If not properly shielded, this
transient noise can cause interference with other low-voltage electronic devices that may be nearby.
Another major problem that may arise is repair and maintenance of a VFD. General technicians can’t provide necessary
technical support in case of any drawback. The problem increases if more than one VFDs are networked together.
VFD’s cost will increase as the system size increases.
9. Are there other methods to control speed other than VFD?
The most widely used large motor is the AC induction motor and for them, the VFD is the best way to control speed, electronically
speaking, however you can use gears and variable speed drive belts…
Previously load banks were used to control speeds of motors by reducing the voltage across the motor. They are a bank of resistors
and basically create voltage dividers such that one is across the load bank, and the other across the motor. The bad thing about
them is they waste a lot of energy. The good thing is that they last a long time.
10. Can VFD be used to control multiple motors?
ANS) Yes same VFD can be used to control multiple motor but there are certain condition that must be met first:
FIRST CONDITION: Each motor must have same speed. If we have one VFD for each motor than there is no issue of running
motors at same speed but since we have only one VFD for multiple motor than they must be operating at same speed.
SECOND CONDTION: Our system will become less reliable because if that one VFD which is controlling all the motor fails
than our whole system will shut down.
Various VFD bypass schemes can be used to overcome this limitation, but these schemes all add cost and complexity to the
system.
THIRD CONDITION: care must be taken when operating the motors. To minimize VFD size, all motors need to be started
up simultaneously. The VFD will ramp all the motors up to speed at a controlled rate, minimizing the inrush current required
by each motor at startup.
11. Difference between SOFT starter and VFD?
Choosing a soft starter or a variable frequency drive often depends on your application. Soft starters are smaller and less
expensive when compared with VFDs in larger horsepower applications. Larger VFDs take up more space and are usually
more expensive than soft starters. That being said, while a VFD is often more expensive up front, it can provide energy savings
of up to 50 percent, thereby producing more cost savings over the life of the equipment. Speed control is another advantage of
a VFD, because it offers consistent acceleration time throughout the entire operating cycle of the motor, not just during startup.
VFDs can also provide more robust functionality than soft starters offer, including digital diagnostic information. It is important
to note that a VFD can initially cost two to three times more than a soft starter. Therefore, if constant acceleration and torque
control is not necessary, and your application requires current limiting only during startup, a soft starter may be a better solution
from a cost standpoint.
12. What is IGBT?
IGBT is has the combined features of both MOSFET and bipolar junction transistor (BJT). It is gate driven like MOSFET and
has current voltage characteristics like BJTs. An IGBT, or insulated gate bipolar transistor, is a solid state device (with no
moving parts). It is a switch that is used in order to allow power flow in the On state and to stop power flow when it is in the
Off state. An IGBT works by applying voltage to a semiconductor component, therefore changing its properties to block or
create an electrical path. Much like an Solid State Relay but built for higher power applications.
It switches electric power in many modern appliances: variable-frequency drives (VFDs), electric cars, trains, variable speed
refrigerators, lamp ballasts, air-conditioners and even stereo systems with switching amplifiers.
13. Why is IGBT used in inverter modules of VFD?
1. The high impedance gate of an IGBT means it is comparatively simple to turn it ON and OFF quickly by
controlling the gate.
2. The IGBT has a fast switching speed. This minimizes switching losses and allows for high switching
frequencies which is good for motor harmonic and noise reduction.
3. It can have a high current-carrying capacity. IGBT modules are available with maximum rated collector current Ic(max)
exceeding 100A
4. The IGBT has a wide Reverse Bias Safe Operating Area (RBSOA) which means it can be reliably protected against load
short circuits by desaturation detection circuits.
TOPIC 5: NET METERING
Q1) what is net metering?
ANS) net metering is an agreement between a consumer who installs a Solar PV power plant on his premise and the Electricity
Distribution Company that allows the solar PV system owner to sell excess solar energy to the utility company
Following 2 cases take place in this scenario:
Case 1: If at any moment of time, if solar energy generation (kWh) is less than the load requirement at that time, the difference
of energy is taken from the main grid and the meter runs forward, as usual. In this case, the system owner is charged for the
units (kWh) consumed from the main grid. E.g. During early morning or during late evening/night.
Case 2: If at any moment of time, if solar energy generation (kWh) is more than the load requirement at that time, the excess
solar energy is fed back to the main grid and the meter now runs backward. In this case, the system owner gets credit for the
units (kWh) fed back to the main grid. E.g. During peak sunshine hours (afternoon)
When you generate extra electricity (more than what you can use) the meter moves backwards.
Thus, at the end of the billing period:
If case 1 > case 2, then the owner is charged for the difference of units as per usual retail tariff
If case1 < case 2, then the difference of units is either carried forward to the next billing period or the owner is paid for the
difference of units as per the tariff decided by the concerned utility
In short, the owner pays/gains for the ‘net’ energy used over the designated period of time.
Q2) what is feed in tariff?
ANS) Feed-in tariffs (FIT) are fixed electricity prices that are paid to renewable energy (RE) producers for each unit of energy
produced and injected into the electricity grid. These rates are usually set high by the utility to encourage consumer to generate
more electricity.
Q3) meters used in net metering?
ANS) the meters used in this case read the ‘net’ energy consumed by the system owner. These meters have the ability to move
in both directions: backward and forward However, we can also use two energy meters – one to measure solar energy
generation and the second to measure the units consumed from the utility grid.
Q4) is net metering suitable for Pakistan?
Yes it is suitable for Pakistan. 2 most important advantages of net metering are:
1. Financial benefit for the system owner: Since the system owner is charged for the net energy consumed from the utility
grid, the owner gets financial benefits. E.g. if energy generation < energy consumed: owner pays just for the net amount. If
energy generation > energy consumed: the owner gets credit for excess generation.
2. Avoid the use of batteries: In a grid connected solar PV system, any excess energy generated can be fed back to local
utility grid and can be taken back at later stage when required. Thus, there is no need to store the surplus energy in batteries
for later use, thus, avoiding the heavy costs of batteries. Also, since batteries are eliminated, the maintenance costs of the
system also reduce to a great extent.
Q5) what are the issues related to net metering?
There are many issues related to net metering:
1)
People are not aware how the net metering works, how bill will look like, settlement procedures & cycles and how much
bill they will finally get after settlement.
2) Most of the rooftops are not suitable for solar due to shading.
3) High upfront cost and payback is more than 5 years
Q6) how is excess of electricity produced by solar energy fed to the grid?
Solar panels converts sun lights into electricity. Electricity produced by solar panels is Dc in nature hence an inverter is used
to convert DC to AC. This inverter is called as grid tie inverter which sends electricity to the grid through electricity lines.
TOPIC 6: BLACKOUT AND TRIPPING OF TRANSMISSION LINES
Q1) what does tripping means in transmission lines?
ANS) Tripping means the interruption in electricity supply. An electric line is tripped if it starts carrying the fault current. It
is a protective measure which essentially isolates the faulty lines from the rest of the healthy sections
Q2) why does fog causes tripping of transmission lines?
Two conductors paired as transmission lines are at sufficient distance from each other to prevent arcing between them. Such
distance can become ineffective when fog increase
Due to the Fog, moisture content between the two transmission conductors increases which provides the low resistance current
flow path and hence conductivity increases between lines. That's why flashover occurs between the transmission lines and
large current flow them and tripping occurs.
Fog adds moisture to pollution or salt contamination on insulators creating a conductive path. Fog on its own with no pollution
is generally not a problem.
Q3) what is cascaded failure in terms of transmission lines?
Large blackouts are often caused by a series of events called as a cascade failure. Cascade failures are initiated by a disturbance
of the energy flow in the system. If a fault occur on a transmission line and its trips (disconnected from the other section) than
Failure of transmission line puts additional stress on the on the other part of the system and thus weakens the system.
Additionally, the trigger event may cause voltage or frequency swings. Increased stress on the network may cause relays to
trip other lines in the system. Voltage and frequency swings are dangerous for generator equipment and may cause protection
devices to shut down generation. Both types of events cause further instability. Once these failures follow up rapidly after one
other, one speaks of a cascading failure. (Cascading means ‘one after the’)
Q4) Difference between blackout and load shedding?
Whenever load or the demand of a particular area is increased beyond the supply than utility deliberately reduces the load of
that area and this is called as load shedding. In load shedding one or two areas are effected
Whereas if because of any fault, large area goes under darkness then this will be termed as blackout. For example during
rainfall, majority of the feeder trips and almost half of the city gets power shortage.
Load shedding is done deliberately whereas blackout can be done deliberately but most of the time they are
ACCIDENTALLY due to some fault.
Q5) what is brown out? What is the Difference between blackout and brownout?
Ans) Brownout is reduction of power to an area and it results in dimming of light whereas blackout is loss of power to big
area.
The name brownout comes from the color of the bulbs which typically turns brown due to dimming when the voltage drops.
In some cases, brownout is done deliberately by utility to prevent blackout. Brownout usually occurs when there is an increase
in power usage and also due to overload on an electrical system
TOPIC 7: BASICS OF POWER SYSTEM 1
Q1) what are power systems?
ANS) The process of transferring electricity from generation end to consumer end is called power systems. It includes
Generation, transmission and distribution.
Q2) why electrical energy is transmitted at higher voltage and not at lower voltage?
ANS) Voltage and current are inversely proportional. The More the voltage, the lesser will be the current. Electrical energy is
transmitted at higher voltage to reduce the copper losses which are current dependent losses (I^2*R) Losses.
If electrical energy is transmitted at lower voltage than current increases and ultimately it will increase the cross sectional area
of conductor and the cost of transmission will rise. So to reduce cost and copper losses, electrical energy is transmitted at
higher voltage.
Q3) Why generation voltage is usually between 11-33kv?
ANS) Generation is even being done at 6.6kv in world. If generation voltage is below 11kv then voltage decreases and current
increases. Ultimately copper losses will increases and the cost too. So i.e. generation voltage is not below 11kv.
It is not higher then 33kv because, generator has certain insulation level and if voltage rises so we need more insulation.
Insulation cost will increases which is undesirable
Q4) how is electrical energy produced?
ANS) Phenomena of electrical energy production is simple and is based on law of energy conversion that energy can neither
be created, nor it can be destroyed, it can only be transformed from one form to another.
Electrical energy is one of the most efficient energy and hence can be produced from various forms of energy such as wind,
solar, nuclear, thermal etc.
Q5) what is the use of TIE LINES in power system?
ANS) Modern day power systems are divided into various areas. The transmission line connecting an area to it neighboring
area is called tie lines. Power sharing between two areas occurs through these lines.
Q6) why can't we store AC in batteries instead of dc?
ANS) Because AC changes its polarity 50 times in a second (when the frequency is 50 Hz). Therefore battery terminals should
change between positive and negative 50 times a second but a battery cannot change its terminals at such rapid speed.
Furthermore if we connect Ac supply to battery, it will charge during positive cycle and discharge during negative cycle.
Positive cycle cancels negative cycle so average current or voltage will be zero
Q7) Difference between transmission and distribution?
1) The process of transferring electricity from generation end to the load centers is called transmission whereas process of
transferring electricity from load centers to consumer end is called distribution.
2) Most important difference between Transmission and distribution is that during transmission, none of the consumers are
provided electricity whether it be an industrial consumer or residential consumer whereas electricity is provided to consumers
in distribution phase.
3) Transmission is done at high voltage levels where as distribution is done at low voltage levels.
Q8) why voltage levels are like 11, 22, 33, 66,132,220kv? Why not 10.5, 38.5, 66.7?
ANS) When an alternator generates voltage, we always use a multiple of 1.11 because for a pure sine wave the FORM
FACTOR is the ratio of rms value of voltage or current with the avg. value of voltage or current and for pure sine wave rms
value of current is Imax/root '2' and avg. value is 2Imax/pie and which comes out to be 1.1;
Therefore we can't have a combination of other then a multiple of 1.1 and that is why used such voltage levels. 11,22,33,66,110
kV
Q9) what is electrical machine?
ANS) a device which is used for electro-mechanical energy conversion or vice versa is called as an electrical machine. If the
device converts electrical to mechanical energy then it is termed as motor, where as if a device converts mechanical to electrical
then it is termed as generator. By changing the input-output configuration, same device can work as either generator or motor.
Q10) what is rated in electrical machine?
ANS) Rated means the optimum level at which machine can operate.
For e.g.: If a motor is rated at 230v, 10A, 50HZ, 1100RPM. It means that it can operate at 230v maximum with supply
frequency at 50 Hz, having a full load current of 10A and a maximum speed of 1100rpm.
Q11) what is load in terms of electrical engineering?
ANS) Any device which consumes electricity and converts it into other forms of energy is called as load. For example: all the
appliances which runs on electricity in our homes are called as loads
Q12) if someone hangs on an electric line. Will he get a shock? He has no contact with other line and doesn't touches
earth?
ANS) No, for current to flow from one point to another there should be potential difference between two points. Hanging in
air holding just one wire doesn’t give you a shock unless you touch earth or another wire.
Q13) what will happen if an electric power distribution lines falls in river? Suppose if a person is swimming in that
river? What will happen to him?
ANS) Water is a conductor of electricity and its conductivity depends on its purity. For e.g.: Pure water has high resistance
and low amount of current will flow through it. Impure water has lower resistance and high amount of current will flow through
it.
If a person is swimming in a river and electric lines falls into it than the person may get a shock if he is in contact with the
ground and water simultaneously. If he is swimming completed isolated from the ground than probably he won’t get a shock
as the circuit is not completed.
Q14) if a person hangs from transmission line. Why doesn't he experience a shock? Even though air below him is
completing the circuit?
ANS) Air acts as an insulator and hence the circuit for the flow of current is incomplete and no current flows through that
person. But if the voltage is high enough and the person is hanging low enough from the ground than even though he is not
touching the ground, he will be electrocuted because of ionization potential.
Apart from it the person hands should be apart from each other and if not than enough potential difference could be created
between hands and current will start flowing, leading to death. So, to survive the person should be hanging with his hands as
close as possible and as high as possible from the ground.
Q15) what is the difference between phase and line voltage?
Line voltage is the voltage between two given phases whereas phase voltage is the voltage between a phase and a neutral
Q16) what is the difference between voltage and current?
Voltage is the “Energy of electric charge”. It is the potential difference in an electric charge between two points whereas
current is “Rate at which electric charge flows”. This can be understood by an analogy. Consider a slide which is at high
height and is steep. Water will flow from it rapidly. Similarly if the slide is at low height so water will not flow at high
speed. Slide represents voltage whereas water represents current. Current cannot flow unless and until there is potential
difference.
Voltage is always measured between two points.
Q17) why ammeter is connected in series and voltmeter in parallel?
Because ammeter has to measure current and ammeter is a device which has very low resistance so that maximum current will
pass through its coil. If it is connected in parallel so very high current will pass through its coil and will burn it because ammeter
has low resistance and current flows through least resistive path. That is why ammeter is not connected in parallel.
Voltmeter has a very high resistance and is connected in parallel because if it is connected in series then no current will be
there in the circuit due to its high resistance.
Q18) Can we determine transmission line voltage by counting number of insulator disc?
Yes, we can have a general idea of voltage by counting insulator disc. Generally an insulator disc carry 11kv so:
1) 33kv voltage will have around 3-4 insulator disc
2) 66kv voltage will have around 6-7 insulator disc
3) 132kv voltage will have around 12-13 insulator disc
4) 220kv voltage will have around insulator disc 20 disc
Q19) what is enameled wire?
Magnet wire or enameled wire is a copper or aluminum wire coated with a very thin layer of insulation. It is used in the
construction of transformers, inductors, motors, speakers, hard disk head actuators, electromagnets, and other applications that
require tight coils of insulated wire. When wound into a coil and energized, magnet wire creates an electromagnetic field.
TOPIC 8 INTERESTING INFORMATION RELATED TO ELECTRICAL
ENGINEERING
Q1) what kills a person? Current or voltage?
ANS) let me explain you with an analogy. If you have the gun but not the bullets than gun is useless. Similarly if you have
bullets but not the gun than gun is useless. So current cannot flow unless there is a potential difference.
Both current and voltage are equally dangerous. Your body doesn’t needs thousands of amperes to get feel electrical shocks
instead studies says that 1mA of current can be felt, 5mA of current is painful, 10mA of current causes muscle
contraction.15mA of current causes loss of muscle control and 70mA of current if passed through heart can even cause death.
So, both voltage and current are equally dangerous.
2)
Which one is more dangerous AC or DC?
ANS) DC has zero frequency. So if someone gets electrocuted by a dc than his body will stuck to the circuit just as if you just
held on to a super duper glue. In the meanwhile all his muscles would contract, heart will stop beating because of contraction
and other major burns and eventually he will die.
AC can kill you with even lesser power. AC needs more insulation than DC since in AC we normally use RMS values to show
the current or voltage where as in dc we uses peak values to show current, voltage FOR EG: 220V DC (Peak value)= 320 V
AC( peak value)
In DC your body would contracts once if touched but in AC, since it has fluctuating frequency (50 Hz) so if a person gets
electrocuted by an AC, he would experience series of muscle contraction. That is like multiple dc shocks in a small gap and
that too current flow in both directions alternatively.
3) How to know if someone is stealing your electricity?
ANS) turn off all the power in your house using the main disconnect beside your breaker box or fuse box. Then go outside and
examine your electric meter. If all your power is turned off, the meter should stop moving. If it has not stopped, someone may
have intercepted your electricity OR the meter could have some problem too
4) Why fans and other electronic appliances makes noise while operating on an inverter?
ANS) The output of an inverter is not a pure sine wave. Harmonics are created in the system when a sine wave distorts i.e. an
electrical device which is fed by an imperfect sine wave at 50 Hz fundamental frequency has harmonics. Also machine has
magnetic materials inside them which produces sound due to magnetic effect called Magnetostriction
5) Why is there a sparking in the ends of a tube light even when it is of?
ANS) Even current is not there, filament is still hot which excites some gas atoms and thy releases photons. This is the reason
why we see sparking in the ends of tube light even when it is off
6) How a human body does conducts an electric current?
ANS) Blood, nerves and certain other parts of body are basically electrical conductors. Nerves must conduct electricity because
that’s basically how they operate. Many of the cells in your body carries ions like calcium in order to perform normal body
functions and blood carries electrolytes that have electrical properties to maintain osmotic pressure and acidity in your body
Another reason is that because water conducts electricity in your body and your body is 75 % water. Hence you will feel
electric shock
7)
Why utility companies generates electricity at 50 Hz? What does 50 Hz frequency means?
ANS) Generally the utility company produces electricity in 3 phase at a frequency of 50 Hz. and each of the three phase gets
charge positively for 0.001 sec and then negative for 0.001 sec in a sine wave manner( 1/50=0.001 sec)
Now when one phase is having positive charge the other may be having negative charge. Therefore ideally every phase wire
becomes positive and negative 50 times in a second and current direction reverses every 0.001sec in same phase. This way
every phase wire mutually give and take current to each other and carry return current.
Every phase wire becomes positive and negative 50 times in a second and you can see it by using a phase tester.
50 Hz means that the bulbs which are connected in our homes will flicker (turn on and off rapidly) 50 times in a second. It
does happens in our homes but we cannot observe this phenomena by our eyes as the flickering is at such a fast pace that our
eyes cannot detect it and it seems like bulbs are not flickering at all.
8)
Why electricity is not generated at 20 Hz or 80 or some other frequency? Why 50 Hz?
IF WE GENERATE ELECTRICITY AT 20 HZ:
20 Hz means that every phase wires gets positive and negative in 0.05 sec (1/20=0.05). Therefore at low frequency, flickering
in bulbs can be detected by our eyes and will be extremely annoying.
IF WE GENERATE ELECTRICITY AT 70 HZ OR ABOVE:
1) Drop across inductor will increase as inductance is given by XL= 2*pie*f*L. A higher drop will cause the line voltage
to sag
2) Corona loss is directly proportional to frequency. Hence losses due to corona will increase
3) At high frequency current tends to flow through the conductor’s surface than the core of the conductor. This effect is
called skin effect and it increases with the increase in frequency.
4) Due to skin effect, heat losses called as “ I square R losses” will increase
Even in distribution phase, high frequency will not be efficient
We use a lot of power electronic based devices. High frequency will cause large switching losses
9) Why 230 volts is for single phase and 430v for 3 phase?
ANS) A Sinusoidal waveform varies with time uniformly. When we say voltage, it is actually the rms value of voltage i.e.
Vm/ root’2’ and it is designed to be 230volts. Current flows in wires because of the potential difference that is created by the
two “phase and neutral wires”. Phase wire being at 230 volts whereas neutral wire being at 0 volts and hence due to this phase
difference, current flows.
But when we talk about three phase voltage, there will be 120 degrees distance between the three phases therefore:
Sin120= 0.866
230+ (230*0.866) = 430 volts
10) What is the difference between High Tension and Low Tension lines?
Tension is a French word for voltage. A Low tension is a low voltage line and high tension is a high voltage line. LT line is
used for supplying voltage for small consumers of electricity such as houses, shops, small offices and other smaller
manufacturing units whereas HT line is used for supplying voltage to bulk consumer of electricity such as industries, hospitals,
flats, universities etc.
1) Low tension is up to 1000v i.e. 1kv
2) High tension is above 11kv
11. What is the difference between grounding and earthing? Are they both same?
Earthing and grounding are different terms for expressing the same concept:
1) Earthing means connecting the dead part (The part which doesn’t carry current under normal condition to earth). For
e.g. electrical equipment frames, enclosure, support
Whereas grounding means connecting the live part (The part which carries current under normal condition to earth).
For e.g. neutral of a power transformer
2) The purpose of earthing is to minimize the risk of receiving an electrical shock if touching metal part when fault is
present. Whereas the purpose of grounding is the protection of power system equipment
12. What is the voltage regulation range in transmission and distribution side? Why is it kept more at distribution side?
Voltage regulation is a measure of change in the voltage magnitude between the sending and receiving end of a component,
such as a transmission or distribution line. Voltage regulation describes the ability of a system to provide near constant voltage
over a wide range of load conditions
Voltage regulation range in the transmission side is 3-5% whereas in distribution side it is 5-10%. Voltage drop range is kept
more in the distribution side because consumer’s equipment’s are connected at the distribution side. So, to protect them, voltage
drop range is kept more in distribution side
Q13) what is Breakdown voltage, di-electric strength, Circuit breaker making and breaking capacity?
1) Breakdown voltage is the minimum voltage which causes a portion of insulator to be electrically conductive.
2) Circuit breaker's breaking capacity is the maximum amount of current which a breaker can safely interrupt without damaging
itself. It is expressed in RMS value
3) Circuit breaker's making capacity is the maximum current which a Breaker experienced in the first cycle of fault. It is maxim
current which breaker can conduct at the instant of closing. It is expressed in Peak values.
4) Di- electric strength is the maximum voltage which can be applied to a material without causing it to breakdown.
Q14) Difference between MCB, MCCB, RCB?
1) MCB: Miniature Circuit breaker. It protects our system from short circuit current. MCB is for devices that uses current of
less than 100 Amps. Its interrupting capacity is up to 18,000 amps. Commonly used in Homes.
2) MCCB: Molded case circuit breaker. It also protects our system from short circuit current but the main difference between
MCB & MCCB is that MCB is used for application that requires less than 100 amps whereas MCCB is used for applications
that requires more than 100 Amps. Its interrupting capacity is from 10,000 to 200,000 amps. Used in industrial application
3) RCCB: Residual current CB. Used to provide protection against electric shock. Rated normally at 30-50 mille amperes
because 70ma is enough to kill a person. So RCCB trips at very low value than that
Q15) Ways of improving power factor?
1) CAPACITOR BANK: Capacitor provide reactive power, so reactive power dependency of the load on main supply is
reduced because load is getting reactive power from cap. Bank and hence pf is improved. The capacitor (generally known as
static capacitor) draws a leading current and partly or completely neutralizes the lagging reactive component of load current
2) CONDENSER: Synchronous motor operating at no load and at over excited condition is called as synchronous Condenser.
It provides leading current which cancel out lagging current of load and hence pf is improved. When such a machine is
connected in parallel with the supply, it takes a leading current which partly neutralizes the lagging reactive component of the
load. Thus the power factor is improved.
3) PHASE ADVANCER: Phase advancers are used to improve the power factor of induction motors. The low power factor of
an induction motor is due to the fact that its stator winding draws exciting current which lags be-hind the supply voltage by 90
degrees. If the exciting ampere turns can be provided from some other a.c. source, then the stator winding will be relieved of
exciting current and the power factor of the motor can be improved. This job is accomplished by the phase advancer which is
simply an a.c. exciter. The phase advancer is mounted on the same shaft as the main motor and is connected in the rotor circuit
of the motor. It provides exciting ampere turns to the rotor circuit at slip frequency. By providing more ampere turns than
required, the induction motor can be made to operate on leading power factor like an over-excited synchronous motor
Q16) why is neutral connected to Earth in a transformer?
Earth is the largest ever capacitor we know about, Ie. It has infinite capacity to store charge!
Now in Electrical system the neutral (as its name says itself) is supposed to carry the Current under unbalanced conditions
only (to stabilize our system and protect the appliances it supplies to)
Ideally the neutral point is supposed to be at Zero Potential, but in practice under some conditions it can carry some potential
difference, so to make this potential near to grounds, we connect the neutral to earth!
If we don’t do it, then the unbalanced currents would flow in the phases and can damage our system supplied by that
particular transformer! (infect you must have heard sometimes about TVs, refrigerators, filament lamps blown by fluctuating
Electricity)
TOPIC 9: REACTIVE POWER COMPENSATION
Q1) what is reactive and active power?
ANS) There are two components of power i.e. Active and reactive power. Active power is the actual power that does work in
the system and hence it is called as “Watt-full power”, whereas reactive power is the power which is required to produce
magnetic field in a machine. It doesn’t do any useful work, hence it is called as “Watt-less power”.
Q2) why reactive power should be compensated in our system?
There are many reasons why reactive power should be compensated in our system. The important reasons are:
1) Reactive power is directly linked with the voltage of the system. When reactive power increases, voltage increases and
when reactive power decreases, voltage decreases. Both over voltage and under voltage can be dangerous for various
equipment’s which are connected in our system
2) One of the most important reason of blackout is the imbalance of reactive power in our system. When there is less reactive
power in our system, it leads to under voltage. This under voltage will cause the tripping of various equipment’s and
generation units. This voltage instability will further lead to blackout.
3) Transformers, generators and other equipment’s requires reactive power to produce magnetic field in them. So when
reactive power increases or decreases, it causes under and over voltage which can be dangerous for these equipment’s.
Q3) what are the effects of reactive power on power factor of our system? What happens due to low power factor?
ANS) Reactive power has indirect effect on power factor. When there is more reactive power in our system, it leads to low
power factor and low power factor has negative effects on our system such as:
1) When there is less power factor, it causes current to be increased in our system. When current increases, it leads to produce
more heating effect in our system which can dangerous for power system equipment’s.
2) More current means more number of electrons. So to accommodate these electrons we need a conductor of greater size.
Conductor size will increase which will ultimately increase the cost.
3) Low power factor is the main reason of voltage drop and this voltage drop creates poor voltage regulation in our system.
Q4) what are the methods to compensate reactive power?
There are many methods to compensate reactive power such as:
1) SHUNT COMPENSATION: In shunt compensation we use shunt capacitors and shunt reactors which are connected in
parallel/shunt. Shunt Capacitor increases voltage level and compensates for reactive power caused by inductive loads and
shunt reactor is mainly used to keep the voltage down by absorbing the reactive power.
2) SERIES COMPENSATION: In series compensation we use series capacitors which are connected in series. Series
compensation is used to reduce transfer reactance between buses, decreases the effective real power losses and increases
the maximum power that would be transmitted and increases voltage stability in a transmission system
3) VOLTAGE REGULATORS AND LOAD TAP CHANGERS: Voltage regulators and Load tap changer (LTC) are
devices that control the voltage by adjusting the voltage as load changes. In tap changing transformer we tap primary or
secondary at various points in order to adjust voltage. Taps and voltage regulators are used to control voltage level in our
system.
4) FACT DEVICES: Static VAR compensators (SVCs), Static Synchronous Compensator (STATCOM), and synchronous
condensers (SC) are part of the flexible AC transmission systems (FACTS). They are power electronic based devices
which are used to control the voltage by supplying or absorbing reactive power
Q5) Fact devices is one of the best solution to control reactive power problem but still they are not used by utilities
Why is that so?
ANS) There are many issues related to installing fact devices in our system. The important ones are:
1) COST: The biggest issue of not installing them in our system is their high cost.
2) PROCUREMENT: procurement means how we will actually get them. Market for SVC is developed and they can
be procured easily however there is less competition existing among companies manufacturing SSSC, TSCS and there
are very few companies which manufactures UPFC( unified power flow controller) and IPFC
3) OTHER ISSUES: There will be not a single fact device which will be inserted in our system. There will be many.
So how these fact devices will communicate with each other. Where we will place these fact devices in our system.
These issues needs to be addresses before installing them.
Q6) what is the Capability Curve of a Synchronous Generator?
A capability curve reflects the maximum active and reactive power that a synchronous generator can deliver at its
terminals The Capability Curve of a Synchronous Generator defines a boundary within which the machine can operate
safely.
Q7) How synchronous generators are responsible for controlling reactive power in system?
Synchronous generators which we mostly use in power systems produce active power mainly. Additionally, they support
voltage by producing reactive power when over-excited, absorbing reactive power when under-excited. In this way, reactive
power is continuously under control. The ability of a synchronous machine to provide reactive power is mainly dependent
upon active power production.
The control over generator reactive power output and terminal voltage are performed through DC current in the rotor of the
generator. If the system voltage is going down, the generator will supply more reactive in order to raise the voltage at a specific
value. If the voltage of system going up then the generator will absorb reactive power to drop the voltage to a certain value.
Automatic Voltage Regulator would take care of this generator behavior by driving the field current in a certain direction to
maintain the system voltage. It actually controls the DC Excitation level in the generator, therefore, reactive power output as
well as the terminal voltage of the generator.
TOPIC 10: CURRENT AND POTENTIAL TRANSFORMER
Q1) what are instrument transformer?
If we want to measure extremely high values of current and voltage than there are two ways of measuring it. One is to use high
capacity instruments which would be obviously costly. Another way is to use the transformation property of current and
voltage.
Current and voltage can be stepped down by using a transformer whose turn’s ratio is known and then measuring the stepped
down current and voltage by a normal ammeter or voltmeter. The original magnitude can be determined by multiplying the
stepped down magnitude with the turn’s ratio. Such specially constructed transformer with accurate turn’s ratio are called as
instrument transformer. There are two types of instrument transformer:
1) Current transformer
2) Potential transformer.
Q2) what are current transformers?
ANS) Current transformer are put in series with the line in which the current is to be measured. They are used to step down
the current to such a level so that it can easily be measured by using an ammeter. Generally they are expressed as primary:
secondary current ratio for e.g.: A 100:5 amp CT will have primary current of 100 Amp’s and secondary current of 5 Amp’s.
Standard secondary rating of CT’s are either 5 or 1 Amp’s
Common application of CT available in market is “clamp meter”.
Q3) why secondary of CT should not be open?
Ans) If the secondary of Ct is open, it means that there will be no current flowing on secondary side and hence no mmf,
whereas current will be flowing in the primary side and there will be mmf produced.
To counter the mmf of primary, there will be no secondary mmf (because of no current on sec). Hence there will be large
amount of mmf present in the current transformer.
This large mmf will produce large flux in the core and will saturate the core. Again, due to large flux in the core the flux
linkage of secondary winding will be large which in turn will produce a large voltage across the secondary terminals of the
CT. This large voltage across the secondary terminals will be very dangerous and will lead to the insulation failure and there
is a good chance that the person who is opening the CT secondary while primary is energized will die due to shock.
Also, because of excessive core flux, the hysteresis and eddy current loss will be very high and the CT will get overheated. As
CT is oil filled, because of overheating, the oil of CT will get boil and start to vaporize. Because of vaporization of CT oil, the
CT housing will get pressurized and blast. This blasting will lead to fire and smoke. Again, it is not the end here but due to
smoke, the nearby lines will trip due to earth fault which may trip the Power Generating Station.
Q4) what are the differences between current and power transformer?
At basic level, they are no different. Both of them work on the principle of electromagnetic induction. But the difference lies
in
their
usage.
Current transformers, which come under the category of Instrument transformers, are mainly used along with other instruments
for the purpose of measurement. As with every other instrument that is used for measurement purpose on electrical circuits, a
current transformers must have a very low impedance in order to not affect the current in the circuit it is measuring by a huge
amount. Also it is very important to ensure that the phase difference between primary and secondary currents is as close to
zero as possible. A current transformer also has very few, or even a single turn on primary and many on secondary. The phase
angle is zero for ideal CT. In practical CT, it will be of the order of 6 degrees in case of simple CT and fraction of a
degree in case of instrument grade CT
Power transformers on other hand are used to transfer power from primary side to secondary side. Here the emphasis is not
that much on reducing the impedance in transformer, nor is it much on reducing phase angle error close to zero. Here emphasis
is laid more on efficiency than accuracy. Secondly, a power transformer has way too many turns on its primary, than a single
turn, although it’s still lesser than those on secondary.
Q5) what are potential transformer?
Potential transformers are also known as voltage transformers and they are basically step down transformers with
extremely accurate turn’s ratio. Potential transformers step down the voltage of high magnitude to a lower voltage which can
be measured with standard measuring instrument. These transformers have large number of primary turns and smaller
number of secondary turns.
A potential transformer is typically expressed in primary to secondary voltage ratio. For example, a 600:120 PT would mean
the voltage across secondary is 120 volts when primary voltage is 600 volts.
Q6) why secondary of pt. should not be short circuit?
Answer: A VT is a "step-down transformer" that steps down voltage from a very high voltage level (200KV,) to a lower level
(110V). Since, the power (P=VI) in a transformer (input and output) is same, the current rises to a very high level. Thus, a very
high resistance is maintained at the secondary terminal to limit the current (which appears as open circuit)... Short circuiting
the secondary would burn out the windings.
Q7) what is burden in potential transformer?
Burden indicates apparent power related to potential transformer and / or of load connected to it. Burden rating of the
transformer provide s maximum value of apparent power, which the transformer can provide under specified
conditions. Actual burden is the actual value of apparent power that is being drawn by the load from the transformer output.
Q8) Difference between CT and PT?
The primary winding of the CT is connected in series with the line caring the current to be measured. Hence it carries of the
full line current.
The primary winding P.T is connected across (parallel) the line of voltage to be measured. Hence the full line voltage is
impressed across its terminal
Q9) define accuracy class, instrument security factor, accuracy limit factor, knee point voltage of current
transformer?
Accuracy Class of Current Transformer: There is always some difference in expected value and actual value of output of
an instrument transformer current error and phase angle error count in CT, as because primary current of current transformer
has to contribute the excitation component of CT core. Accuracy class of current transformer is the highest permissible
percentage composite error at rated current. The standard accuracy classes of current transformer as per IS-2705 are 0.1, 0.2,
0.5, 1, 3 and 5 for metering CT. The accuracy class or simply class of measuring current transformer is 0.1, means the maximum
permissible limit of error is 0.1%, more clearly, if we try to measure 100 A with a 0.1 class CT, the measured value may be
either 100.1 or 99.9 A or anything in between these range. The standard accuracy class for the protection current transformer,
as per IS-2705 are 5 P, 10 P, 15 P.
Here in the protection current transformer, 5 P means 5%, 10 P means 10 %, and 15 P means 15 % error and ′P′ stands for
protection.
Instrument Security Factor
Now when the current in primary is of multiple times of nominal current then CT becomes saturated that means
secondary current will not be proportional to primary current.
Metering CT is designed with ISF which is normally less than 5 or less than 10, that means CT will be saturated
when the current produced is more than 5 or 10 times of the maximum intended output.
ISF or instrument security factor of current transformer is defined as the ratio of instrument limit primary current to the rated
primary current. The instrument limit primary current of metering CT is the value primary current beyond which CT core
becomes saturated.
So to prevent this, the CT is designed with an ISF of around 5. In other words the CT core will saturate when the current it
produces is around 5 times more that the intended maximum output, so that whatever metering circuit you connect to it is not
reduced to “dust or ashes”.
Accuracy Limit Factor
For protection current transformer, the ratio of accuracy limit primary current to the rated primary current is called accuracy
limit factor of current transformer.
ALF is used in the Protection Class CT's. Purpose of protection class CT is to sense the current under fault conditions and the
output is fed to the protective relay or circuit breaker which in turn cut off the power supply.
Remember in protection class CT the current should not saturate under fault condition like in metering CT.
ALF helps to maintain the accuracy of CT under fault condition such that it does not saturate. it is simply the maximum
allowed limit of error in the primary current.
If you see the specification or name plate of a protection class CT, you will find that it is given like 5P10. This CT can be
interpreted as a protection class CT having an accuracy of 5% over a current range of 10 times of normal primary current
rating. Let us elaborate this by taking an example.
Let us consider a CT of ratio 200/5 A, 5P10. This CT will maintain its measurement accuracy of 5% over 10xrated primary
current i.e. 2 kA. This factor of 10, is called Accuracy Limit Factor.
“Thus accuracy Limit Factor can be defined as the ratio of limiting primary current over which accuracy of CT is maintained
to the rated primary current.”
Knee Point Voltage of Current Transformer
Knee point voltage of current transformer is significance of saturation level of a current transformer core mainly used for
protection purposes. The sinusoidal voltage of rated frequency is applied to the secondary terminals of CT, with other winding
being open circuited which increased by 10%, cause the exiting current to increase by 50%.
TOPIC 11: INTRODUCTION TO POWER ELECTRONICS
Q1) what is power electronics and why power electronics is important?
ANS) Electrical energy is one of the most efficient energy that can be converted easily and cleanly is almost all other forms of
energy. This is a unique characteristic of electrical energy.
We have to transform electrical energy again and again for utilizing it properly and this conversion is done through power
electronic devices. So, power electronics is mainly concerned with the conversion of electrical energy as efficiently as possible
into other forms of energy.
Power Electronics is a hybrid field which merges the field of electrical power systems and solid state electronic and its main
application is in energy converters.
EXAMPLES:
1) Electricity generated by solar panels in DC in nature and has it has to be converted to AC by using a power electronic
device named as inverter.
2) Your laptop requires few DC volts to operate and electricity supplied to your house is 220 volts AC in nature. So to
charge your laptop, you use a charger. That charger is also a power electronic device that is converting 230 volts AC
to few volts DC.
3) Mobile phone charger is a power electronic device.
Power electronics is not limited to only high power applications instead we can see many power electronic based applications
in our daily lives.
Q2) what is SCR and why it is widely used in power electronics?
ANS) Silicon controlled rectifier (SCR) is one of the oldest member of thyristors family and hence SCR is also called as
thyristors.
WORKING:
It is similar to diode as it allows current to flow in only one direction. It differs from a standard diode since it has a third
terminal called “Gate terminal” that controls when the SCR will turn on or off.
APPLICATION:
SCR is used wherever you need to control power. Some modern SCR’s can switch power on the scale of megawatts. They are
most important part of HVDC systems, high power inverters.
FIRING ANGLE OF SCR:
The angle at which a thyristors is fired/triggered/turned on is called firing angle. There is no fixed firing angle for SCR. The
command for turning on or off is given to the gate terminal of SCR with the help of firing circuit.
Q3) what is the difference between electronics and power electronics?
The most important difference between electronics and power electronics is that Electronics deals with transmission of signals
in the form of electrical energy. Whereas power electronics deals with transmission of electrical energy itself.
In electronics we are mainly concerned with transmission of signal be it amplifier, oscillator, filters, audio & video transmitters
and receivers etc. (A microphone output would be a voltage varying with time. So the voltage signal represents a person’s
voice, music etc. The interest lies in processing the signals rather than the power supply.
Whereas in Power Electronics, the focus is more on efficient utilization of electrical power
Q4) what are applications of buck converters, boost converters and buck boost converters?
BUCK CONVERTERS: They are step down voltage or step up current DC to DC converters. They are used wherever the
voltage needed is less or has to be stepped down. They can be used in USB, solar chargers, batters chargers, power audio
amplifier etc.
BOOST CONVERTERS: They are step up voltage or step down current DC to DC converters. They are used wherever the
voltage needed is more or has to be stepped up. They can be used in hybrid electrical vehicles, regulated dc supplies,
regenerative braking of dc motors etc.
BUCK BOOST CONVERTERS: They are the combination of buck and boost converters and can be used to step up or step
down voltage
Q6) what are the applications of power electronics in daily lives?
There are a number of applications of power electronics in our daily lives such as:
1) Ups
2) Inverter
3) We can find many power electronic components in modern cars such as such as ignition switch, control module, vehicle
speed sensor, steering sensor and other component
4) Mobile phone or laptop charger
5) Electrical drives.
Q7) what are power electronic converters?
Majorly there are five types of power electronic circuits, each having different purposeRectifiers - converts fixed AC to variable DC
Choppers - converts fixed DC to variable DC
Inverters - converts DC to AC having variable amplitude and variable frequency
AC Voltage Controllers - converts fixed AC to variable AC at same input frequency
Cyclo converters - converts fixed AC to AC with variable frequency
Q8) what is the biggest disadvantages of power electronic converters?





ANS) during conversion harmonics are injected into our system and this is perhaps the biggest disadvantage of power electronic
converters. Harmonics are generated in the output voltage and current of the converter and also in the input current to the
converter. Now these harmonics create a lot of trouble on both the sides.
On the load side if we have motors, harmonics cause problems such as excess heating, more acoustic noise, commutation
issues in DC motors, etc. Hence nowadays we have special VFD motors which are designed to better handle the effects of
harmonics. Apart from this we also have filter circuits to limit the harmonics to the load.
On the supply side also harmonics create a lot of trouble. The performance of other equipment’s connected to the same supply
is seriously affected. Harmonics in supply lines also leads to radio interference with communication lines, audio and video
equipment’s. Apart from this the input side transformer also gets overheated and its efficiency gets reduced. Harmonics also
increases skin effect in the cables and hence more heating. Thus we need to install filters in the input side also
Q10) what is the difference between an inverter and UPS?
There is no much difference between a UPS and an inverter.
UPS: UPS battery gets continuously charged by the incoming AC supply at our homes. UPS converts the power from battery
into AC and supplies to the equipment. It will continue to supply as long as the battery doesn’t gets discharged.
INVERTER: The main power AC is supplied to the inverter, and it is transformed into DC simultaneously, which keeps
charging the battery continuously. There is a sensor and relay structure that always monitors the ON or OFF status of the main.
As soon as there is a power failure, the relay actuator activates the inverter switch. Every other action is similar to the UPS,
but because of the sensor and relay process, there is a delay while activating the switch.
Q11) what is the difference between an inverter and a converter?
ANS) the purpose of inverter is to invert DC to AC and either increasing or decreasing it to a required value whereas the
purpose of converter is to just to change the voltage levels. They either increases or decreases the voltage without changing
the nature. An AC will still remain AC whereas DC will remain DC.
Q11) how does an inverter technology saves bills? Why an inverter ac does is more efficient than normal split ac?
NORMAL AIR CONDITIONER:
Normal air conditioner takes the indoor air, cools it down and throws it back to the room. In a normal AC the compressor is
either off or on. When it is on, it works at full capacity and consumes full electricity it is designed to consume. When the
thermostat reaches the temperature level set in the AC, the compressor stops and the fan (in AC) continues to operate. When
the thermostat senses that the temperature has increased, the compressor starts again.
INVERTER TECHNOLOGY:
The inverter technology works like an accelerator in a car. When compressor needs more power, it gives it more power. When
it needs less power, it gives less power. The idea is, with an appropriate 'sensor' that measures the temperature in the room, the
speed can be adjusted to the required level, and the system need not work at full capacity all the time. This means better
efficiency and lower power consumption. With this technology, the compressor is always on, but draws less power or more
power depending on the temperature of the incoming air and the level set in the thermostat. The speed and power of the
compressor is adjusted appropriately. An AC draws a considerable amount of power when it cycles its compressor to the one
state. Also, if the compressor is totally shut off, it will turn itself back on only when the thermostat is triggered
So it kind of automatically adjusts its capacity based on the requirement of the room it is cooling. Thus drawing much less
power and consuming lesser units of electricity.
One more very good feature of the INVERTER AC is that even when it does need to start the motor, it does it in a very
controlled fashion, gently ramping up the power, so that a transient (sudden) load is not felt on the power feeder line
TOPIC 12: VOLTAGE FLUCTUATIONS
Q1) why voltage fluctuate in your house?
There can be many reasons to it.

The most common is overloading. If the area, where you live, draws more load current than specified by the
authority, then there will be low voltage in your area. This due to the limited capacity of the distribution transformer
used.

It can also be caused due to use of various electronic equipment’s like, reactor, inverters, etc. due to the harmonics
created by them.

Another reason can be due to use of heavy inductive loads like, ac, refrigerator, also motors that draws high amount
of current from supply line and as a result voltage dips occurs If you use heavy loads simultaneously, then there
will be voltage dip.

Or else, it can be caused due to damaged wiring, or earthing system being faulty etc. They dissipate unnecessary
power, thereby causing voltage drop.
Q2) which devices in our homes need voltage stabilizer? What does voltage stabilizer actually does?
ANS) Freezer, microwave oven, computer, refrigerators, and air conditioners are common devices in your homes that requires
a voltage stabilizer.
As the name suggests, voltage stabilizers stabilize the voltage, which means if the supply voltage fluctuates or varies (is less
or more than the acceptable range), it brings it to the desired range. The components of voltage stabilizer include a transformer,
relays, and electronic circuitry. If the stabilizer senses the voltage drop in incoming voltage, it enables the relay so as to add
more voltage from transformer to compensate the loss of voltage
When the incoming voltage is more than normal value, stabilizer activates another electromagnetic relay such that it reduces
the voltage to maintain the normal value of voltage.
For home needs, 200VA, 300 VA, 500 VA, 1 KVA, 2 KVA, 3 KVA, 4 KVA, 5 KVA, 8 KVA and 10 KVA rated voltage
stabilizers are suitable. For industrial and commercial purposes, high power rated servo stabilizers are needed.
Q3) Can an air conditioner which is labeled 220-230v run on 170v?
No, and if it tried to run on the lower voltage, the high currents would likely burn out the compressor motor.
Q4) what to do in case of voltage fluctuations?
Over voltage and under voltage can be dangerous for various equipment’s connected in your home. When voltage is low, so
your equipment will try to draw more current and hence will result in overheating of equipment. Long time over voltage will
result in Permanent damage to the equipment and Insulation damage to the windings whereas Long time under voltage will
result in Malfunctioning of the equipment and reduced speed of motor
In most of the cases, under voltage occurs in Pakistan and the most obvious way to detect is by dimming of lights, fans speed
slowing down etc. In case of voltage fluctuation, do these things:



Refrigerators are AC motor driven appliances that draw large currents during voltage drop conditions which may lead to
overheating of windings. So, turn them off.
It is said that majority of the LED/LCD TV’s have built in voltage stabilizer in them. Yes, they do have but it’s better to
switch them off.
In case of lighting equipment, low voltage drop reduces the lumen output (illumination) that will further reduce the life of
the lamp. Turn them off for few minutes
Q5) Does LED TV requires voltage stabilizer?
More and more electrical appliances these days have started incorporating electronic circuitry for power management. With
electronic circuits like SMPS (Switch Mode Power Supply), voltage management has become much easier. Most LED
televisions in the market today can work between 110 V and 290V and adding a voltage stabilizer does not help. Your TV can
still work if the voltage is low.
If the voltage often fluctuates in your homes than its better to use it
Q6) how can I choose a right sized stabilizer for my application?
The most important thing is to know the nature of the load connected to the stabilizer. First you must note down the power (or
Watts) for all the appliances that will be connected to a stabilizer. The sum total of the power consumption (or Watts) will give
you the load on the stabilizer in watts. But most stabilizer sizes are in VA (Volt Ampere) or kVA (kilo Volt Ampere which is
equal to 1000 Volt Ampere). Although to get the actual VA (or Volt Ampere) from Watts (W) you will have to do some
measurements, but to give a rough approximation, you can increase the Watts value by 20% to get the approximate VA size
that you may need. Watt rating will be written on the device.
So for e.g. if AC connected to your stabilizer is 1.9kw then you can take a 2500 VA or 2.5 kVA stabilizer.
TOPIC 13: GENERAL ELECTRICAL INFORMATION
Q1) Do LED bulbs really save energy?
For incandescent Light Bulbs, only 0.5-1% of the electrical energy consumed is transformed into light. If you use Compact
Fluorescent Light bulbs, this number jumps to 4-5%. If you use current LED Lighting technology, your electrical-optical
conversion efficiency goes to 20-30%, depending on your color temperature (2700-5000K).
Q2) why fans and other electronic appliances makes noise while operating on an inverter?
ANS) The output of an inverter is not a pure sine wave. Harmonics are created in the system when a sine wave distorts i.e. an
electrical device which is fed by an imperfect sine wave at 50 Hz fundamental frequency has harmonics. Also machine has
magnetic materials inside them which produces sound due to magnetic effect called Magnetostriction
Q3) Why is there a sparking in the ends of a tube light even when it is off? Is it dangerous?
ANS) Even current is not there, filament is still hot which excites some gas atoms and thy releases photons. This is the reason
why we see sparking in the ends of tube light even when it is off. It’s not dangerous for light.
Q4) why some appliances use a 2-pin plug and others have 3-pin plug? Appliances such as TVs and DVD players,
mobile phones use a 2-pin plug, while other appliances such as refrigerators, microwave ovens, ACs use a 3-pin plug?
ANS) Electronic devices such as TV operate on low current and do not require being earthed/ grounded. Whereas 3 pin plug
is used for heavy appliances such as the air-conditioners, refrigerator, and microwave oven
Electrical appliances with metal bodies are required to be properly earthed for protection against possible electric shock. If
there is a fault current (leakage) inside the appliance, the whole metal case becomes conducting and if someone touches it by
mistake, current will flow through his or her body, resulting in a shock which can also become sometime life threatening. The
purpose of ground pin is you from electrical shocks
Q5) Why are conductors used in high tension transmission lines are stranded?
The main reason we are going for Stranded conductors instead of Solid conductors is to avoid the “SKIN EFFECT” .Due to
skin effect, the alternating current has a tendency to flow near the outer part of the conductor. So to avoid this skin effect we
are using Stranded conductors where the implications are reduced as compared to the solid conductors.
Q6) “bring a wire of 3/29, 7/29 “You might have heard these words from electricians? What does it means actually?
The conductor material, insulation, size and the number of cores, specifies the electrical wires. These are important parameters
as they determine the current and voltage handling capability of the wires. The wires may be of single strand or multi strand.
Wires with combination of different diameters and the number of cores or strands are available.
For example: The conductors are specified as 3/20, 3/22,….7/20 etc.
The numerator indicates the number of strands while the denominator corresponds to the diameter of the wire in
SWG (Standard Wire Gauge). SWG 20 corresponds to a wire of diameter 0.914 mm, while SWG 22 corresponds to a wire of
diameter 0.737 mm.
3/29 is generally used for lights and fans. 7-29 for ac
Q7) why sine wave is used for representation of ac quantities?
Since, we have elements such as inductor and capacitors in our circuit so output will be differentiated or integrated form of
input. If we use square, triangular or other waves so our output will be of different nature. But if we use sine wave. So, output
won't be of different nature. Since derivative of sine is cos. same for integration.
So, output will be of same nature as input. We need output signals of same nature as input. Since, for finding the transfer
function, both input and output quantities should be of same physical form or of same domain.
Transformation such as of Laplace and Fourier becomes easy, if we use sine wave to represent AC quantities
Q8) what is the difference between a sensor and a transducer?
The main difference between sensor and transducer is that a transducer is a device that can convert energy from one form to
another, whereas a sensor is a device that can detect a physical quantity and convert the data into an electrical signal. Sensors are
also a type of transducers
Q9) why ac is rated in tons?
Tons" is a measure of how quickly an air conditioner can remove heat from the air. The actual unit is 'tons of ice per day'.
The capacity of air conditioner is measured in tons. A Ton of air conditioner capacity involves the amount of heat energy
required to melt one ton of ice in a 24-hour period. It means that if you have an Air Conditioner of 3 tons, then it possesses
the power to melt down 3 ton of ice in 24 hours.
TOPIC 14: COOLING METHODS OF TRANSFORMERS
Q1) what are the reasons for heating production in a transformer?
Transformer is a device which is used to transform one voltage level to another. The main source of heat generation is
copper losses also called as Heat losses or “I Square R losses”. There are other reasons too such as eddy current loss and
hysteresis losses but Copper losses are the real culprit for heating in a transformer.
If this heat is not dissipated than the temperature of transformer will rise and will ultimately be harmful for its insulation and
other parts.
Transformers can be divided into two major types:
1)
Dry type transformers.
2)
Oil immersed transformers.
Q2) why is oil used in a transformer?
1) There is no electrical connection between transformer primary and secondary winding, there is a magnetic connections
and hence oil is used as an insulator between primary and secondary winding.
2) Oil serves as a cooling medium in a transformer. Transformer loads shifts ups and down and hence lots of heat is generated
in winding. Oil has a very good heat transfer coefficient i.e. it removes heat easily and doesn’t conducts electricity at all
so no electrical short circuit will occur.
3) Bucholz relay operates by detecting the oil vapors and hence oil acts a medium of detecting internal faults in a transformer
Q3) why it is needed to change the transformer oil periodically?
Transformer oils are subject to electrical and mechanical stresses during operation. The oil gets contaminated due chemical
interactions with windings and other solid insulation which gets accelerated by temperature of the windings. Thus the original
chemical properties of transformer oil change would change and render it ineffective for its intended purpose after many years
necessitating a change.
DRY TYPE TRANSFORMERS: They are small rating transformers which uses natural air for cooling purposes
1) AIR NATURAL: atmospheric air is sufficient for its cooling purpose and they are generally small transformers up to 3
MVA
2) AIR BLAST: They are rated more than 3 MVA and natural air method is not sufficient for their cooling instead air is
first filtered to remove the dust particles in it and then it is forcibly apply on the core and windings with the help of fans and
blowers. This method can be used for transformers up to 15 MVA.
OIL IMMERSED TRANSFORMERS: These transformers requires air and as well as oil for cooling
1) Oil natural Air natural (ONAN)
2) Oil natural Air forced (ONAF)
3) Oil forced Air forced (OFAF)
4) Oil forced Water forced (OFWF)
ONAN: This is the simplest cooling method. In this the heat generated by windings and core is transferred to oil. Hot oil will
flow in the upward portion of transformer tank and its place will be taken by cold oil. This hot oil which comes to upper side,
will dissipate heat in the atmosphere by natural convection and radiation in air and will become cold. This method can be used
for transformers up to about 30 MVA.
ONAF: Rate of heat dissipation can be made faster by applying forced air flow on the dissipating surface. In this method
cooling fans are mounted near the radiating surface and may be provided with an automatic starting mechanism such that when
temperature exceeds a certain limit, they will be automatically turned on. This method is used for transformers up to 60MVA
OFAF: In this method oil is forced to circulate within the transformer tank by means of oil pumps. The oil is further cooled
down by using cooling fans and blowers. In OIL NATURAL AIR FORCE method, rate of dissipation of heat from the oil is
natural and hence slower but in this method, rate of dissipation is much faster as compared to ONAF
OFWF: In this method, oil is forced to flow through the water heat exchanger by using a pump where the heat dissipated by
oil is transferred to water. In this method fans and blowers are used to increase the rate of heat dissipation. The heated water
is taken away to cool in separate coolers. This type of cooling is used in very large transformers having rating of several
hundred MVA’s.
TOPIC 15: GRID AND SUBSTATIONS
Q1) what is the difference between a sub and grid station?
Ans) A grid is something where two or more transmission lines are connected to transmit the power further. There may or
may not be change in voltage. Grid station is mostly used in context of transmission. Basic requirement of a grid is that it has
two or more than two transmission line. Grid means division. It is a point where we are distributing power. Whereas substation
can be step up, step down, switching. Simply a transformer is a substation.
Q2) what are different types of substation?
Ans) There are 4 major types of substation:
1) Generation substations: Which can be found along with power plants.
2) Switching substation/Transmission substation: Located in long distance transmission networks and farther away from load
centers. Usually involves large amount of power transfer, higher voltages and large capacity transformers.
3) Distribution substation: Steps down voltages from lower transmission levels (138/69kV etc.) to distribution voltages
(25/15/11kV etc.) and is located near load centers
4)
Customer substations: Similar to distribution substations but intended to serve one 1 or a couple of customers (such as
industries)
Q3) what are feeders?
ANS) Feeders are basically conductors of large current carrying capacity. As the name implies they are used to feed or
supply power to different areas. Feeders connect substation to different areas where power is to be supplied to different
consumers. According to purpose and requirements, feeders are divided into following types:




Radial feeder
Ring feeder
Parallel feeder
Meshed feeder
Q4) what is the need of batteries at grid stations?
ANS) in grid stations a special battery room is provided. Batteries in substation are essential as these are used for Auxiliary
power source. When there is no power from main station, batteries are used to supply power to the meter and other equipment.
They are used to provide an uninterrupted & reliable power supply to control the switch gears & Monitor the status of Feeders
in case of power failure
Battery rooms are also found in electric power plants and substations where reliable power is required for operation
of switchgear, critical standby systems, and possibly black start of the station.
Q5) why do we use stones instead of grass or sand on substation?
ANS) There are many reasons of using stones in substation instead of sand or grass because:
1) Power transformer has oil as insulating and cooling medium. If oil leakage takes places during operation or during changing
the oil in transformer so oil spillage can catch fire and this thing is prevented by using stones placed on the ground in substation.
2) Stones absorbs heat radiated by the radiator during cooling of transformer.
3) In plain grounds where grass grows, grass will form moisture and it will be the cause of leakage current flowing. Stones
prevent this thing by not allowing grass to grow.
4) During short circuit step and touch potentials increases, so to reduce these potentials, when operators works on substation,
stones are provided
NOTE:
1) STEP POTENTIAL: Potential developed between the feet’s on the ground of a human or animal during short circuit is
called step potential. This results in flowing of current in the body that ultimately leads to an electric shock
2) TOUCH POTENTIAL: Potential developed between the ground and the body of an equipment during short circuit is
called touch potential.
Q6) is electricity send to our home from the grid? How a grid does knows what amount of electricity is to be send to a
house?
ANS) Electricity is not send to our homes from grid instead the equipment in our homes draws electricity because of their
resistance. More appliances will draw more electricity
Q7) what is steel gantry in grid station?
ANS) Gantry tower guide transmission lines into the substation.
Q8) what’s the purpose of isolators, circuit breakers, arrestors, Current transformer, potential transformer in
substation?
1. POTENTIAL TRANSFORMER: Potential transformer measures the voltage through the transmission lines. A control
cable is connected to it, so that if P.T detects any high or undesirable value of voltage through the TX lines. Then it can
send a control signal to the respective relays and trips the circuit breaker to disconnect the supply for safety precautions.
2. ISOLATORS: They are closed under normal circumstances. In case of any fault, isolator opens and any damage can be
avoided. They physically isolates the line from the supply. They can also be open if we want to perform any maintenance
task
3. CURRENT TRANSFORMER: Current transformer measures current through the transmission lines. It also has a cable
which is for same purpose as of PT (mentioned above).
4. SURGE ARRESTORS: Surge arrestors is used to avoid faults due to severe weather circumstances. In case of heavy
lightning shocks, surge arrestor’s passes these shocks to the ground to avoid heavy losses
5. LINE DISCONNECTORS: They allow transmission lines and equipment within the substation to be safely isolated for
maintenance work.
6. POWER TRANSFORMER: Power transformers increase or reduce voltage, depending on whether the substation is an
entry point to the transmission network or an exit to the distribution network. . It also has a Surge arrestors protect
equipment within the substation from any voltage spikes on the transmission lines.
7. CIRCUIT BREAKERS: They are used to break the circuit in cases of fault or for maintenance reasons.
Q9) why it is not allowed to go with an UMBRELLA in grid station especially during rainy seasons?
Ans) normally when you visit the grid station of 220kv or above, you can feel induction effect, you can feel hairs on your hand
will rise automatically due to induction. In simple the “GOOSE BUMP EFFECT”. During rainy seasons, you can witness it
much more than normal as charged lines are present above and due to moisture present in air because of rain. If you carry an
umbrella, the induction may cause a direct effect on the tip of the umbrella due to sharp metallic point which is highly sensitive
and conductive in nature
Q10) how many grid stations are there in k-electric network?
ANS) K-Electric’s transmission system comprises 1,251.82 km of 500, 220kV, 132kV and 66kV lines, with 64 grid stations
and 138 power transformers.
TOPIC 16: NEUTRAL WIRE AND UNBALANCE CURRENT
Q1) Difference between ground wire and neutral wire?
ANS) Neutral wire is a return path for unbalance current. Electrically neutral is zero potential with respect to live potential.
While ground wire is used for safety purpose. It provides a low impedance path for the fault current to flow to the ground,
instead of our body and thus saving us from dangers of electrocution.
Q2) if the neutral wire is at zero potential than how does it provide return path for the current?
ANS) Neutral is a reference point in circuit and when we say it has zero potential, it means that”
“ If the neutral wire is at zero volts then the live wire will be at 220 volts”
Neutral is 0 volts with respect to earth. It means it has same voltage as of the ground. In reality Neutral doesn’t have any
voltage. We just assume it to 0 volts so that we can use it for comparison purpose
Q3) what is the difference between balance and unbalance loads?
ANS) Balance loads are those in which power is splitted equally among all phases i.e.
Load on Phase A= Load on Phase B = Load on Phase C
EXAMPLES: Three phase Motors, Converters; Rectifiers using 3 phase input supply are
common examples of balanced loads.
Unbalanced load=the power loading on each phase is unequal.
Load on phase A=/=Load on phase B=/=Load on phase C
EXAMPLES: A single phase / 2 phase loads on a three phase system is called
Unbalanced load.
A balanced system is more efficient because the heating losses are minimized per watt of power transfer.
If the load is unbalanced, there is a neutral or imbalance current flow. Current imbalance can cause voltage unbalance and if
the system is not monitored well for neutral conductor, probability of failure will be high.
Q4) Can neutral wire be made thinner than a phase wire?
Ans) Yes neutral wire can be made thinner than a phase wire in three phase system because normally three phase systems are
balanced and even though they are not balanced, still the current in neutral would be very low and hence in three phase system.
Neutral wire can be made thinner than phase wire but not in single phase because in single phase all the current which comes
from phase wire goes back through neutral wire. Hence neutral wire should be equal to phase wire in single phase system.
Q5) what is unbalance current?
During balanced condition there will be no current flowing through the neutral line and hence there is no need of the neutral
terminal. But when there will be unbalanced current flowing in the three phase circuit, neutral is having a vital role. It will take
the unbalanced current through to the ground and protect the transformer or other equipment
Neutral or unbalance current which flows through neutral impedance raises the load side neutral potential with respect to
source neutral point which is normally earthed and at zero potential. This means at load side there will be potential difference
between neutral and earth. This can be unacceptable for certain equipment
Unbalanced currents result in higher operating temperature, shortened motor life, and efficiency reduction.
Q6) is neutral terminal grounded in our homes?
ANS) No, neutral terminal is not grounded in our homes. If the supply's neutral is broken, then the power will go through the
live conductor and then to the equipment. Since, there is no neutral available for the power hence it will take another path
"ground" which is connected to neutral at the main DB and it results in main voltage present at the metal casing or frame of
the equipment. Earth and neutral are bonded together at main switch board by law in some countries. Earthing the neutral has
a number of advantages in built up areas and is safer than non-ground referenced neutrals provided the right protection is on
place.
Q7) what should be the ground to neutral voltage?
ANS) The potential difference between earth and neutral lines in our homes should be 0 volts ideally. The tolerance permitted
is 10% of 230 volts. In other words, earth to neutral potential difference measured using a voltmeter should be between 0-23
volts
If potential difference doesn’t lie in this range than:
1) May be earthing is not done properly.
2) There may be a fault in the wiring due to which the potential of neutral has risen above its nominal value
Q8) why is there no neutral for High tension lines? Or why transposition of HT lines is required?
ANS) Neutral lines as the name implies carries the neutral/unbalance currents. T/L currents are balanced out using
transposition. Without transposition, the system begins to develop some unbalance between phases, with a resulting build up
in neutral current. T/L currents are balanced out using transposition. So there is no need for neutral in HT lines
2. Since transmission lines are in a way isolated from load therefore there is no need to connect neutral wire in transmission
phase because all the load is connected on distribution side.
Q9) what happens if I connect a bulb between live and ground wire and not neutral? Will the bulb glow?
ANS) it will glow but it is not advisable. Normally the neutral is grounded and will have same zero potential or near to zero.
But in most of the installations the grounding is not proper and neutral will show some potential. The brightness of the bulb
will be lower
Q10) If a bulb is connected between two phases, will it glow or not?
ANS) The bulb is rated at 220 volts and if we connect the bulb between two phases than voltage will rise to 440 volts. The
blub will glow much higher than usual but for the time being as high potential will burn out the filament wire of the bulb.
Q11) why is it said to wear a rubber slipper while working with electrical circuits?
ANS) Rubber is a very bad conductor of electricity and as long as you are wearing a rubber slipper, it will insulate you from
the ground and possibly no current will flow from the wire through your body to ground.
TOPIC 17: POWER FACTOR IMPORTANCE
Q1) what is active, reactive power and apparent power?
Before understanding power factor, we should first know about active, reactive and apparent power.
ACTIVE POWER: Active power is the actual power which does work in the system.
REACTIVE POWER: According to Newton’s law of motion “Every action has an equal and opposite reaction”. So, if active
power is produced than as a reaction some other power will be produced too. That power is called reactive power. So what
actually it does?
Suppose, if we have pure inductor and capacitor connected in the system as a load, than they gets charged during first cycle
and will gets discharged in the next, it gives the same amount of energy back to the source. It doesn't consume any power.
Whatever energy it stores, will return that back to the source. So if you only connect pure inductive or capacitive load at your
home then you don't have to pay any bill because whatever energy it stores, will return back to the source.
Electrical load consist of resistive, inductive and capacitive components. Resistive component uses the active power but
inductive/capacitive components just get charged and discharged. So in every cycle because of inductive and capacitive
components, some power just goes to and fro, doesn't do any useful work. This power, which is just going to and fro is known
as reactive power
APPARENT POWER: it is the phasor sum of active and reactive power.
Q2) what is power factor?
Power factor is the ratio of real power to apparent power. It indicates how much power is being converted to useful work. For
example if the power factor of a distribution transformer is 0.6 it means that only 60% of the power supplied to the transformer
is supplied to the load and the rest is used up to setup flux in core of the transformer which is obviously a problem for the
power generation company as now it needs to generate 40% more power so that user gets the amount of power it demands. Its
value changes from -1 to 1.
1) If power factor=0:
It means that active power=0 and load draws only reactive power. Such loads are mostly inductive in nature.
2) If power factor=1:
It means that reactive power=0 and load draws only active power. Such loads are resistive in nature.
3) If power factor=-1:
It means that the loads is generating power and is feeding back to source. In this case, load acts as a generator.
Q3) How to explain the significance of "Power Factor" to a non-technical person?
Take a packet of LAYS or any other chips. The chips present in it will be the active power and the air present in the packet
will be reactive power. Although the air is of no use to us. Still it is present to keep the chips fresh in the packet.
The more the chips (active power) in the packet, the happier the consumer will be. Similarly if the air is more as compared to
chips, then it will disappoint the consumer.
Q4) Can power factor be negative?
Theoretically power factor cannot be negative. Since power factor is the cosine of angle between voltage and current i.e. cos
(Theta V- Theta I). If the current lags voltage then the difference between voltage and current will be positive and pf will be
positive. Similarly, if current leads voltage then this difference will be negative but due to cosine function property " cos
(theta) =cos (- theta) ". Pf will become positive. So theoretically in both cases pf will be positive either leading or lagging
case
But practically it can be negative in one condition only when load starts supplying power back to the source and the load
starts acting as a generator.
However, the fact that PF is negative means that active power is negative too.
For a simple example think of a DC motor. When the battery is connected, the motor is the load and uses energy and has a
positive power factor (which is equal to the motor's efficiency in converting electrical energy to mechanical energy). when you
disconnect the battery, the motor is still running for some time and because DC motors usually have permanent magnets the
motor will act like a generator and feeds power to the circuit (In this case in the opposite direction).
Q5) whether +0.85 or -0.85 is lagging or leading pf?
There is nothing like + or - for power factor, 0.85 can be leading or lagging only depending on the load. For example: If the
ratio is 0.85 lagging it means 15% power does not result in actual work directly and is used by inductive elements.
If the ratio is 0.85 leading it means 15 % power is used by capacitive elements that don’t result in actual work
Q6) Does DC supply has pf?
DC supply has nothing to do with power factor because. Power factor is the cosine of angle between the current and voltage.
This angle between voltage and current will be introduced by either inductance or capacitance. There is nothing called
inductance for DC circuit as the flux linkage is not changing at all. So inductance ruled out. Now, coming to capacitance, for
DC circuit, you know that capacitance does not allow the flow of DC current.
Therefore, you see that there is no any element to introduce angle between voltage and current. Thus there is no concept of
power factor for DC circuit
Q7) what happens to the system reactive power when inductive load is connected? Whether reactive power will
increase or decrease?
When you connect an inductive load (for e.g. Motor) to any system, it consumes reactive power. This leads to decreases in
reactive power in the system. Inductive load will add extra inductive reactance and hence more magnetizing current would be
drawn from supply. As a result, the load current will lag behind the load voltage and hence we can say reactive power decreases,
reducing power factor.
Q8) Why is pf of transformer poor at no load?
On no load condition the current is drawn mainly to magnetize the core called as magnetizing current which is highly lagging
due to inductance effect, thus power factor at no load is highly lagging phenomenon
Q9) why induction machines always runs on lagging power factor?
First we should know about pf:
CASE 1: If current and voltage are in phase than it’s a purely resistive circuit
CASE 2: If current lags voltage by 90 degrees than it’s a purely inductive circuit
CASE 3: If current leads voltage by 90 degrees than it’s a purely capacitive circuit
Induction machines has windings on rotor and windings have their own inductance and resistance. Therefore induction
machine will always runs on lagging pf.
Q10) what are the effects of low power factor on the system?
Active power in the system is given by:
Active power = VIcosθ
Where, cosθ is the power factor of the system.
Hence, if power factor is less than it will lead to the increase in current for the same value of active power.
Ø When current increases, it leads to the increase in the heat losses which are also called as copper losses. Because of these
losses, heating increases.
Ø Increase in the current actually means the increase in the number of electrons, so to accommodate these electrons in a
conducting wire. We need a conducting wire of larger size. When size of the conductor increases, it leads to the increase in
cost of the conductor which is undesirable in many projects.
Ø Low power factor will result in large voltage drop which will ultimately cause poor voltage regulation
Q11) if a synchronous motor has leading pf, does this motor consume or supply reactive power or real power?
When synchronous motor is operated at leading power factor, it means it is over-excited. So it will deliver reactive power and
if operated at lagging power factor then it will be under-excited and absorb reactive power.
Synchronous motor will operate at leading power factor when Rotor is overexcited such that back emf (Eb which is generated
in stator due to dc excitation of rotor) is greater than Supply voltage (V), then Synchronous motor is operating on leading
power factor. At this time resultant flux is greater than that is required for unity power factor, then this extra flux will generate
reactive power so motor will generate reactive power
TOPIC 18: POWER CABLES
Q1) what is the difference between a wire and a cable?
A wire is a single conductor (material most commonly being copper or aluminium) while cable is two or more insulated wires
wrapped in one jacket. Multiple conductors that have no insulation around would be classified as a single conductor.
Q2) what are the most important parameters while selecting a cable?
There are various types of cable available but the choice of particular type depends on the operating voltage and service
requirement. However, a cable must fulfill some of the basic requirement. Those requirements are as follows
Short Circuit Rating:
It happens frequently that the conductor size necessary for an installation is dictated by its ability to carry short-circuit current
rather than sustained current. During a short –circuit, there is a sudden inrush of current for a few cycles followed by a steadier
flow of current for a short period until the protection switchgear operators, normally between 0.1-0.3 seconds.
Current Carrying Capacity:
Current carrying capacity is an important aspect is the selection of the optimum size of conductor. The safe current carrying
capacity of an underground cable is determined by the maximum permissible temperature rise. The cause of temperature rise
is the losses that occur in a cable which appear as heat.
Voltage Drop:
The allowable maximum voltage drops from source to load is another aspect of power cable conductor design.
As per ohm’s law V = IR. The first is the choice of material used for the wire. Copper is a better conductor than and will have
less voltage drop than aluminum for a given length and wire size. Wire size is another important factor in determining voltage
drop. Larger wire sizes (those with a greater diameter) will have less voltage drop than smaller wire sizes of the same length.
Q3) how cables are classified on the basis of voltages?





LT cables: Low-tension cables with a maximum capacity of 1000 V
HT Cables: High-tension cables with a maximum of 11KV
ST cables: Super-tension cables with a rating of between 22 KV and 33 KV
EHT cables: Extra high-tension cables with a rating of between 33 KV and 66 KV
Extra super voltage cables: with maximum voltage ratings beyond 132 KV
Q4) why do we use stranded conductors? What are strands?
Instead of having one solid conductor, we usually have multiple strands in a cable. Strands are basically thin wires of small
cross sectional area.
Basic, reason of using stranded conductor is to make the conductor flexible. If we use a single solid conductor. It does not have
sufficient flexibility. It becomes difficult to transport a single solid conductor of long length over the distance. To eliminate
this drawback, conductor is formed by using several thin wires of small cross section. By making the conductor stranded, it
becomes flexible. Which makes stranded conductor suitable to be coiled easily to transport it over long distance.
Generally, in successive layer, the stranding is done in opposite direction to preceding layer. This mean, if the strands of one
layer are twisted in clockwise direction, the strands of next layer will be twisted in anticlockwise direction and so on ‘x’ is
number of layers in conductor.
Q5) what is skin effect in cables and how stranding is helpful in reducing it?
Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current
density is largest near the surface of the conductor, and decreases with greater depths in the conductor. Current passes through
the skin of the conductor and not from the center.
By using stranded conductors surface area increases and therefore a better current carrying capacity for AC
Q6) what are the components of an underground cable?
An underground cable consists of conductor, insulation, sheath, bedding, Armour and serving
Conductor > Insulation > Metallic Sheath > Bedding > Armour > Serving.
An easy way to remember this is CIMBAS
1)
Cores or Conductors
The number of core or conductors in a cable depends upon its use. For example, a three core cable is used for three phase
system. The conductors are made of copper or aluminum and are stranded to provide flexibility.
2)
Insulation:
A suitable thickness of insulation is provided on each core or conductor so that the conductors of underground cable may
withstand the operating or designed voltage. The thickness of insulation on the core increases as the operating or design voltage
is increased. Normally the insulation materials are made of impregnated paper, varnish cambric or rubber mineral compound.
3)
Metallic Sheath:
Metallic sheath surrounds the insulation on the core. It is provided to protect the insulation from moisture, gases, oil, liquids
etc. present in the soil and atmosphere. It is normally made of lead or aluminum.
4)
Bedding:
Bedding surrounds the sheath. It is made of fibrous materials like jute or hessian tape. The purpose of bedding is to protect the
sheath from mechanical injury due to armoring. You can think it like a shock absorber.
5)
Armoring
Armoring is provided over the bedding to protect the cable from mechanical injury during its handling and lying. It consists of
one or two layers of galvanized steel wire or steel tape. In some cable, armoring is not provided.
6)
Serving:
To protect the armoring from atmospheric condition, a layer of fibrous material like jute is applied on the armoring. This layer
is called serving
Thus, we observe that the main working part of underground cable are core / conductor and Sheath. Other parts of cable, just
protect the conductor and sheath from mechanical injury or chemical attack. This does not mean that, they are not important.
Bedding, Armoring and Servings are very important parts of cable else cable won’t work as its insulation will degrade.
TOPIC 19: HARMONICS
Q1) what are linear and nonlinear loads?
LINEAR LOAD:
When a sinusoidal voltage is applied to a certain type of load, the current drawn by the load is proportional to the voltage.
These loads are referred to as linear loads. Examples of linear loads are resistive heaters, incandescent lamps etc.
NON LINEAR LOAD:
A load is said to be non-linear when the current it draws does not have the same waveform as the supply voltage. Examples of
nonlinear loads are battery chargers, electronic ballasts, variable frequency drives, computers, instrumentation, actuators,
Fluorescent, HID, LED lamps and switching mode power supplies etc.
Q2) what are harmonics and how they are produced?
When the load is nonlinear than it contains distortion in current and voltage waveform. These distortions are produced by
harmonics and because of them sinusoidal waveform deviates from its original waveform.
A pure sinusoidal voltage is a conceptual quantity produced by an ideal AC generator built with finely distributed stator and
field windings that operate in a uniform magnetic field. Since neither the winding distribution nor the magnetic field are
uniform in a working AC machine. So, ideal sinusoidal waveform is not possible.
Harmonics are voltages or currents that operate at a frequency that is an integer (whole-number) multiple of the fundamental
frequency. So given a 50Hz fundamental waveform, this means a 2nd harmonic frequency would be 100Hz (2 x 50Hz), a 3rd
harmonic would be 150Hz (3 x 50Hz), a 5th at 250Hz, a 7th at 350Hz
So in other words, we can say that “harmonics” are multiples of the fundamental frequency and can therefore be expressed
as: 2ƒ, 3ƒ, 4ƒ
Q3) what is the difference between current and voltage harmonics?
CURRENT HARMONICS:
Current Harmonics are caused by non-linear loads such as thyristor drives, induction furnaces, etc. The effect of these loads is
the distortion of the fundamental sinusoidal current waveform alternating at 50Hz they also contribute to the Copper losses
(I2R) losses in the system. However, current harmonics do not affect the remainder of the loads in the system which are linear.
They only impact the loads which are causing them i.e. non-linear loads. Remember that, only fundamental frequency current
can provide real power. Current delivered at harmonic frequencies doesn't deliver any real power to the load
VOLTAGE HARMONICS
Voltage Harmonics are caused by the current harmonics which distort the voltage waveform. Current distortion interacts with
system impedance creating voltage distortion. These voltage harmonics affect the entire system not just the loads which are
causing them. Their impact depends on the distance of the load causing the harmonics from the power source. If other harmless
loads are connected between the source and harmonics causing loads, these innocent loads will also be affected by the
harmonics.
Q4) What are positive, negative and zero sequence harmonics?
Harmonic sequence refers to the phasor rotation of the harmonic voltages and currents with respect to the fundamental
waveform.
Harmonics are grouped into positive (+), negative (-) and zero (0) sequence components. Positive sequence harmonics
(harmonic numbers 1, 4, 7, 10, 13, etc.) produce magnetic fields and currents, rotating in the same direction as the fundamental
frequency. Negative sequence harmonics (harmonic numbers 2, 5, 8, 11, 14, etc.) develop magnetic fields and currents that
rotate in a direction opposite to the fundamental frequency set. Zero sequence harmonics (harmonic numbers 3, 9, 15, 21, etc.)
do not develop usable torque, but produce additional losses in the machine. Zero sequence harmonics are those harmonics
which doesn’t rotate at all because they’re in phase with each other
Q5) what are the effects of positive, negative and zero sequence harmonics?
Generally, positive sequence harmonics are undesirable because they are responsible for overheating of conductors, power
lines and transformers due to the addition of the waveforms.
Negative sequence harmonics on the other hand circulate between the phases creating additional problems with motors as the
opposite phasor rotation weakens the rotating magnetic field required by motors, and especially induction motors, causing
them to produce less mechanical torque.
Another set of special harmonics called “triplens” (multiple of three) have a zero rotational sequence. Triplens are multiples
of the third harmonic (3rd, 6th, 9th …), etc., hence their name, and are therefore displaced by zero degrees. Zero sequence
harmonics circulate between the phase and neutral or ground.
Unlike the positive and negative sequence harmonic currents that cancel each other out, third order or triplen harmonics do not
cancel out. Instead add up arithmetically in the common neutral wire which is subjected to currents from all three phases.
The result is that current amplitude in the neutral wire due to these triplen harmonics could be up to 3 times the amplitude of
the phase current at the fundamental frequency causing it to become less efficient and overheat.
Q6) Why aren't even harmonics significant when compared to odd harmonics in power system?
AC is a sinusoidal waveform which have equal positive and negative half. A odd harmonic can be simply described as a
sinusoidal waveform terminating in odd pi points(for ex-pi, 3 pi, 5 pi) thus it have odd no. of cycles(1 cycle for pi, 3 cycles in
3 pi(2 positive cycles & 1 negative cycle). So for odd harmonics, there is a positive half cycle still left which does not have a
negative half cycle to cancel it.
That does not happen for even harmonics. They have equal number of positive as well as negative half cycles
Even harmonics are smaller in motors and transformers and tend to cancel in neutral wire but the odd currents add up and
increase neutral current temp rise above expectations
Q7) what are the effects of current harmonics?
Effects of current harmonics:
The effect of current harmonics on distribution systems can be serious, primarily because of the increased current flowing in
the system.
·
For example, a 1,000kVA, 480V transformer is rated to deliver 1,200A rms. But the more harmonic current this
transformer has to supply, the less fundamental current it can provide for powering loads. In other words, because the harmonic
current doesn't deliver any power, its presence simply uses up system capacity and reduces the number of loads that can be
powered.
·
Harmonic currents also increase I2R heat losses in transformers and wiring. Since transformer impedance is frequency
dependent, increasing with harmonic number, the impedance at the 5th harmonic is five times that of the fundamental
frequency. So each ampere of 5th harmonic current causes five times as much heating as an ampere of fundamental current.
§ In a system powering phase-to-neutral connected loads, detrimental effects are again due to the harmonic currents using up
system capacity and reducing the number of useful loads that can be powered. Third harmonic currents cause a further
detriment, because they're additive in the neutral conductor. When many computers, which are nonlinear loads, are connected,
the neutral current — primarily 3rd harmonic — can be larger than any of the phase currents. The third phase currents, are
introduced at 120 harmonic of each phase is identical, being three times the frequency and one-third of a (fundamental) cycle
offset. Case studies in commercial buildings generally show neutral currents between 150% and 210% of the phase currents
Q8) what are the effects of voltage harmonics?
Effects of voltage harmonics. Besides overheating, the other major effect of current harmonics on an electrical system is the
creation of voltage harmonics
·
Harmonic voltage distortion causes increased eddy current losses in motors in the same way as in transformers. Eddy
currents are circulating currents in the conductors induced by the sweeping action of the leakage magnetic field on the
conductors The increase in transformer eddy current loss due to harmonics has a significant effect on the operating temperature
of the transformer. However, additional losses arise due to the generation of harmonic fields in the stator, each of which is
trying to rotate the motor at a different speed either forwards or backwards. High frequency currents induced in the rotor further
increase losses.
·
5th harmonic voltage distortion can cause serious problems for 3-phase motors. The 5th harmonic is a negative sequence
harmonic, and when supplied to an induction motor it produces a negative torque. In other words, it attempts to drive the motor
in a reverse direction and slows down its rotation. So the motor draws more 50-Hz current to offset the reverse torque and
regain its normal operating speed. The result is overcurrent in the motor, which either causes protective devices to open or the
motor to overheat and fail. For this reason, removing 5th harmonic current from systems powering 3-phase loads is often a
high priority in industrial facilities.
·
Harmonic voltages can produce excessive vibration in both single and three-phase motors. Vibration leads to higher than
normal wear and tear on bearings, and can even impact motor shaft reliability.
Q9) how to reduce the effects of harmonics?
Unfortunately, non-linear loads will always produce harmonics when connected to an AC voltage supply. This means it is
impossible to eliminate them completely, but they can be prevented from propagating throughout an installation by using
harmonic filters. There are both passive and active filters.
PASSIVE FILTERING
A passive filter basically has an inductor and a capacitor tuned for a specific frequency. When connected in parallel with a
non-linear load, a passive filter draws in harmonic currents of that specific frequency and prevents them from affecting other
pieces of equipment.
Passive filters are suitable for high-power applications, but they have one drawback: an individual filter is required for
each harmonic frequency.
ACTIVE FILTERING
Unlike a passive filter, an active filter monitors the harmonic currents and injects a current of equal but opposite magnitude to
cancel them out.
Instead of using an inductor and capacitor tuned for a specific frequency, an active filter uses power electronics and can operate
over a wide range of frequencies. However, this technology is more sensitive than a passive filter and is limited in terms of the
electric power than can be handled.
HYBRID FILTER
As implied by its name, a hybrid filter combines both technologies to offer the benefits of active and passive filtering in a
single device.
Q10) what is total harmonic distortion?
Total harmonic distortion (THD) is a measurement that tells you how much of the distortion of a voltage or current is present
in the signal due to harmonics. It is an important aspect in power systems and it should be kept as low as possible. Lower THD
in power systems means higher power factor, lower peak currents, less heating, less emissions, less core loss in motors and
higher efficiency.
It is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency.
Determining THD for a system depends on many factors. Those factors include system impedance, transformer impedance,
ratio of non-linear to linear power-consuming devices, and pre-existing harmonic distortion levels
Q11) what level of THD (total harmonic distortion) is acceptable in a commercial electrical installation?
IEEE Standard 519 deals with this in depth, but there are commercial applications where the acceptable THD can be as low as
3% for voltage and 5% for current.
TOPIC 20: PV SYSTEMS
Q1) what is solar PV system?
Solar power system is one of renewable energy system which uses PV modules to convert sunlight into electricity. The
electricity generated can be either stored or used directly, fed back into grid line.
Q2) what is photovoltaic (solar electricity) or "PV"?
So, photovoltaic could literally be translated as light-electricity. And that's just what photovoltaic materials and devices do;
they convert light energy to electricity.
Q3) how can we get electricity from the sun?
When certain semiconducting materials, such as certain kinds of silicon, are exposed to sunlight, they release small amounts
of electricity. This process is known as the photoelectric effect. The photoelectric effect refers to the emission, or ejection, of
electrons from the surface of a metal in response to light. Sunlight is made up of photons, or particles of solar energy. When
photons strike a PV cell, they may be reflected or absorbed, or they may pass right through. Only the absorbed photons generate
electricity. When this happens, the energy of the photon is transferred to an electron in an atom of the PV cell.
Q4) why photovoltaic cells produce dc current?
Solar cells, generally consisting of 2 layers of silicon (semi-conductor material) and a separation layer, are wired together and
assembled into panels or modules. When the cells are exposed to sunlight, photons from the sun interact with electrons in the
upper silicon layer, basically knocking them loose from their associated atoms. The loose electrons are attracted to atoms in
the lower layer of silicon and travel through the wire to get there. This movement of electrons from one side of the cell to the
other through the wire is electrical current.
Q5) why do solar PV cells use semiconductors and not conductors, which have significantly lower band gap?
Solar cells or photo-voltaic (PV) cells work on the principle of photo electric effect. In photo electric effect the photons
strike the element surface and the energy of the photons is transferred to the electrons present in the valence band of the
element. This energy is good enough to excite the electrons from valence band to conduction band and hence give rise to free
electrons. These free electrons form the base for the electric current in the circuit.
The band gap in the descending order is as follows:
1. Insulators
2. Semiconductors
3. Conductors
In insulators, the energy band gap between the valence band and conduction band is very large. Generally the photons from
the sunlight do not possess this amount of energy to excite the electrons from valence band to conduction band.
In semiconductors, this gap is less and photons can do the task of exciting the electrons without any problem.
In conductors, these band almost overlap each other and hence there are always free electrons.
But free electrons does not mean flow of current. Free electrons moving in a specific direction is known as current. In
conductors, the free electrons move randomly and hence the relative speed is 0.
In semiconductors, due to the presence of PN junction, there is a small electric field present within the semiconductors
without which the solar cells won’t work. To make the electrons move in the circuit, there must be some driving force.
This driving force is “Electric Field in the PN junctions, the electrons and holes try to mix with each other but they are not
able to completely mix, otherwise the junction will become neutral and will be of no use. When few electrons and holes
combines with each other and rush towards the opposite charge, repulsive force comes into play due to the heavy rush of
similar charges.
A potential barrier is formed and when equilibrium is attained, we find the presence of small amount of electric field within
it. This acts as a driving force for the free electrons, all in one direction.
That is why we cannot use conductors in PV cell
Q6) What Happens With a Solar PV System at Night and On Cloudy Days?
Even though solar panels are not actively producing much at night (maybe just a little bit if the moon is bright enough),
appliances are still getting power in your home. How?
Your home is connected to a main utility grid. Think of the grid like a giant bank that you can draw power from or even deposit
power to from your solar power system. During the day, extra power that your system produces but doesn’t use gets sent back
into the electric grid that everyone shares. Your utility company will pay you a set amount for energy that you “sell” back to
them. And during the nighttime when your system is not producing enough to power your appliances, you can draw power
back from the grid. This process is called net-metering.
For some areas that do not have an efficient electrical grid to rely on (such as rural areas), a battery back-up is also a good
choice. Power is generated during the day and then banked in the batteries to be used when needed. This is a great option for
agricultural systems.
Q7) what happens with a solar PV system during power outages?
Most grid-connected PV systems shut down to prevent back feeding electricity into de-energized power lines that may have
fallen or that line crew members may be working on. It’s important to have this shut-down feature to prevent injuries—and
even death— to those working on the line.
Back feeding is flow of electrical energy in the reverse direction from its normal flow. For example, back feeding may occur
when electrical power is injected into the local power grid from a source other than a utility company generator.
Q8) Do solar panels work in cloudy weather?
But, do solar panels work in cloudy weather? Yes… just not quite as well on a cloudy day, typical solar panels can produce
10-25% of their rated capacity. The exact amount will vary depending on the density of the clouds, and may also vary by the
type of solar panel One cloudy day isn’t as important as the amount of sunshine over a full year…
When you’re looking at how solar power can help you save money on your electric bill, you’ll be considering how much
sunshine you get over an entire year, not any particular day
Q9: How long do photovoltaic (PV) systems last?
A PV system that is designed, installed, and maintained well will operate for more than 20 years. The basic PV module
(interconnected, enclosed panel of PV cells) has no moving parts and can last more than 30 years. The best way to ensure and
extend the life and effectiveness of your PV system is by having it installed and maintained properly. Experience has shown
that most problems occur because of poor or sloppy system installation.
Q10: What's the difference between PV and other solar energy technologies?
A: There are four main types of solar energy technologies:
1. Photovoltaic (PV) systems, which convert sunlight directly to electricity by means of PV cells made of semiconductor
materials.
2. Concentrating solar power (CSP) systems, which concentrate the sun's energy using reflective devices such as troughs
or mirror panels to produce heat that is then used to generate electricity.
3. Solar water heating systems, which contain a solar collector that faces the sun and either heats water directly or heats
a "working fluid" that, in turn, is used to heat water
Q11) how much does a solar PV system cost?
The price of PV components varies depending on the size of the system (generating capacity), type and quality of the
components purchased, and complexity of the system selected. The good news for consumers is that the cost of PV has declined
dramatically, while the technology has improved, equally dramatically. Installation costs depend on the size and complexity
of the system, but also on the home layout and construction.
Price is directly proportional to power consumption. 300 watt solar panel cost around 2 lacs, 3kw cost around 6 lacs and 5 kW
cost around 10 lacs
Q12) what is a charge controller?
A charge controller is an essential part of nearly all power systems that charge batteries, whether the power source is PV, wind,
hydro, fuel, or utility grid. Its purpose is to keep your batteries properly fed and safe for the long term.
The basic functions of a controller are quite simple. Charge controllers block reverse current and prevent battery overcharge.
Photovoltaic panels work by pumping current through your battery in one direction. At night, the panels may pass a bit of
current in the reverse direction, causing a slight discharge from the battery. (Our term "battery" represents either a single
battery or bank of batteries.) The potential loss is minor, but it is easy to prevent. Some types of wind and hydro generators
also draw reverse current when they stop (most do not except under fault conditions).
Most batteries need around 14 to 14.5 volts to get fully charged.
Q13) How Can I Know If a Solar PV System Will Work On My House?
To begin, you can look at factors such as which direction your home faces, the condition of your roof, and obstructions such
as trees and other buildings that may block the sun during the peak generation period of 9:00 a.m. to 3:00 p.m. Solar
contractors can provide a more detailed information on this. . Shading on the system can significantly reduce energy output
Q14) how do I (we/you) determine how large a system I'll need?
A: The size of the system is usually directly proportional to the amount of power you use
Q15: Roof-mounted or ground-mounted?
A: It depends. Roof-mounted tends to be less expensive as they require no support structures and are most often not visible
to passersby. On the other hand, ground-mounted systems can usually be oriented and tilted to optimize production.
Q16: Do I need batteries?
A: Is back-up power critical to your business? For most businesses, the answer is “no” and if you don't have a back-up
solution today, an ordinary grid-connected PV system will leave you in the same position. However, if you require back-up
power can design a battery storage solution for your PV system to automatically switchover in the event of a power outage.
Q17: I have plans for expansion. Can I plan for this now?
A: We frequently take into account future growth plans when designing your system. The choice of inverter can be
influenced by near-term expectations about power usage and in some cases it may make sense to include excess capacity
now and simply add more modules later.
Q18: What are the maintenance procedures and costs?
A: Solar PV systems are solid-state technology, have no moving parts and require no maintenance beyond cleaning, which
can typically be done with a garden hose. Most systems should be cleaned once or twice a year, concentrated in the dry
season. Systems in agricultural areas will likely require 2-3 cleaning per year.
Inverters are also solid-state and require little to no maintenance beyond regularly checking the cooling fan outlets and
cleaning when necessary. Mounting hardware is either aluminum or stainless steel or high-quality ASA resin (same as car
side mirrors) and is rust-proof.
Q21: Does cleaning the modules require any special expertise or equipment?
A: No special skills or equipment needed. At the completion of each installation, we’ll provide our cleaning guidebook and
train your team on how to clean your solar panels. We can also offer scheduled cleaning programs
Q22: What parts will break first?
A: The inverter(s), which typically have a useful life of 15-20 years, will be the first thing to fail.
Q23: What about rust and oxidization?
A: All hardware is made for outdoor use and is made of either rust-proof stainless or galvanized steel, aluminum or highgrade outdoor-rated plastic.
Q24. Is it possible to power a household entirely by solar power?
A. Yes, many people power their remote homes completely with solar energy in a standalone solar system, however certain
high load appliances are sometimes powered by an alternative means. Usually, they also adopt a conservation lifestyle
Q25 How durable are solar panels?
A. They are very durable. All of the solar panels Crystal Bay Solar carries are water proof and our high quality Unisolar
crystalline silicon panels are unbreakable. We don't recommend that you step on them though
28) Technical - Charging Time-Charging time depends on the capacity of the battery, how much energy remains in the battery, and intensity of sunlight.
Charging Time=Battery Capacity divided by Solar Panel Output.
So your battery will take longer to charge with a smaller PV panel -- If you're impatient you can get a larger
panel to charge it quicker. It's good to get a battery with a power indicator built in or get a voltage meter.
31) What is a Solar Panel?
A solar panel is the main part of the solar system. It absorbs the sun’s rays and converts it to electricity. This is done through
photovoltaic cells, which are hundreds of semiconductors combined to form a circuit. When the light shines on the cells, they
convert it into DC current. When there is no more light, the cells stop producing electricity
A residential solar panel measures 65 inches by 39 inches (L x W), while a commercial solar panel is usually 77 inches by 39
inches in width. The depth of both is 1.8 inches. The residential panel is made up of 60 photovoltaic cells, while a commercial
panel can have anywhere from 72 to 96 photovoltaic cells. A solar panel can generate anywhere from 50 W to 270W depending
on how bright the sun is. This equates to almost 10kWh daily and about 38kWh energy per month.
The solar panels have either 10 or 25 years of warranty, depending on which brand you choose. The inverter has a year
warranty, and the battery has 6 months warranty. Once the system has been installed, the estimated payback time on the
investment is almost 2-3 years.
32) Solar in Pakistan?
1) Frequent load shedding, high electricity bills, power blackouts and never ending UPS battery issues! All of these probably
sound familiar to an average Pakistani. In the past few years, one viable alternative to counter power and energy problems
has emerged in the form of solar panels. While still not very common, solar panels are increasingly becoming popular
across the country, both in urban areas and in rural regions which have access to very little or no electricity at all
2) Number of projects at the macro level across the country are underway like the Quaid-e-Azam solar park in Bahawalpur,
which will be completed later this year, covers an area equivalent to 500 soccer fields and will generate 1000MW of
electricity.
3) The solar energy technologies have not been exploited on a large scale for a number of reasons such as, high cost, lack of
motivation and inadequate demonstration of effective use of the technology. Thar in Sindh and entire Baluchistan province
is considered ideal for utilization of solar energy
4) Light is the only requirement for these houses located in remote areas of the province and the electric requirement for each
house is 100 watt at maximum. Extension of grid lines for such small power requirements would certainly be very
uneconomical and local power generation could be the best solution. In case, diesel generators are used, transportation of
fuel to such remote areas and maintenance is again costly proposition therefore solar energy seems an attractive option for
these areas.
5)
Baluchistan province is particularly rich in solar energy. It has the highest annual mean sunshine duration in the world.
6) Pakistan has huge potential for alternative energy generation, an estimated 2.9 million megawatt (MW) f solar energy
and 340,000 MW of wind, according to Pakistan Alternative Energy Board as well as some 100,000 MW of
hydropower. But around 70 per cent of its electricity is generated through dirty fossil fuels – primarily oil and gas
7) Pakistan producing more than 1,000MW of clean energy
8) As up to 49 per cent of electricity produced in Pakistan is used in the domestic sector, the government should be
providing more incentives for households to switch to solar which could in turn reduce the country’s reliance on fossils
and reduce its carbon footprint,
9)
“The biggest issue is that the government doesn’t support the sector,”
10) CPEC also an emissions factor
11) With huge investment flowing in from China under the China-Pakistan Economic Corridor, there are plans to add more
than 10,000 MW to the national grid. Work is currently underway on 17 priority projects.
TOPIC 21: BILL READING
We get our electricity bill monthly but most of the people have no idea of what they are being charged for. What is slab system,
what effects slab system has on our electricity bill, what are fixed charges, demand charges, how electricity is being billed. So,
i will try to make you understand how k-electric charges for electricity you consume.
Given below is a sampled k-electric bill
We will break this bill into various sections:
1) Tariff: Fixed charges, demand charges
2) Electricity Consumption
3) Cost of electricity
4) Slab system
5) Sanctioned load
6) Government and bank charges
TARIFF:
Your tariff is the amount charged for providing electricity by the utility. It includes both fixed and variable charges.
FIXED CHARGES
They are not based on how much energy you use. They will be separately identified on your bill, and is often called the
‘daily supply charge’ or ‘service to property’ charge. These charges are for the poles, meters, wires etc. used by the utility to
provide you electricity
VARIABLE CHARGES:
They are also called as ‘consumption charge’ is the amount you pay for each unit of electricity you use. It is listed on your
bill as Rs per kilowatt hour (Rs/kWh) for electricity
ELECTRICITY CONSUMPTION:
There are 2 kinds of power in Electrical systems; real and reactive power. Kilowatt is the basic unit of real power. It is the
product of current and voltage.
One of the most basic formulas is Energy=Power*Time
So, using this formula, we can represent Electrical Energy as the product of Electrical power and time
Electrical Energy= Electrical Power*Tim
When you use any device in you home that requires electricity, the usage is measured by kilowatt hour (kWh)
Electricity consumption is measured in terms of the number of units of electricity that your household has expended in a
billing period which is usually one month. It is measured in kilo watt hours. Your electricity meter outside your house provides
a reading for the number of units consumed at any given point in time.
K electric sends a meter readers to note down the number of units consumed by a particular consumers. Meter reader would
use to come on specified date which was mentioned on k electric bill. Now k electric has installed new energy meters (smart
meters) in some areas. So these meters won't need any manual intervention to read number of units consumed by a consumer
The highlighted text indicates the due date (before which we have to pay electricity bill else we will have to pay extra amount),
issue date (date on which bill was issued), meter reading date.
Below image shows the billing record of previous months
To calculate the monthly unit consumption of a household, a meter reader subtracts the current month’s reading with the
previous month’s reading. The current month and previous month’s units are present in all the k electric bills supplied. These
are highlighted in the image below. In the sample k electric bill below, the total units consumed are 84 (18,312 – 18,228).
COST OF ELECTRICITY:
Cost of electricity is what you pay to your electricity supply company K-Electric etc. Calculating the cost of electricity is a
little tricky because a variable tariff structure is in place instead of a flat per unit cost. So the tariffs will be different if
consumption is under 50 units compared with if your consumption is between 50 and 100 units and so forth. Essentially,
NEPRA (National Electric Power Regulatory Authority) has created different tariff slabs, which are determined by the unit
consumption for a particular household. Simply put, the more units you consume, the cost of electricity per unit will go up.
SLAB SYSTEM IN K ELECTRIC:
In KE following are the slab units that are being followed.
 Up to 50 units = Rs.2.00 per unit
 1-100 units = Rs. 5.79 per unit
 101-200 units = Rs. 8.11 per unit
 201-300 units = Rs. 10.20 per unit
 301-700 units = Rs. 16.00 per unit
 Above 700 units = Rs. 18.00 per unit
EFFECTS OF SLAB CHANGING:
If 1-100 units are being consumed by a consumer than he will be charged for 5.79 rp per unit. Similarly if a consumer
consumes 101-200 units, he will be charged for 8.11 per unit. So, one has to keep the record number of units consumes
because a slight change in the units consumed can change the slab system and ultimately we will have to pay more
BILLED VALUE:
The picture below (highlighted) shows your bill calculation, where your billed value is found by using this formula:
Billed Value= Units consumed* Slab rate
SANCTIONED LOAD:
Sanctioned load is the maximum amount of load which you are allowed to use at home. If you exceed this load rating than
you will be charge penalty for this.
IN below picture the area which is circled is showing sanctioned load... Here it is written 1 i.e. our sanctioned load is up to
1kw. Sanctioned load varies according to requirement of consumer
(ignore the arrow part)
GOVERNMENT CHARGES:
Different government and other charges are mentioned on k electric bill
General Sales Tax (GST)
General Sales Tax (GST) is levied on the total cost of electricity. GST is usually 17% to 18% depending on the province or
federal territory.

OTHER CHARGES

TVL charges of PKR 35 are standard across all electricity bills in Pakistan

Fuel price adjustment

BANK CHARGES

RENT

others

( Below image shows the charges section)
TOPIC 22: TRANSIENTS IN POWER SYSTEM
Q1) what are transients in power system?
ANS) Transient means temporary or intermittent, someone or something that comes and goes. Sinusoidal wave deviates from
its normal form whenever there is a sudden change in the system such as by faults, addition or removal of heavy loads, power
outages. This short-duration abrupt deviation from the normal sine wave is called transient. They are an instantaneous change
in the state leading to a burst of energy. Transients that exceed the normal voltage are called spikes. Transients that drop below
normal voltage are called sags. They are momentary changes in voltage and current that occurs over a short period of time.
Voltage transients lasts for 50 micro second and current transients lasts for typically 20 microsecond.
Q2) what do you mean by electromagnetic and electromechanical transient for a power system?
ANS) There're two types of transients: electromagnetic and electromechanical transients. Although they both are interrelated
and have a slight difference between them.
Electromagnetic transients that deal with the changes in voltages and currents are usually caused by closing or opening of
circuit breakers, power electronic equipment’s, equipment failure or faults, lightning strokes etc. restricted to events on and
along the transmission and distribution system
Electromechanical transients are caused by mismatch between power production and consumption causing the generator to
either speed up or slow down compared to its normal rotation speed. The reason for that is usually a disturbance in a system
such as the outage of a nearby transmission line.
Unlike electromagnetic transients, which are very fast, electromechanical transients take longer time because of inertia of huge
generator shafts. In which energy is transformed from electrical to mechanical and vice versa. So big hydro, steam turbines
etc. will involve transients as the load they supply changes. Also motors will start and their load will change as they run and
these will create transients both in the electrical and mechanical system alike.
Q3) Where do Transient come from?
Transients can be generated internally, or the can come into a facility from external sources.
EXTERNAL SOURCES:

Lightning is the most well-known of the externally generated transients. Most lightning transients are not actually the
result of direct lightning strikes they are most often "induced" onto conductors as lightning strikes near the power
line. The large electric fields generated during a discharge can couple into the power system, creating induced
transients. A cloud-to-cloud discharge can generate a 70 Volts per meter electric field. On a 1/2-mile length of
transmission line this is equal to a 56,000-volt transient--and it didn't even touch the line!

Other externally generated transients may also be imposed on power lines through normal utility operations. Switching
of facility loads, opening and closing of disconnects on energized lines, switching of capacitor banks, re-closure
operations and tap changing on transformers can all cause transients.

Another common source of transients is if you share a transformer with other users, any transient activity generated
on his premises will be seen at your electrical main. Remember, you are both physically connected at the secondary
side of the transformer.
INTERNAL SOURCES:

The vast majority of transients are produced within your own facility. The main culprits are device switching, static
discharge, and arcing.
Each time you turn on, turn off, load, or unload an inductive device, you produce a transient. Examples of inductive
loads are motors and transformers. A motor with a faulty winding, commutator, or other insulation faults can produce
a continuous stream of transients. Even transformers can produce a large transient, particularly when energizing.
Copiers and laser printers, for example, are notorious transient generators – as are heating and air conditioning systems,
generators, inverter, pumps.

Arcing can generate transients from a number of sources. Faulty contacts in breakers, switches, and contactors can
produce an arc. Faulty connections and grounds can produce arcing.
Q4) what are the effects of voltage sag on our system?
ANS) Voltage sags or dips are brief reductions in voltage, typically lasting from a cycle to a second or so, or tens of
milliseconds to hundreds of milliseconds. Voltage Sags are caused by abrupt increases in loads. Voltage sags can arrive from
the utility or they can generated inside a building. For example, in residential wiring, the most common cause of voltage sags
is the starting current drawn by refrigerator and air conditioning motors.
Sags do not generally disturb incandescent or fluorescent lighting, motors or heaters. However, some electronic equipment
lacks cannot suffer these voltage sags and can be damaged
Q5) what is voltage swell? What happens when excessive reactive power flows in our system?
ANS) Excessive reactive power will result in voltage swell and Voltage Swell is defined by IEEE 1159 as the increase in the
RMS voltage level to 110% - 180% of nominal, at the power frequency for durations of ½ cycle to one (1) minute. It can cause
control problems and hardware failure in the equipment, due to overheating that could eventually result to shut down. Also,
electronics and other sensitive equipment are prone to damage due to voltage swell.
Q6) what is transient stability in a power system?
The ability of a synchronous power system to return to stable condition and maintain its synchronism following a relatively
large disturbance arising from very general situations like switching ON and OFF of circuit elements, or clearing of faults etc.
is referred to as the transient stability in power system
Q7) what are the effects of transient activity?
1) Motors will run at higher temperatures when transient current are present. This type of disruption produces motor
vibration, noise, and excessive heat. Motor winding insulation is degraded and eventually fails. Transients produce
hysteresis losses in motors that increase the amount of current necessary to operate the motor
2) Transient activity causes early failure of all types of lights. Fluorescent systems suffer early failure of ballasts, reduced
operating efficiencies, and early bulb failures. One of the most common indicators of transient activity is the premature
appearance of black "rings" at the ends of the tubes
3) The facility's electrical distribution system is also affected by transient activity. Transient degrade the contacting
surfaces of switches, disconnects, and circuit breakers. Intense transient activity can produce "nuisance tripping" of
breakers by heating the breaker and "fooling" it into reacting to a non-existent current demand. Electrical transformers
are forced to operate inefficiently because of the hysteresis losses produced by transients and can run hotter than
normal.
4) This also has a harsh effect on the IC (Integrated circuits) and can result in getting them burnt out
5) Transient over-voltages can cause breakdown of insulation, resulting in either a temporary disturbance of device
operation or instantaneous failure
Q8) what is the difference between the terms “Surge Arrester”, “Transient Voltage Surge Suppressor (TVSS)” and
now “Surge Protective Devices (SPD)”?
Up until the third edition of the ANSI/UL 1449 standard was introduced and put into effect in 2009 there we re various
terms used when referencing devices intended to limit the effects of transient surge events. Surge arresters less than
1000 volts, commonly referred to as secondary surge arresters, were originally developed for and applied to the line
side of the power distribution system to protect utility supplied equipment and building wiring. Surge arresters were
intended to protect the system structure and not necessarily the connected equipment and loads.
A TVSS as it was defined must be applied to the load side of the main service. Unlike a surge arrester, a TVSS was intended
to protect the sensitive electronics and microprocessor based loads by having a more limiting effect on transient voltages.
With the adoption of ANSI/UL 1449 – Third Edition, both the terms “Secondary Surge Arrester” and “TVSS” were
done away with and replaced with a more general term of “Surge Protective Devices (SPD)”. With this new term there
was a need to identify the proper SPD for its intended application so UL also introduced the different Types of SPDs.
For those applications similar to where a surge arrester would have been used on the line side of the system, it would
now require a Type 1 SPD. In those applications where you once placed TVSS devices on the load side of the sys tem, these
same installations require a minimum of a Type 2 SPD.
The electrical industry has followed these changes. The NEC 2008 Sec 285 has updated their terminology eliminating
the term TVSS and describes the proper application of Type1 and Type2 SPDs.
Q9) Does transient activity occurs only in power distribution lines?
Transient overvoltage’s do not occur solely in power distribution lines, but are also common in any line formed by metal
conductors, such as telephony, communications, measurement and data.
Q10) what is the difference between voltage surge and voltage spike?
A very brief change in voltage is called a spike. A longer-lasting change is called a surge
Q11) what are SPD’s and how do they work?
ANS) Surge Protective Devices (SPDs) are designed to protect sensitive equipment from damage caused by lightning and
electrical transient surges
This device is connected in parallel on the power supply circuit of the loads that it has to protect
This is the most commonly used and most efficient type of overvoltage protection.
SPD connected in parallel has a high impedance. Once the transient overvoltage appears in the system, the impedance of the
device decreases so surge current is driven through the SPD, bypassing the sensitive equipment.
TOPIC 23: AUTO TRANSFORMER
1) What is auto transformer? What is the difference between an auto transformer and a conventional two winding
transformer?
Auto transformer also called as auto-former is an electrical transformer where primary and secondary shares same common
single winding. So basically it’s a one winding transformer
The main difference between conventional two winding transformer and auto-transformer is their working principle and hence
application. In an auto -transformer energy transfer is mainly through conduction process and only a small part is transferred
inductively
Auto-transformer works on the principle of induction and conduction together.
(Two winding transformer works on principle of electro-magnetic induction only)
NOTE:
Conduction involves a flow of electric charge due to an electric field. To conduct a current from one conductor to another,
the two conductors should be in contact. In conduction, current is generated when an electric field passes through the
conductor.
In induction, a current can be made to flow in a conductor by keeping it in the vicinity of another conductor, carrying a
constantly varying current. In induction, current is generated when a magnetic field changes around a conductor.
2) Why is an auto transformer not used as distribution transformers as it is more efficient than a normal
transformer?
ANS) Distribution transformers are normally 11 kV/ 400 Voltage level transformer. Auto-transformer is typically used when
the voltage ratios are close to 1. (Max < 2). They are used for stepping down voltage from 220kv to 132kv. As the ratio on
distribution side is too high from (11000 to 400v) therefore auto transformer is not used on distribution side.
3) Why autotransformer is used when voltage ratio is maximum 2?
ANS) for transformation ratio = 2, the size of the auto transformer would be approximately 50% of the corresponding size
of two winding transformer. For transformation ratio say 20 however the size would be 95 %. The saving of cost is
appreciable when the ratio of transformer is low, that is lower than 2.
4) What is transformation ratio in auto transformer?
Transformation ratio is the ratio of secondary voltage to the primary voltage and is equal to the ratio of the number of turns
in the secondary winding to the number of turns in the primary winding
K = N2/N1 = E2/E1
K = N2/N1 = E2/E1
5) Why the short circuited current in an auto transformer is more compared to a two winding transformer?
ANS) 2 winding transformer has got electrical isolation. That is there is no electrical short between primary and secondary
winding in auto transformer. Let’s take an example:
Consider a 10-kva transformer rated at 2300/230 volts which will have a leakage impedance of say 4 %. If it is connected as
an auto transformer stepping down from say 2530 to 2300 volts which corresponds to a transformation ratio of 1.1
(2530/2300=1.1) , the impedance drop will be:
4*(0.1/1.1) = 0.364%
Its leakage impedance is 9.1 % of the leakage impedance of 2 winding transformer
Therefore short circuit current in an auto transformer is more compared to a two winding transformer and in order to protect
auto transformer from the hazards of high amount of short circuit current, a current limiting is inserted.
6) What are the Advantages of auto transformer over distribution transformer?
ANS) Autotransformer is a transformer where primary and secondary shares same common single winding. So basically it is
a one winding transformer.
The cost and as well as size is reduced. They are cheaper, lighter in weight, lower leakage reactance, lower losses and lower
excitation current.
Autotransformers are clearly much cheaper than conventional double wound transformers of the same VA rating. When
deciding upon using an autotransformer it is usual to compare its cost with that of an equivalent double wound type.
This is done by comparing the amount of copper saved in the winding. If the ratio “n” is defined as the ratio of the lower
voltage to the higher voltage, then it can be shown that the saving in copper is: n.100%. For example, the saving in copper
for the two autotransformers would be:
7) What are Disadvantages of Auto transformer?
 The main disadvantage of an autotransformer is that it does not have the primary to secondary winding isolation of a
conventional double wound transformer. Then an autotransformer cannot safely be used for stepping down higher
voltages to much lower voltages suitable for smaller loads.
 If the secondary side winding becomes open-circuited, current stops flowing through the primary winding stopping the
transformer action resulting in the full primary voltage being applied to the secondary terminals.
 If the secondary circuit suffers a short-circuit condition, the resulting primary current would be much larger than an
equivalent double wound transformer due to the increased flux linkage damaging the autotransformer.
 Since the neutral connection is common to both the primary and secondary windings, earthing of the secondary
winding automatically Earth’s the primary as there is no isolation between the two windings. Double wound
transformers are sometimes used to isolate equipment from earth.
 The leakage flux between the primary and secondary windings is small and hence the impedance is low. This results
into severer short circuit currents under fault conditions.

8) What are the applications of auto transformer?
Autotransformer has many uses and applications including the starting of induction motors, used to regulate the voltage of
transmission lines, in audio applications and impedance matching between such as a low impedance microphone and a high
impedance amplifier input.
Auto transformer is used as variac in laboratory or where continuous variable over broad ranges are required.
9) Should there be only one tapping from auto transformer?
The Autotransformer can also be constructed with more than one single tapping point. Auto-transformers can be used to
provide different voltage points along its winding or increase its supply voltage with respect to its supply voltage V P as
shown.
TOPIC 24: TYPES OF INSULATORS
Q1) why are insulator used?
It is obvious that if overhead power lines are not properly insulated from their support poles/towers, the current will flow
towards the ground through the poles/towers which also become hazardous.
Q2) what are pin insulators? Why there is a groove at top of them? Where they are placed? At what voltage they can
be used? Why they have a rain shed with them?
Pin type insulators or pin insulators are popularly used in electric distribution systems up to 33 kV voltage level. They are
secured on the cross arms of the pole to carry power lines. There is a groove on the upper end of a pin insulator for housing
the conductor. Conductor wire is passed through this groove and secured by binding with the same wire as of conductor.
A pin insulator is usually made from porcelain, but glass or plastic may also be used in some cases. As pin insulators are almost
always employed in open air, proper insulation while raining is also an important consideration. A wet pin insulator may
provide a path for current to flow towards the pole. To overcome this problem, pin insulators are designed with rain sheds or
petticoats. Beyond operating voltage of 33kV, pin insulators become too bulky and uneconomical. That during raining the
outer surface of the rain shed becomes wet but the inner surface remains dry and non-conductive
Q3) what are suspension insulators? What are disc insulators and why they are called disc insulators? Can we tell
voltage of line by looking at them? What are their advantages?
As it is already mentioned above, pin insulators become too bulky an uneconomical beyond 33 kV. So, for voltages higher
than 33 kV, suspension insulators are used. A suspension insulator consists of a number of porcelain discs connected to each
other with metal links in the form of a string. Line conductor is suspended at the bottom end of the suspension string which is
secured to cross-arm of the tower. Each disc in a suspension insulator string is designed for a low voltage, say 11 kV. The
number of discs in a string depends on the working voltage. Suspension insulators are preferred for transmission lines. In
suspension insulator numbers of insulators are connected in series to form a string and the line conductor is carried by the
bottom most insulator. Each insulator of a suspension string is called disc insulator because of their disc like shape Mechanical
stresses on the suspension insulator is less since the line hanged on a flexible suspension string.
ADVANTAGES OF SUSPENSION INSULATORS:

Each unit of disc is designed for a low voltage, say 11 kV. Hence, depending upon the working voltage, desired number
of discs can be connected in series to form an insulator string suitable for particular voltage.
 If any of the discs in insulator string is damaged, it can be replaced easily. Replacement of the whole string is not required.
 In case of increased demand on the line, the line voltage can be increased and the additional insulation required for the
raised voltage can be easily provided by adding the desired number of discs in the insulator strings.
 As the line conductors are suspended by suspension strings, they run below the earthed cross-arms of the towers. This
arrangement provides partial protection from lightning.
 The suspension arrangement provides greater flexibility to the line. Suspension insulators are allowed to swing so that
they can take up the position where mechanical stresses are minimum.
Q4) what are strain insulators? What is the difference between strain and suspension insulators? Why they are used at
dead end?
At a dead end of a transmission line or at a corner or sharp curve, the transmission line is subjected to a great tensile load. In
order to sustain this great tension, strain insulators are used at dead ends or sharp corners. For high voltage transmission lines,
stain insulator consists of an assembly of suspension insulators. A strain insulator is an electrical insulator that is designed to
work in mechanical tension (strain). For low voltage lines (less than 11 kV), shackle insulators are used as strain insulators.
"Suspension" and "Strain" insulators are the same thing; when used in the vertical position (with the conductor hanging below)
they are called "suspension"; when used in the horizontal, to dead-end a conductor, they are called "strain"
Q5) what are shackle insulators?
Shackle insulators are used in low voltage distribution lines as strain insulators. A shackle insulator can be used vertically as
well as horizontally and it can be directly fixed to a pole with a bolt or to the cross arm. However, the use of such insulators is
decreasing after increasing the use of underground cables for distribution purpose.
Some additional types of insulators are: post insulators, glass insulators, polymer insulators, long-rod insulators, stay insulators
etc.
TOPIC 25: PARALLEL OPERATION OF TRANSFORMER
Q1) what is parallel operation of transformer?
Two transformers are connected in parallel means that the two primary windings are connected to supply bus and the two
secondary windings are connected to load bus-bars
Q2) why parallel operation of transformer is required?

Increased Load: When load is increased and it exceeds the capacity of existing transformer, another transformer may
be connected in parallel with the existing transformer to supply the increased load.
 Non-availability of large transformer: If a large transformer is not available which can meet the total requirement of
load, two or more small transformers can be connected in parallel to increase the capacity.
 Increased reliability: If multiple transformers are running in parallel, and a fault occurs in one transformer, then the
other parallel transformers still continue to serve the load. And the faulty transformer can be taken out for the
maintenance.
 Transportation is easier for small transformers: If installation site is located far away, then transportation of smaller
units is easier and may be economical.
Q3) what are the conditions for parallel operation of transformer?

Voltage ratio of all connected transformers must be same: If the voltage ratio is not same, then the secondary will not
show equal voltage even if the primaries are connected to same bus bar. This results in a circulating current in secondary
and hence there will be reflected circulating current on the primary side also. In this case, considerable amount of current is
drawn by the transformers even without load.
Same Polarity: In a transformer each of the primary terminals becomes alternately positive and negative with respect to the
other and the state is true about the secondary terminals If the transformer is to be used alone, the polarity is not important but
if the transformer is to be used in parallel with another the instantaneous polarity is important because the terminals having
identical instantaneous polarity have to connected together.
Polarity of all transformers that run in parallel, should be the same otherwise huge circulating current that flows in the
transformer but no load will be fed from these transformers. Polarity of transformer means the instantaneous direction of
induced emf in secondary. If the instantaneous directions of induced secondary emf in two transformers are opposite to each
other when same input power is fed to both of the transformers, the transformers are said to be in opposite polarity. If the
instantaneous directions of induced secondary emf in two transformers are same when same input power is fed to the both of
the transformers, the transformers are said to be in same polarity.
Same Phase Sequence: The phase sequence or the order in which the phases reach their maximum positive voltage, must be
identical for two parallel transformers. Otherwise, during the cycle, each pair of phases will be short circuited.
Frequencies must be identical

Equal per unit leakage impedance:
If the ratings or line voltages are equal their per unit leakage impedance’s should be equal in order to have equal load sharing
of the both transformers. If the ratings are unequal then the transformer which has less rating will draw more current and it
leads to unequal load sharing. It may also lead to mismatch in line voltages due to voltage drops. In other words, for unequal
ratings, the numerical values of their impedance’s should be in inverse proportional to their ratings to have current in them in
line with their ratings.
A difference in the ratio of the reactance value to the resistance value of the impedance results in different phase angles of the
currents carried by the two parallel transformers. Due to this phase angle difference between voltage and current, one
transformer may be working on high power factor and another transformer may be working on lower factor. Hence real power
sharing is not proportional between the two transformers
TOPIC 26: PARALLEL OPERATION OF GENERATORS
Q1) How synchronous generator works?
Synchronous generator are called as synchronous because here rotor and magnetic field rotate with the same speed, because
the magnetic field is generated through a shaft mounted permanent magnet mechanism and current is induced into the stationary
armature
There are two parts of an alternator stator and rotor, rotor is the rotating part in which excitation windings are wound, this part
is rotated using any mechanical setup either a diesel engine or a turbine driven by steam or water(hydro projects). In the
excitation winding we supply DC voltage. The stator is stationary part on which 3 phase windings are wound as star winding
(mostly for large generator). So when we supply DC voltage to the rotor winding then electro magnet is made in the winding
and when this magnet is rotated then we have a rotating magnetic field which is intersected by the conductor in the stator and
this intersection of magnetic field in the stator coil results in generation of EMF(Electro Motive Force), this EMF when
measured at the terminal of open circuit generator shows the voltage and when generator is connected to any load be it of any
type the EMF causes the current to flow and hence power is drawn from the generator depending upon the type of load and it
power factor. Thus it could be concluded that generator actually generates EMF. Thus, for each winding the current flows in
one direction for the first cycle and then in another direction in the other cycle with a time lag of 120 degrees.
There are some conditions to be satisfied for parallel operation of the alternator. Before entering into that, we should
understand some terms which are as follows.



The process of connecting two alternators or an alternator and an infinite bus bar system in parallel is known as
synchronizing.
Running machine is the machine which carries the load.
Incoming machine is the alternator or machine which has to be connected in parallel with the system.
Q2) what are the Condition for Parallel Operation of Alternator?
In order to synchronize a generator to the grid, four conditions must be met:
1. Phase Sequence
2. Voltage Magnitude
3. Frequency
4. Phase Angle
The figure below shows an alternator (generator 2) being paralleled with a running power system (generator 1).These two
machines are about to synchronize for supplying power to a load. Generator 2 is about to parallel with the help of a switch, S1.
This switch should never be closed without satisfying the above conditions.
VOLTAGE:
If the generator voltage is higher than the grid voltage, this means that the internal voltage of the generator is higher
than the grid voltage. When it is connected to the grid the generator will be overexcited and it will put out MVAR.
If the generator voltage is less than the grid voltage, this means that the internal voltage of the generator is lower than
the grid voltage. When it is connected to the grid the generator will be under-excited and it will absorb MVAR.
To make the terminal voltages equal. This can be done by adjusting the terminal voltage of incoming machine by changing the
field current and make it equal to the line voltage of running system using voltmeters
PHASE SEQUENCE:
There are two methods to check the phase sequence of the machines. They are as follows
A small induction motor can be connected alternately to the terminals of each of the two generators. If the motor rotates in
the same direction each time, then the phase sequences of both generators are the same. If the phase sequences are different,
the motors will rotate in opposite directions. In this case, two of the conductors on the incoming generator must be reversed.
Here we can see three light bulbs are connected to the terminals of the switch, S1. Three light bulbs connected across the
terminals of the switch connecting the generator to the system. When the phase changes between the two systems, the light
bulbs become bright when the phase difference is large and dim when the phase difference is small. When the systems have
the same phase sequence, all three bulbs become bright and dim simultaneously. If the systems have opposite phase
sequence, the bulbs get bright in succession.
Their output voltages must be in phase. A mismatch in the phases will cause large
opposing voltages to be developed. The worst case mismatch would be 180° out of phase, resulting in an opposing
voltage between the two generators of twice the output voltage. This high voltage can cause damage to the generators and
distribution system due to high currents.
FREQUENCY:
Next, we have to check and verify the incoming and running system frequency. It should be nearly the same. This can be done
by inspecting the frequency of dimming and brightening of lamps.
The frequency of the oncoming generator should be slightly higher than the frequency of the running system. A frequency
meter is used until the frequencies are close. The frequency of the oncoming generator is adjusted to a slightly higher frequency
to ensure that when it is connected, it will come on-line supplying power as a generator, instead of consuming it as a motor.
This can cause an overload in the generators and the distribution system.
PHASE ANGLE:
A confirmation that the two systems are in phase can be achieved by watching the three light bulbs. The systems are in phase
when the three light bulbs all go out (because the voltage difference across them is zero).
This simple scheme is useful, but it is not very accurate. A synchroscope is more accurate. It is a meter that measures the
difference in phase angle between the phases of the two systems. The phase difference between the two a phases is shown by
the dial. When the systems are in phase (0° phase difference), the dial is at the top. When they are 180° out of phase, the dial
is at the bottom.
The phase angle on the meter changes slowly because the frequencies of the two systems are slightly different. Since the
oncoming generator frequency is slightly higher than the system frequency, the synchroscope needle rotates clockwise because
the phase angle advances. If the oncoming generator frequency is lower than the system frequency, the needle rotates
counterclockwise. When the needle of the synchroscope stops in the vertical position, the voltages are in phase and the switch
can be closed to connect the systems.
However, the synchroscope provides the relationship for only one phase. It does not provide information about the phase
sequence.
The whole process of paralleling large generators to the line is done by a computer. For small generators, the operator performs
the paralleling steps.
TOPIC 27: BUCHOLZ RELAY
Buchholz relay is a safety device which is generally used in large oil immersed transformers (rated more than 500 kVA). It is
a type of oil and gas actuated protection relay. It is used for the protection of a transformer from the faults occurring inside the
transformer, such as impulse breakdown of the insulating oil, insulation failure of turns etc.
CONSTRUCTION:
Buchholz relay consists of an oil filled chamber. There are two hinged floats, one at the top and other at the bottom in the
chamber. Each float is accompanied by a mercury switch. The mercury switch on the upper float is connected to an alarm
circuit
and
that
on
the
lower
float
WORKING PRINCIPLE OF BUCHHOLZ RELAY
is
connected
to
an
external
trip
breaker.
Whenever a fault occurs inside the transformer, such as insulation failure of turns, breakdown of core or excess core heating,
the fault is accompanied by production of excess heat. This excess heat decomposes the transformer insulating oil which results
in production of gas. The generation of gases depend on intensity of fault. Gas bubbles tend to flow in upward direction towards
conservator and hence they are collected in the bucholz relay which is placed on the pipe connecting the transformer tank and
conservator.
Hence, when minor fault occurs, the connected alarm gets activated. The collected amount of gas indicates the severity of the
fault occurred. During minor faults the production of gas is not enough to move the lower float. Hence, during minor faults,
the lower float is unaffected.
During major faults, like phase to earth short circuit, the heat generated is high and a large amount of gas is produced. This
large amount of gas will similarly flow upwards, but its motion is high enough to tilt the lower float in the bucholz relay. In
this case, the lower float will cause the lower mercury switch which will trip the transformer from the supply, i.e. transformer is
isolated from the supply.
The amount of gas collected can be viewed through the window provided on the walls of the chamber. The samples of gas are
taken and analyzed. The amount of gas indicates the severity of and its color indicates the nature of fault occurred. In case of
minor faults the float at the bottom of the chamber remains unaffected because the gases produced will not be sufficient to
operate it.
During the occurrence of severe faults such as phase to earth faults and faults in tap changing gear, the amount of volume of
gas evolves will be large and the float at the bottom of the chamber is tilted and the trip circuit is closed. This trip circuit will
operate the circuit breaker and isolates the transformer.
Advantages of Buchholz Relay
Buchholz relay indicates the internal faults due to heating and it helps in avoiding the major faults.
Severity of the fault can be determined without even dismantling the transformer.
If a major fault occurs, the transformer can be isolated with the help of bucholz relay to prevent accidents.
TOPIC 28: SKIN AND PROXIMITY EFFECT
Q1) what is skin effect?
Tendency of current to pass through the outer surface of conductors rather than inner one is called as skin effect. In skin
effect maximum flux density exists at the outer surface and minimum at the center
Q2) why skin effect occurs in transmission line?
The main cause of skin effect is the non-uniformity of flux linkage. The phenomenon of skin effect can be explained as
follows:
Consider a multi—stranded conductor composed of ‘n’ number of strand (filaments). The AC current flowing through the
inner strands produce flux which links (enclose) the inner strands only. But, the flux produced by the flow of AC current
through the outer strands, links (enclose) not only the outer strands but also the inner strands. Thus, the flux linkage per ampere
to the inner strands is higher than the flux linkage per ampere to the outer strands. This causes the inductance (and hence
inductive reactance’s) of inner strands to be much higher than for the outer strands. Thus, higher the impedance lower » is the
flow of current. As a result, the outer strands carry more current than the inner strands.
Q3) why skin effect exist in ac only and not in dc?
Dc has a frequency of zero hertz and its inductive reactance will be zero. Hence maximum current will pass through the
conductor and will have no opposition i.e. no skin effect
Q4) how skin effect can be reduced and on what factors skin effect depends?
Skin effect primarily depends on the three important factors,
1. Operating frequencies
2. Size of conductor
3. Type of conductor.
Higher the operating frequency, the greater is the skin effect. The diameter of conductor has a direct relation with skin effect.
Bigger the diameter of conductor, higher is the skin effect. The type of conductor also decides the level of skin effect. The skin
effect is less in case of a stranded conductor than a solid conductor.
Q5) what is proximity effect?
When two or more conductors are placed near to each other, then their electromagnetic fields interact with each other. Due to
this interaction, the current in each of them is redistributed such that the greater current density is concentrated in that part of
the strand most remote from the interfering conductor.
If the conductors carry the current in the same direction, then the magnetic field of the halves of the conductors which are close
to each other is cancelling each other and hence no current flow through that halves portion of the conductor. The current is
crowded in the remote half portion of the conductor. Proximity effect can be reduced by selecting the core and number of turns
that optimizes the number of layers... An increased number of layers decreases the losses after the first selection. When one
conductor carries alternating current, then there is alternating (constantly varying flux) linked with the nearby conductor in
vicinity. This causes changes in current density of both conductors. The eddy currents are also induced in the conductor in
vicinity.
Q6) How to reduce Proximity Effect?
The proximity effect can be reduced by using the ACSR (Aluminum Core Steel Reinforced) conductor. In ACSR conductor
the steel is placed at the center of the conductor and the aluminium conductor is positioned around steel wire.
The steel increased the strength of the conductor but reduced the surface area of the conductor. Thus, the current flow mostly
in the outer layer of the conductor and no current is carried in the center of the conductor. Thus, reduced the proximity effect
on the conductor.
When the conductors carry the current in the opposite direction, then the close part of the conductor carries, the more current
and the magnetic field of the far off half of the conductor cancel each other. Thus, the current is zero in the remote half of the
conductor and crowded at the nearer part of the conductor.
TOPIC 29: CAPACITOR BANK
Q1) what is a Capacitor Bank?
A Capacitor Bank is a group of several capacitors of the same rating that are connected in series or parallel with each other to
store electrical energy. The resulting bank is then used to counteract or correct a power factor lag or phase shift in an alternating
current (AC) power supply. Inductive devices draws lagging current which is lagging voltage by 90 degrees and capacitor
produces leading current. So, they cancel out and neutralized the component and hence pf increases. Capacitors also reduce
the total current drawn from the distribution system and subsequently increase system capacity
Q2) What Does a Capacitor Bank Work?
Capacitor banks work on the same theory that a single capacitor does; they are designed to store electrical energy, just at a
greater capacity than a single device. An individual capacitor consists of two conductors which are separated by a dielectric or
insulating material. When current is sent through the conductors, an electric field that is static in nature then develops in the
dielectric which acts as stored energy. The stored energy is not maintained indefinitely, as the dielectric present between the
plates allows for a certain amount of current leakage which results in the gradual dissipation of the stored energy.
Q3) why capacitor banks are rated in KVAR?
Basically Capacitor bank is used for 3 purposes
1. To improve power factor of the system
2. To improve voltage regulation.
3. To improve system stability
In all the three applications capacitor provides Reactive power (which deals with voltage) to the system. So, Rating of
capacitor bank is in KVAR (it deals with voltages)
Power factor correction capacitors are rated in electrical units called “vars”. One VAR = one Volt Ampere of Reactive
power. VARs are units of measurement for indicating how much reactive power the capacitor will supply. The capacitor
kVAR rating shows how much reactive power the capacitor will supply. Each unit of kVAR supplied will decrease the
inductive reactive power demand by the same amount
Q4) what are the applications of capacitor banks?
REACTIVE POWER COMPENSATION:
Capacitor banks are capable of supplying reactive power. Capacitor banks are used wherever there is a requirement of reactive
power. They are used to supply reactive power to the load and hence the loads reactive power dependency on the main utility
supply is reduced and burden on the utility to supply reactive power to the load is reduced. Capacitor banks can supply reactive
power. So, if some factory consists of motors and other inductive loads that consume reactive power, we install capacitor bank
near it. This relieves power plant in some far off place from supplying reactive power to that factory.
Q5) what are the disadvantages of capacitor banks?

Capacitor BANKS has no effect on the system during dynamics condition such as post fault voltage recovery

The reactive power produced by a capacitor bank is in direct proportion to the square of its terminal voltage, and if the
system voltage decreases, the capacitors produce less reactive power, when it is most needed, while if the system voltage
increases the capacitors produce more reactive power, which exacerbates the problem

When load levels are high, a shunt capacitor system is beneficial. But when the load drops. Capacitor can do more harm
than good. An excess of capacitance in system leads to higher voltage than desired voltages during less load conditions.
C=Q/ V

Capacitor banks usually have COUPLE of large step size so they don't provide particularly precise control. Whereas on
the other end fact devices can supply the exact amount of VARs needed.

There are harmonics generation associated with capacitor while switching. In fact they are harmonics generator. Moreover,
switching reduces capacitor lifetime. Transients are associated with capacitor switching.
TOPIC 30: HOW TO CALCULATE CAPACITANCE OF A CAPACITOR BANK?
1) A 3 Phase, 5 kW Induction Motor has a P.F (Power factor) of 0.75 lagging. What size of Capacitor in kVAR is
required to improve the P.F (Power Factor) to 0.90?
Motor input = P = 5 kW
Original P.F = Cosθ1 = 0.75
Final P.F = Cosθ2 = 0.90
θ1 = Cos-1 = (0.75) = 41°.41; Tan θ1 = Tan (41°.41) = 0.8819
θ2 = Cos-1 = (0.90) = 25°.84; Tan θ2 = Tan (25°.50) = 0.4843
Required Capacitor kVAR to improve P.F from 0.75 to 0.90
Required Capacitor kVAR = P (Tan θ1 – Tan θ2)
= 5kW (0.8819 – 0.4843)
= 1.99 kVAR
And Rating of Capacitors connected in each Phase
1.99/3 = 0.663 kVAR
2) A Single phase 400V, 50Hz, motor takes a supply current of 50A at a P.F (Power factor) of 0.6. The motor power
factor has to be improved to 0.9 by connecting a capacitor in parallel with it. Calculate the required capacity of
Capacitor in both kVAR and Farads.
Motor Input = P = V x I x Cosθ
= 400V x 50A x 0.6
= 12kW
Actual P.F = Cosθ1 = 0.6
Required P.F = Cosθ2 = 0.90
θ1 = Cos-1 = (0.60) = 53°.13; Tan θ1 = Tan (53°.13) = 1.3333
θ2 = Cos-1 = (0.90) = 25°.84; Tan θ2 = Tan (25°.50) = 0.4843
Required Capacitor kVAR to improve P.F from 0.60 to 0.90
Required Capacitor kVAR = P (Tan θ1 – Tan θ2)
= 5kW (1.3333– 0.4843)
= 10.188 kVAR
2) To find the required capacity of Capacitance in Farads to improve P.F from 0.6 to 0.9 (Two Methods)
Solution #1 (Using a Simple Formula)
We have already calculated the required Capacity of Capacitor in kVAR, so we can easily convert it into Farads by using this
simple formula
Required Capacity of Capacitor in Farads/Microfarads
C = kVAR / (2 π f V2) in microfarad
Putting the Values in the above formula
= (10.188kVAR) / (2 x π x 50 x 4002)
= 2.0268 x 10-4
= 202.7 x 10-6
= 202.7μF
TOPIC 31: FACT DEVICES
Q1) what are fact devices?
With the advancement of technology, transmission systems throughout the world are undergoing changes in order to efficiently
provide electricity to consumer. Transmission systems are becoming overloaded and one of the ways to overcome this situation
is to build new transmission lines. However this is not the best of the solutions since it is costly to build new transmission lines,
difficult and also time consuming. This is where FACT devices comes into picture.
Facts are basically power electronic devices which can be inserted in series, in parallel or sometimes both in series and parallel
to improve transmission capability, system stability, and power quality, minimize transmission losses etc. Three main variables
that can be directly controlled in the power system to impact its performance. These are: • Voltage • Angle • Impedance. They
are used for controlling impedance, dynamic control of voltage and phase angle of high tension transmission lines. Flexible
Alternating Current Transmission System (FACTS) is a static equipment used for the AC transmission of electrical energy. It
is meant to enhance controllability and increase power transfer capability
Q2) what are the benefits of fact devices?
1 FASTER THAN CONVENTIONAL SOLUTION:
Electronic based switches use in FACTS device have less switching time than the conventional mechanical switches. FACTS
Devices using electronic based switches are more flexible and fast reacting causes many advantages like enhancement of
transmission capacity, control of power flow, improvement of transient stability, voltage stability and control. With using of
appropriate type and rating of FACT devices transmission capacity enhance up to 40 – 50 %.
2. IMPROVE TRANSMISSION SYSTEM RELIABILITY:
One of the things to be considered is that Fact devices cannot prevent faults in our power system instead they can only limit
the impact of fault and prevent fault from entering the healthy section of our system.
For example, when a major load rejection occurs in our system. It results in an increase in voltage of the line which ultimately
leads to tripping of line. To counteract this over voltage problem and to avoid tripping of more lines, Fact devices such as
Static Var compensator (SVC) or Statcom can be used.
3. IMPROVE TRANSIENT STABILITY OF THE SYSTEM:
Instability increases in our system whenever sudden load changes occurs, power outages, opening or closing of circuit breakers
or various equipment’s connected in our system. This can lead to reduction in power flow or even line can trip. Fact devices
improves the transient stability of our system and reduce the risks of line trips
4. IMPROVE POWER QUALITY:
Electricity which is reaching to the consumer should be fluctuation free, should have constant voltage and frequency. There
are many industries which heavily rely on constant supply having constant voltage and frequency and the variation of these
parameters can lead to interruptions in their manufacturing processes which results in huge economic losses. Fact devices can
be a handy solution to overcome this problem.
Q3) what are the advantages of fact devices over capacitor bank?
Although Reactive Power Compensation Can Be Achieved By Shunt Capacitor but they have many disadvantages.

Capacitor BANKS has no effect on the system during dynamics condition such as post fault voltage recovery

The reactive power produced by a capacitor bank is in direct proportion to the square of its terminal voltage, and if the
system voltage decreases, the capacitors produce less reactive power, when it is most needed, while if the system voltage
increases the capacitors produce more reactive power, which exacerbates the problem

When load levels are high, a shunt capacitor system is beneficial. But when the load drops. Capacitor can do more harm
than good. An excess of capacitance in system leads to higher voltage than desired voltages during less load conditions.
C=Q/ V

Capacitor banks usually have COUPLE of large step size so they don't provide particularly precise control. Whereas on
the other end fact devices can supply the exact amount of VARs needed.

There are harmonics generation associated with capacitor while switching. In fact they are harmonics generator. Moreover,
switching reduces capacitor lifetime. Transients are associated with capacitor switching.
Q4) what are the types of facts controller?
TYPES OF FACTS CONTROLLERS:
FACTS controllers are classified into four categories, which include series controllers, shunt controllers, series series
controllers, and series-shunt controllers.
 SERIES CONTROLLER: These Controllers consists of capacitors or reactors. They are basically variable impedance
devices whose reactive or capacitive impedance can be changed to damp various oscillations that can take place in our
power system. This is done by introducing an appropriate voltage in series with the line. If the line voltage is in phase with
the line current, than series controller absorbs or produces reactive power, while if line voltage is not in phase with the line
current, than controllers absorb or generate real and reactive power.
Examples of Series controllers are Static Synchronous Series Compensator (SSSC), Thyristor-Switched Series Capacitor
(TSSC). Series controllers control current, power flow in the system and damps various oscillations occurring in the system

SHUNT CONTROLLER: Shunt controllers are used to introduce current into the system at the point where they are
connected. If the current injected is in phase with the line voltage, the controller adjusts reactive power while if the current
is out of phase then the controller adjusts real power. Examples of shunt controllers are STATCOM, TSR, TSC, and SVC.
They can be effectively used to control the voltage by introducing active or reactive current into the system.
 SHUNT-SERIES CONTROLLER: In Shunt -series controller the shunt component injects a current into the system
while the series component injects a series voltage. Example is unified power flow controller( UPFC)
 SERIES-SERIES CONTROLLER: Series-series controller can be connected in two ways. In one way we can connect
series controllers which will operate in a coordinated manner in a multiple-line transmission system. Second way provides
separate reactive power and real power control for each line of a multiple-line transmission system. Example of seriesseries controller is Interline Power Flow Controller (IPFC), which helps in balancing both the real and reactive power
flows on the lines
Q5) what are the steady state applications of fact devices?
1- WHEN LOW VOLTAGE AT HEAVY LOAD OCCURS:
To overcome this problem we have to supply reactive power and the conventional solution to overcome it was to add Shunt
capacitor. But with the invention of fact devices such as SVC, TCSC, STATCOM, this problem can be solved much faster as
compared to conventional solution.
2-WHEN HIGH VOLTAGE OCCURS:
To overcome this problem we have to absorb reactive power and the conventional solution to overcome it was to add Shunt
reactor. Fact devices such as SVC and STATCOM can be used to solve this problem
3-WHEN LINE OR TRANSFORMER OVERLOADING OCCURS:
To overcome this problem we have to reduce overload and the conventional solution to overcome it was to add another line
or transformer or add series reactor. Fact devices such as TCSC, UPFC can be used to solve this problem.
Q6) what are the dynamic applications of fact devices?
TRANSIENT STABILITY:
When a fault occurs in our system the electrical power transmitted by the generator decreases very much while the mechanical
input power to the generator remains constant and as a result the generator continuously accelerates. During the fault, the
generator terminals experiences voltage sag of higher value. This voltage is not recovered even though after the fault clearance
due to lack of reactive power,
When fact devices such as statcom is connected to the system, it adjusts its firing angle according to the system requirements.
Firing angle is that angle at which statcom is on or it is triggered. During normal operating condition, the firing angle remains
zero and Statcom does not supply reactive power to the system. When the fault occurs firing angle changes instantly and
reactive power is exchanged between the system and statcom. Firing angle returns to zero again when the fault is cleared.
By connecting Statcom to the system rotor speed and power angle remains at their normal operating values even during fault
condition. Voltage sag at the generator terminal is reduced. Shaft oscillations and torsion forces will be reduced to almost the
normal operating value
2- DAMPING:
Oscillations exists in our system due to electromechanical dynamics of machines and are called as electromechanical
oscillations. To reduce these oscillations one solution is to use power system stabilizer which would damp generator rotor
oscillations but the problem is that because of them great variation in voltage profile exists. Other solution is using fact devices
such a SVC, STATCOM, TCSC, UPFC etc. which would not only damp these oscillations but also maintain steady state
voltage of the system.
3- POST CONTINGENCY VOLTAGE CONTROL:
In power systems, a contingency is when an element of the electric grid fails. The element that fails could be a generator,
transmission line, substation, transformer, etc. and power systems engineers perform contingency analyses to see the effect of
a particular element failing
In case of faults, the equilibrium of the system usually gets disturbed and if the system does not operate within constraints i.e.
voltage magnitude, the current and power flow over the lines are not in suitable ranges than system is said to be in emergency
condition. To bring back system voltage in its normal operating ranges, fact devices such as SVC, STATCOM, UPFC, and
TCSC can be used. They are used for dynamic (fast changing) voltage support
Q7) comparison of svc vs statcom?
SVC VS STATCOM



Although SVC fulfils the same task as STATCOM, The ability to provide more reactive power during fault situations is
one of the most important advantages of the STATCOM over the SVC, which helps for faster recovery of the system
during faults situations.
STATCOM normally exhibit a faster response than SVC, because of the voltage source converter technology
Due to lower space requirements, low switching frequencies, and better performance, STATCOM is expected to be a better
economical option.
Q8) disadvantages of fact device?
Issues related to fact devices are:
 They have high cost.
 Procurement of fact device is another major issue. Market for SVC is widely developed and can be procured easily. While,
very limited competition exists regarding the procurement of TCSC and STATCOM and very few companies manufacture
UPFC and IPFC.
 Their appropriate placement in the network is also an important issue. Secondly the coordination of multiple fact devices
in the network is also a major concern
Q9) what are the advantages of fact device over synchronous condenser?
Q10) CONCLUSION:
Power supply industry throughout the world is undergoing massive changes. As the demand of electricity is increasing, power
supply companies are finding new ways to meet supply and demand of electricity. Facts have proven to be a good solution to
this problem.
These devices are required when there is a need to respond to dynamic (fast-changing) network conditions. Although the
conventional solutions are normally less expensive than FACTS devices, but problem is they are very limited in their dynamic
behavior. Most importantly usually one fact device can solve multiple problems, which would otherwise need to be solved by
several different conventional solutions. In conclusion fact devices are more flexible than most of the conventional solution.
TOPIC 32: CIRCULATING CURRENT
Consider two power sources having different voltages connected in parallel and that parallel combination is then connected to
a load. It seems that Kirchhoff's Voltage Law, which states that the voltage across all parallel branches of a circuit must be the
same, is violated. But it is not.
This is where Circulating Current come into the picture. KVL is satisfied by the presence of Circulating Current.
Circulating current flows in the loop shown above in such a manner that it discharges the power source having the greater
voltage and charges the voltage source having the smaller voltage till the voltage level of the two sources becomes equal, at
which point the circulating current stops flowing. This is the case when the power sources are batteries.
When the power sources are generators, instead of charging/discharging as in the case of batteries, the generator having greater
voltage feeds power to the generator having lower voltage causing the latter to act as a motor.
In general, in a circuit, circulating current are undesirable because the circulating current constitutes a component of power
that is generated but not utilized by the load. This power is instead wasted as heat in the internal resistance of the power sources.
On the other hand, circulating current helps in parallel operation of generators/alternators by ensuring load is shared by the
generators in proportion to their KVA rating and that synchronization with the bus bar is maintained when there is a change in
load or in the prime mover input of any one generator.
The circulating current in a transformer is a current flowing internally which is there when the transformer energized.
TOPIC 33: STAR DELTA STARTER
Q1) why do we use starter?
Most induction motors are started directly on line, but when very large motors are started that way, they cause a disturbance
of voltage on the supply lines due to large starting current surges.
To limit the starting current surge, large induction motors are started at reduced voltage and then have full supply voltage
reconnected when they run up to near rotated speed.
During starting the motor windings are connected in star configuration and this reduces the voltage across each winding 3.
This also reduces the torque by a factor of three.
After a period of time the winding are reconfigured as delta and the motor runs normally. Star/Delta starters are probably
the most common reduced voltage starters. They are used in an attempt to reduce the start current applied to the motor
during start as a means of reducing the disturbances and interference on the electrical supply. Traditionally in many supply
regions, there has been a requirement to fit a reduced voltage starter on all motors greater than 5HP (4KW) Since the Torque
is Proportional to Square of voltage. A high starting torque is not possible.
Q2) what is star delta starter?
During starting (from zero to slightly less than rated speed), motor current depends mainly on the supply voltage. More the
supply voltage, more is the starting current. When connected in star, starting current is approximately 1/3 of the starting current
in delta connection.
If changeover from star to delta is done too early (at too low speeds), motor current remains high for larger time duration (from
the instant of changeover till the motor speed increases close to rated speed)
If changeover from star to delta is done too late, motor current remains high unnecessary after reaching speed close to the rated
speed.
Hence changeover from delta to star has to be done as soon as the speed increases close to rated speed. This can be done by
using timer and setting it to the time equal to time required to reach rated speed.
Simple to make the automatic switching from star into delta. Generally, this switching will happen when the motor cross
beyond 75%–85% of the rated speed
Q3) what are the components of star delta starter?
The Star/Delta starter is manufactured from three contactors, a timer and a thermal overload. The currents through the
winding are 1/root 3 (58%) of the current in the line.
There are two contactors that are close during run, often referred to as the main contractor and the delta contactor. These
are AC3 rated at 58% of the current rating of the motor. The third contactor is the star contactor and that only carries star
current while the motor is connected in star.
The current in star is one third of the current in delta, so this contactor can be AC3 rated at one third (33%) of the motor
rating.
Q4) states of star delta starter?
Q5) working of star delta starter?
The ON push button starts the circuit by initially energizing Star Contactor Coil (KM1) of star circuit and Timer Coil (KT)
circuit. When Star Contactor Coil (KM1) energized, Star Main and Auxiliary contactor change its position from NO to NC.
When Star Auxiliary Contactor (1) (which is placed on Main Contactor coil circuit ) become NO to NC it’s complete The
Circuit of Main contactor Coil (KM3) so Main Contactor Coil energized and Main Contactor’s Main and Auxiliary
Contactor Change its Position from NO to NC. This sequence happens in a friction of time.
After pushing the ON push button switch, the auxiliary contact of the main contactor coil (2) which is connected in parallel
across the ON push button will become NO to NC, thereby providing a latch to hold the main contactor coil activated which
eventually maintains the control circuit active even after releasing the ON push button switch.
When Star Main Contactor (KM1) close its connect Motor connects on STAR and it’s connected in STAR until Time Delay
Auxiliary contact KT (3) become NC to NO.
Once the time delay is reached its specified Time, the timer’s auxiliary contacts (KT)(3) in Star Coil circuit will change it s
position from NC to NO and at the Same Time Auxiliary contactor (KT) in Delta Coil Circuit(4) change its Position from
NO To NC so Delta coil energized and Delta Main Contactor becomes NO To NC. Now Motor terminal connection change
from star to delta connection.
A normally close auxiliary contact from both star and delta contactors (5&6)are also placed opposite of both star and delta
contactor coils, these interlock contacts serves as safety switches to prevent simultaneous activation of both star and delta
contactor coils, so that one cannot be activated without the other deactivated first. Thus, the delta contactor coil cannot be
active when the star contactor coil is active, and similarly, the star contactor coil cannot also be active while the delta
contactor coil is active.
The control circuit above also provides two interrupting contacts to shut down the motor. The OFF push button switch
break the control circuit and the motor when necessary. The thermal overload contact is a protective device which
automatically opens the STOP Control circuit in case when motor overload current is detected by the thermal overload
relay, this is to prevent burning of the motor in case of excessive load beyond the rated capacity of the motor is detected by
the thermal overload relay.
At some point during starting it is necessary to change from a star connected winding to a delta connected winding. Power
and control circuits can be arranged to this in one of two ways – open transition or closed transition.
TOPIC 34: PARTS OF TRANSFORMER

CORE
The core acts as support to the winding in the transformer. It also provides a low reluctance path to the flow of magnetic
flux. It is made of laminated soft iron core in order to reduce eddy current loss and Hysteresis loss. The composition of a
transformer core depends on such as factors voltage, current, and frequency.
WHY ARE WINDINGS MADE OF COPPER?
Copper has high conductivity. This minimizes losses as well as the amount of copper needed for the winding (volume & weight
of winding).
Copper has high ductility. This means it is easy to bend conductors into tight windings around the transformer's core, thus
minimizing the amount of copper needed as well as the overall volume of the winding.

WINDING
Two sets of winding are made over the transformer core and are insulated from each other. Winding consists of several turns
of copper conductors bundled together, and connected in series.
Winding can be classified in two different ways:
. Based on the input and output supply
. Based on the voltage range
Within the input/output supply classification, winding are further categorized:
. PRIMARY WINDING - These are the winding to which the input voltage is applied.
. SECONDARY WINDING - These are the winding to which the output voltage is applied.
Within the voltage range classification, winding are further categorized:
. HIGH VOLTAGE WINDING - It is made of copper conductor. The number of turns made shall be the multiple of the
number of turns in the low voltage winding. The conductor used will be thinner than that of the low voltage winding.
. LOW VOLTAGE WINDING - It consists of fewer number of turns than the high voltage winding. It is made of thick copper
conductors. This is because the current in the low voltage winding is higher than that of high voltage winding.
Input supply to the transformers can be applied from either low voltage (LV) or high voltage (HV) winding based on the
requirement.

INSULATING MATERIAL:
Insulating paper and cardboard are used in transformers to isolate primary and secondary winding from each other and from
the transformer core.
Transformer oil is another insulating material. Transformer oil performs two important functions: in addition to insulating
function, it can also cool the core and coil assembly. The transformer's core and winding must be completely immersed in
the oil. Normally, hydrocarbon mineral oils are used as transformer oil. Oil contamination is a serious problem because
contamination robs the oil of its dielectric properties and renders it useless as an insulating medium.

CONSERVATOR
The Conservator tank is a tank fitted above the level of top cover of Oil Filled transformer to allow for expansion and
compression of the Oil. As goes its name, it CONSERVES the amount of oil filled in the transformer, allowing for expansion
and contraction.
Used As Reserve Tank for transformer oil to provide availability of oil.
used for cooling purpose of oil , heated oil flow in upward direction and reaches to conservator hence temperature of oil
decreased.
When transformer is loaded and when ambient temperature rises, the volume of oil inside transformer increases.
A conservator tank of transformer provides adequate space to this expanded transformer oil
 BREATHER
The breather controls the moisture level in the transformer. Moisture can arise when temperature variations cause expansion
and contraction of the insulating oil, which then causes the pressure to change inside the conservator. Pressure changes are
balanced by a flow of atmospheric air in and out of the conservator, which is how moisture can enter the system.
If the insulating oil encounters moisture, it can affect the paper insulation or may even lead to internal faults. Therefore, it is
necessary that the air entering the tank is moisture-free.
The transformer's breather is a cylindrical container that is filled with silica gel. When the atmospheric air passes through the
silica gel of the breather, the air's moisture is absorbed by the silica crystals. The breather acts like an air filter for the
transformer and controls the moisture level inside a transformer. It is connected to the end of breather pipe.

TAP CHANGER
The output voltage of transformers vary according to its input voltage and the load. During loaded conditions, the voltage on
the output terminal decreases, whereas during off-load conditions the output voltage increases. In order to balance the
voltage variations, tap changers are used. Tap changers can be either on-load tap changers or off-load tap changers. In an onload tap changer, the tapping can be changed without isolating the transformer from the supply. In an off-load tap changer, it
is done after disconnecting the transformer. Automatic tap changers are also available.

COOLING TUBES
Cooling tubes are used to cool the transformer oil. The transformer oil is circulated through the cooling tubes. The circulation
of the oil may either be natural or forced. In natural circulation, when the temperature of the oil rises the hot oil naturally rises
to the top and the cold oil sinks downward. Thus the oil naturally circulates through the tubes. In forced circulation, an external
pump is used to circulate the oil.

BUCHHOLZ RELAY
The Buchholz Relay is a protective device container housed over the connecting pipe from the main tank to the conservator
tank. It is used to sense the faults occurring inside the transformer. It is a simple relay that is operated by the gases emitted
during the decomposition of transformer oil during internal faults. It helps in sensing and protecting the transformer from
internal faults.

EXPLOSION VENT
The explosion vent is used to expel boiling oil in the transformer during heavy internal faults in order to avoid the explosion
of the transformer. During heavy faults, the oil rushes out of the vent. The level of the explosion vent is normally maintained
above the level of the conservatory tank.

BUSHINGS:
You know that transformer has 2 windings inside the core. And you need a connection from these two windings to outside for
further use. This is where Bushing comes in to play. Bushing is an insulated device that allows an electrical conductor to pass
safely
through
an
(usually)
earthed
conducting
barrier
wall
of
a
transformer.
This Bushing is made up of Porcelain material
They are basically insulator. The wavy shape is to maximize surface path length and minimize surface leakage, corona, and
eventual arcing from
exposure
to
year-round
weather
conditions,
dust,
air
pollution
etc.
If continuous shape is given, when the rain falls over the surface, water particles crawl down over the bushing and cause short
circuit.
Transformer Bushings are specially designed electrical terminals for taking out transformer winding ends or leads through the
openings provided on the top cover of the of transformer tank thus facilitating connections to the incoming and outgoing lines
If we are not using any Bushing then there will be chance of Direct Arcing between the line and the body due to breakdown
of the medium.
TOPIC 35: DC MACHINES
Q1) Can I identify a motor or generator by just seeing it?
Yes, you can identify whether it’s a motor or a generator. In terms of design of the machine, there is no difference between a
motor and a generator. Difference lies in how a machine is connected and controlled.
IDENTIFICATION TECHNIQUE OF A GENERATOR:
If you see a prime mover (such as turbine, steam engine, and another motor) connected to shaft, it’s a generator.
IDENTIFICATION TECHNIQUE OF A MOTOR:
Whereas if you see a mechanical load (such as pump, fan) connected to the shaft, then its most probably a motor
Q2) Working principle of dc motor?
The working principle of dc motor is “Whenever a current carrying conductor is placed in a magnetic field, it
experiences a mechanical force”. The direction of this force is given by Fleming’s left hand rule.
When armature windings are connected to dc supply, current sets up in the winding. Magnetic field may be produced by
field winding (by electromagnetic induction) or by using a permanent magnet. In this case, current carrying armature
conductor experiences force due to magnetic field.
Q3) Classification of dc machines?
Every dc machine can work as a motor or a generator. Hence this classification is valid for both. Dc machines are classified
on the basis of field excitation method. This makes two categories:
1) SEPERATELY EXCITED: field winding is fed by some external source
2) SELF EXCITED: field magnet winding is supplied current from the output of the generator itself is called a selfexcited generator
A) SERIES WOUND : Field winding is connected in series with armature
B) SHUNT WOUND : Field winding is connected in parallel with armature
C) COMPOUND WOUND:
• SHORT SHUNT in which only shunt field winding is in parallel with the armature winding.
• LONG SHUNT in which shunt field winding is in parallel with both field and armature winding.
Q4) what is the difference between dc motor and dc generator?
ANS) Difference between dc motor and dc generator is that, in dc motor emf is created across its terminal by an external
source where as in dc generator, emf is created across its terminals by principals of dynamically induced emf
Q5) What is the working principle of motors and what is armature reaction?
WORKING PRINCIPLE OF MOTOR:
Whenever a current carrying conductor is placed in a magnetic field it produces a turning or twisting movement called torque
ARMATURE REACTION:
The effect of armature flux on main flux is called as armature reaction. Armature flux may suppose or oppose main flux
TOPIC 36: RCCB AND MCB
Q1) what is RCCB and its purpose?
ANS) RCCB is essentially a current sensing device which is used to protect low voltage circuits in case of faults. It contains
a switch that switches off whenever a fault occurs in a circuit.
Purpose of using RCCB is to protect an individual from the risk of electrical shocks, electrocutions and fires that are causes
due to faulty wiring or earth fault.
They are particularly useful in situations when there is a sudden earth faults occurring in the circuit for e.g. A person
accidently comes in contact with an open live wire in circuit. In such situations, in the absence of RCCB in the circuit, an
earth fault may occur and the person is at the risk of receiving an electrical shock.
However if the same circuit is protected by an RCCB, it will trip the circuit in fraction of second, thus preventing the person
from receiving an electrical shock. Therefore, it is a good and safe practice to install RCCB in your circuit.
Q2) how does RCCB works?
ANS) The working principle of RCCB is simple. During ideal situations, the current flowing in the circuit through live wires
should be same as the current from the neutral. The live wire and the neutral wire are wound around iron cores in opposite
directions. When the appliance is working correctly all the electrical current entering the appliance via the live wire leaves
the appliance through the neutral wire and the magnetic fields generated around the iron cores cancel out.
In case of earth fault, current finds a passage to earth through accidental means (such as accidental contact of an open wire to
earth). As a result the returning current from neutral is reduced. This difference in the current is called as residual current.
RCCB is designed in such a way that it continuously senses and compares the difference in current (residual current)
between live and neutral wires. Any small change in the current values on account of such event would trigger the RCCB to
trip off the current.
Q3) What are the types of RCCB’s?
There are two basic types of RCCB’s:
1) 2 pole RCCB: They are used in case of single phase supply that involves only a live and neutral wire. It contains two
ends where the live and neutral wires are connected. A rotary switch is used to switch the RCCB back to on or off
position. A test button helps to periodically test the rccb in functionality
2) 4 pole RCCB: They are used in case of three phase supply that involves 3 phase wires and one neutral wires. Its
operation is similar to 2 pole RCCB
RCCB which are available in the market are of 30mA, 100mA, 300mA etc.
Q4) how sensitive RCCB is?
Ans) RCCB are primarily designed for protection against earth fault and its consequences to human life such as electric
shock.
As per studies, a person is able to sustain electrical shock only to the magnitude up to 30mA.Thus RCCB is designed such a
way that it will trip off the circuit even for small change in residual current value upto 30m A. Its response time is usually
within 40msec.
Q5) what is the difference between MCB and RCCB?
Ans) In simple words, Function of MCB (miniature circuit breaker) is to protect the circuit wires from any damage, whereas
RCCB protects the life of an individual from electrical shocks.
RCCB in particular detects faults on electrical leakage current and trips to prevent any electrical shock. In addition they acts
as the main disconnecting switch upstream of any derived mcb.
MCCB provides protection against overload. Overload means an excess current drawn which will lead to the tripping the
circuit breaker. Once the current exceeds 5 times the rated MCB current value within a tenth of a second.
When your electric load is fault and there is a short circuit, a lot of current is drawn and mcb trips. But an electric shock to an
human being may not always draw an excessive current that is so high to trip MCB and protect you. This is where RCCB
comes in
RCCB has both live and neutral wire. Connection is like this. (live wire from RCCB to load neutral wire from load to
rccb). So it forms a close loop. MCB has only the live wire in it and the return doesn’t go to MCB. So, RCCB forms a series
close circuit. In a series circuit, current remains same. So the current that comes in from the live wire of RCCB is the current
that goes back to the neutral.
This is true until a human being or any other conductive device comes in contact with the wire. The current that comes in
isn’t the same current as the current that goes back,
since some current is going through you .RCCB has a mechanism
that trips when there is imbalance in current in and current out.
Q6) what is the difference between over loading and over current?
ANS) Over current is simply the short circuit where the current increases from its allowed value whereas over loading means
increasing the number of loads connected to the system which would increase the current and produces heating effect in the
system. Over loading can produce Over current but generally Overload protection is protection against overheating
Q7) Does RCCB protects against over loading?
Ans) Rccb will protect against a shock but won’t protect against overload because even in an over load, same current flows
through live and neutral. RCCB doesn’t care if the magnitude is high or low. It only works if some of the current is leaking
out somewhere.
So if you need an MCB+RCCB function. Go for RCBO (residual current breaker with overload protection.
TOPIC 37: LOSSES IN TRANSFORMER
Q1) what are the losses in transformers?
In any electrical machine, 'loss' can be defined as the difference between input power and output power. An electrical
transformer is a static device, hence mechanical losses (like windage or friction losses) are absent in it. A transformer only
consists of electrical losses (iron losses and copper losses).
1)
A)
B)
2)
core loss(fixed for given frequency and given material of core)
hysteresis loss
eddy current loss
copper loss(variable with load current)
Efficiency= OUTPUT/INPUT = OUTPUT/INPUT+Pi+Pcu)
Q2) what are copper losses? Why they are called variable losses?
Copper loss is the term often given to heat produced by electrical currents in the conductors of transformer windings, or other
electrical devices.
When the transformer is loaded, current flows in primary and secondary winding, there is loss of electrical energy due to the
resistance of the primary winding, and secondary winding and they are called variable losses. These losses depend upon the
loading conditions of the transformers. Copper loss is solely dependent on the primary and secondary currents, which are
dependent on loading on transformer. As load on transformer is not constant, copper loss is called as variable loss.
These losses are present both in the primary and secondary windings of the transformer and depend upon the load attached
across the secondary windings since the current varies with the variation in the load, so these are variable losses.
Q3) how do transformers reduce copper losses (I2R)?
The windings of the transformer are made thick so that the resistances are minimized. Increasing the cross-sectional area of
the conductor, improving the winding technique, and using materials with higher electrical conductivities, such as copper).
Q4) what are core losses in transformer and why they are called fixed losses?
Core loss occur due to the core of transformer .as core of transformer is made up of ferromagnetic material so these losses
occur. The iron loss is also called as constant loss as it is dependent on frequency and maximum flux density, which remain
constant. Hysteresis loss depends upon both voltage and frequency, while the eddy current loss mainly depends upon
voltage. As for transformers, normally we keep the supply voltage and frequency constant in regards to maintain the stability
and reliability.
It is known that for a transformer,
V = 4.44 f Φm N = 4.44 f Bm A N
Where
A = area
Bm α (V/f)
.. .
.......... For constant A and N
Thus as voltage changes, the maximum flux density changes and both eddy current and hysteresis losses also changes.
As voltage increases, the maximum flux density in the core increases and total iron loss increases.
As frequency increases, the flux density in the core decreases but as the iron loss is directly proportional to the
frequency hence effect of increased frequency is to increase the iron losses.
Q5) what is eddy current loss?
In transformer we provide alternating current in primary which produces alternating flux in core, this flux links with
secondary of transformer and induces emf in secondary. It may be possible that flux also links with some other conducting
parts of transformer like ferromagnetic core or iron body and induces local emf in these parts of transformer which will
cause a circulating current to flow in these parts causing heat loss. These currents are called eddy current and this loss is
called eddy current loss. Like any current they have associated magnetic flux. It is this flux which opposes the generating
flux so causing energy loss in the circuit producing the original generating flux.
Q6) how eddy current losses can be reduced?
In order to reduce the eddy current loss, the resistance of the core should be increased.
In devices like transformers, the core is made up of thin sheets of steel, each lamination being insulated from others by a
thin layer of varnish.
As the laminations are thin, they will have relatively high resistance
Large resistances between the sheets confines the eddy currents to the thin sheets. Each lamination sheet will have an
eddy current circulates within it. Tiny eddy currents still exist, but only within each thin sheet, so are greatly reduced.
The sum of individual eddy current of all the laminations are very less compared to that of using single solid iron core.
Q7) what are hysteresis loss in transformer?
We all know that the Ferro magnetic materials are made up of magnetic domains, when this material is placed in the
absence of magnetic field the magnetic domains are aligned in random directions, but when it is subjected in to uniform
magnetic field the magnetic domains are aligned in the direction of magnetic field.
Every magnetic material has some domains or small magnetic parts consisting of plus and minus poles in it. In normal
condition, these domains remains in an arbitrary position. But when subjected to a magnetic field, these domains comes parallel
with the magnetic lines of forces (flux). When again the magnetic source is removed the domains goes back to their same
arbitrary state.
Now, as the transformer works in alternating current, there are reversals of every cycles. So, the domains also changes
their positions w.r.t the cycle. Due to the frequent changing of the positions, there is some certain amount of work done.
For this reason, there is some amount of electrical energy wasted. This wasted energy is known as Hysteresis Loss. We
cannot completely eliminate the hysteresis losses but we can minimize the hysteresis losses by choosing the low hysteresis
coefficient material.
Q8) how do I reduce hysteresis loss in transformer?
Hysteresis losses taking place in transformer are directly proportional to area of hysteresis loop of the material which is used
for manufacturing of transformer core.
Hence hysteresis loss can be reduced by using material having least hysteresis loop area.
Hence silicon steel or high grade steel is used for manufacturing of transformer core as it is having very less hysteresis loop
area.
Q9) why copper is not used in core of the transformer?
You need the core to have a high permeability for magnetic flux. This ensures that all the magnetic flux lines are channeled
through the core and not through the dielectric.
The reason for that is quite simple. The transfer of power in a transformer depends on the amount of flux that entirely passes
through the core, instead of completing its path through the dielectric. If that happens, the core is either saturated, or the
material is not of enough permeability to ensure the proper completion of the magnetic circuit.
Copper cannot be used because of the low permeability it offers to magnetic flux lines
Q10) why is iron chosen as the material for the core of the transformer? Why don't we use aluminum?
A material with higher permeability and less conductivity is chosen for the transformer core.
High permeability so that it can easily allow the magnetic field through it and low conductivity so that the strength of Eddy
current is less and hence losses are less.
Such materials are iron , steel (steel has better permeability than iron) , it is ferromagnetic material and have high permeability,
but it also have good conductivity hence to decrease the conductivity 4- 5 % of silicon is added to it , hence the material is
called silicon steel.
Aluminium doesn't have these magnetic properties hence aluminium is not used , aluminium is paramagnetic material ,it has
good conductivity which is not needed in transformer core since then Eddy current losses will be more .
TOPIC 38: OPEN AND SHORT CIRCUIT TEST IN TRANSFORMER
Q1) what is open circuit test in transformer?
The connection diagram for open circuit test on transformer is shown in the figure. A voltmeter, wattmeter, and an ammeter
are connected in LV side of the transformer as shown. The voltage at rated frequency is applied to that LV side with the help
of a variac of variable ratio auto transformer. The HV side of the transformer is kept open. Now with the help of variac, applied
voltage gets slowly increased until the voltmeter gives reading equal to the rated voltage of the LV side. After reaching at rated
LV side voltage, all three instruments reading (Voltmeter, Ammeter and Wattmeter readings) are recorded.
The ammeter reading gives the no load current Ie. As no load current Ie is quite small compared to rated current of the
transformer, the voltage drops due to this current that can be taken as negligible. Since, voltmeter reading V1 can be considered
equal to secondary induced voltage of the transformer, the input power during test is indicated by watt-meter reading. As the
transformer is open circuited, there is no output, hence the input power here consists of core losses in transformer and copper
loss in transformer during no load condition. But as said earlier, the no load current in the transformer is quite small compared
to full load current, so copper loss due to the small no load current can be neglected. Hence, the wattmeter reading can be taken
as equal to core losses in transformer.
Q2) what is short circuit test in transformer and how it is done?
The connection diagram for short circuit test on transformer is shown in the figure. A voltmeter, wattmeter, and an ammeter
are connected in HV side of the transformer as shown. The voltage at rated frequency is applied to that HV side with the help
of a variac of variable ratio auto transformer. The LV side of the transformer is short circuited. Now with the help of variac
applied voltage is slowly increased until the ammeter gives reading equal to the rated current of the HV side. After reaching at
rated current of HV side, all three instruments reading (Voltmeter, Ammeter and Watt-meter readings) are recorded. The
ammeter reading gives the primary equivalent of full load current IL. As the voltage applied for full load current in short circuit
test on transformer is quite small compared to the rated primary voltage of the transformer, the core losses in transformer can
be taken as negligible here.
Let’s say, voltmeter reading is Vsc. The input power during test is indicated by watt-meter reading. As the transformer is
short circuited, there is no output; hence the input power here consists of copper losses in transformer. Since, the applied
voltage Vsc is short circuit voltage in the transformer and hence it is quite small compared to rated voltage, so core loss due
to the small applied voltage can be neglected. Hence the wattmeter reading can be taken as equal to copper losses in
transformer
Q3) Why is a short-circuit test, in the case of transformers, done with the high-voltage side only?
The high voltage side has lower rated current. The SC test requires rated current to flow through the winding. As for very
high rating transformers the difference in rated current of HV and LV side could be huge, it is easier to conduct test at lower
currents.
Consider a 1 MVA 415V/ 11,000 V transformer
If you were to perform SC test on LV side the current would be (1 x 10^6)/ (1.732 x 415) = 1391.2 A
Whereas if you were to perform the Test on HV side the Current Would be (1 x 10^6)/ (1.732 x 11000) = 52.48 A
As you can see the Current on the HV side is much lower so SC test is conducted on the HV side S.C
The Problem with LV side is that for test purposes getting a source for delivering 1391 A is very difficult.
Just imagine the size of variac needed to deliver the same. It will be huge Uneconomical & Impractical
Q4) why are open-circuit tests in transformers done from the LV side?
First keep in mind, open circuit and short circuit test can be conducted at any side. But it's better to use convenient side for
test due to following advantages.
1. For LV side, rated voltage required will be less compare to High voltage side. So low range voltmeter and
wattmeter sufficient to conduct the test.
2. In no load test, no load current is only 4 to 6 percentage of full load current, so for LV side rated current is high
and no load current which only 4 or 6 percentage will be high compare to no load current in case of high voltage
winding. So that's why no load current can be accurately measured.
Its main reason is that open circuit test is conducted to find the core/iron loss of a transformer (neglecting copper loss). If you
conduct this test on the high voltage side you are to supply a high voltage and hence it draws more power. In this case the
copper losses is to be taken into account for and the wattmeter doesn’t gives the iron loss but a sum of iron loss and copper
loss.
TOPIC 39: ZONES OF PROTECTION
Q1) what is zone of protection?
Protection zone is defined as the part of the power system which is protected by a certain protective scheme. It is established
around each power system equipment. When the fault occurs on any of the protection zones then only the circuit breakers
within that zone will be opened. Thus, only the faulty element will be isolated without disturbing the rest of the system.
The circuit breakers are placed at the appropriate points such that any element of the entire power system can be disconnected
for repairing work, usual operation and maintenance requirements and also under abnormal conditions like short circuits. Thus
a protective covering is provided around rich element of the system.
Q2) why zones of protection should be overlapping?
If there were no overlapping in the protective zone, then the failure occurs in the equipment will not lie in any one of the zones
and hence no circuit breaker would be tripped. The fault occurs in the unprotective system will damage the equipment and
hence disturb the continuity of the supply.
The overlapping of protective zones is done to ensure complete safety of each and every element of the system. The zone
which is unprotected is called dead spot. The zones are overlapped and hence there is no chance of existence of a dead spot in
a system.
If there are no overlaps, then dead spot may exist, means the circuit breakers lying within the zone may not trip even though
the fault occurs. This may cause damage to the healthy system.
Q3) what is primary and backup protection?
The protection provided by the protective relaying equipment can be categorized into two types as:
a) Primary protection
b) Back-up protection
The primary protection is the first line of defense and is responsible to protect all the power system elements from all the types
of faults. The backup protection comes into play only when the primary protection fails.
In the event of failure or non-availability of the Primary Protection some other means of ensuring that the fault is isolated must
be provided. These secondary systems are referred to as Back-up Protection.
Q4) Does primary and backup relay detect fault at different time?
No they both detect fault at the same time but since we have inserted delay in backup relay that is why it doesn’t works first
and it will work if primary relay fails to operate.
Q5) why relay at the farthest end is fastest in radial system?
Because it does not have to work as a backup for any other relay
Q6) what is the difference between contactor and a relay?
Contactors and Relays perform the same task of switching a circuit. If you see from the application point of view, you would
have seen contactors placed in control panels of industrial motors or other heavy loads. Whereas, relays are used in low
voltage applications such as switching a LED or tube-light or even actuating a circuit breaker.
Both operate under similar principles. The difference comes if we see from the application perspective. Contactors are used
for high voltage switching purposes whereas relays are used for low voltage switching.
Relays are used in circuits with lower ampacity. (Max 20A) whereas the Contactors are used in circuits with low and higher
ampacity up to 12500A
TOPIC 40: FEEDER AND TRANSMISSION LINE
Q1) what is the difference between feeder and transmission line?
1) In electric power distribution, voltage power line transferring power from a distribution substation to the distribution
transformers is called feeder. To connect the consumer/load end with the substation, we have feeders. There are no tapping
taken out of them. They just connect the consumer area with the substation. Whereas transmission line connects the
generating station to grid station.
2) Transmission line is normally starts from 80 km long to 240 km. Normally feeder 4 to 8 km long and rated 11 kV.
3) Feeder is the source point from where you are receiving. It is like a tap point for collecting water if considered as an
analogy. Whereas transmission line is the conductor that we pave for transmitting power…which is like pipe line for the
flow of water.
4) In essence, A Transmission line is used in reference to a transmission system/substation and a feeder with respect to a
distribution system/substation.
5) A transmission line is called a feeder whenever it directly feeds consumer. Examples are of decentralized power or of
distributed generation where the generation itself takes place at the load end.
Q2) what is tapings and why no tapings is taken from feeder?
Tapping refers to connecting a couple of wires to take power to the premises of each consumer along the route. Where
consumers take current from a power line, there will be a drop in remaining current as you progress away from the power
station. It's saying that feeders don't have consumers tapping on along the route; feeders carry power between networks, and it
is the local network that is responsible for distributing that power to consumers.
TOPIC 41: DISTANCE RELAY ZONES
Q1) why protection of transmission line is important and in how many zones distance protection is divided?
The protection of transmission line from any type of fault is very important because any mis-operation or mal operation of
protection relays can give a great effect on the stability of power system entirely. One of the main protection used to protect
transmission line is distance or impedance relay. It uses the impedance measurement technique to measure the apparent
impedance as seen by the relay at the relaying point. The inputs for distance relay are three phase current and voltage phasors
during fault occurrence. Transmission line is divided into several zones of protection normally zone 1, zone 2 and zone 3
.Distance relay acts as main protection for faults within zone 1 while for zone 2 and zone 3, it acts as backup protection for
adjacent line.
Q2) what is zone 1 reach and why it is kept at 80 % of line?
Zone-1 is meant for protection of the primary line. Typically, it is set to cover 80% of the line length. Zone-1 provides fastest
protection because there is no intentional time delay associated with it. Operating time of Zone-1 can be of the order of 1 cycle.
Zone 1 reach is normally set only up to 80% - 90% of the protected line. It is not set 100% of the protected line to avoid relay
from under reached or over reached due to current and voltage measurement errors, transient effect and inaccuracy in
transmission line parameters
Zone 1 does not cover the entire length of the primary line because it is difficult to distinguish between faults which are close
to bus B like fault at F1, F2, F3 and F4. In other words, if a fault is close to bus, one cannot ascertain if it is on the primary
line, bus or on back up line.
CTs and PTs have limited accuracy. During fault, a CT may partially or complete saturate. The resulting errors in
measurement of impedance seen by relay, makes it difficult to determine fault location at the boundary of lines very
accurately.
2. An accurate apparent impedance measurement during fault occurrence by distance relay is very important because false
measurement might result in delayed tripping signal sent by distance relay. There are several factors which can lead to
inaccurate apparent impedance measurement such as high fault resistance, mutual inductance of parallel line, line charging
capacitance and transient effects due to switching of Flexible Alternating Current Transmission System (FACTS) devices.
3. Fault resistance can be high or low depending on the nature of fault. Even a small fault resistance value can make the relay
to be under reached when it is used to protect short transmission line. The relay also might be under reached when the fault
is near to remote substation terminal. Delayed tripping of circuit breaker due to under reached of distance relay will make
the power system in stress for a longer time.
1.
Hence a relay which is supposed to measure the fault impedance may see an impedance more than that of the actual impedance
or it may see an impedance less than the actual impedance. In otherwise the relay may under reach or it may over reach. Hence
to take care of the situation and to avoid the relay overreach (to avoid the relay to see the fault in the next line from the
substation) an allowance is usually made in the setting. Hence if it is intend to protect a line in between two stations, the setting
is usually done to cover 70-80 percent of the line
Q3) why do we keep the setting of Zone-2 150%?
The Zone 1 protection is set to cover 70 to 80 % of the line to be protected. The zone 1 reach is set to this level to avoid any
overreach of the relay due to the error introduced by the CT or in general, measurement. Hence the remaining 20 to 30 % of
the line is still left unprotected. This portion is covered by another Zone called Zone 2. To avoid any error in reach
measurement, the zone 2 is set to cover the remaining 20 t0 30 % plus say 50% of the next shortest line from the remote
substation. As the Zone 2 is set to cover ideally some portion of the next line also. Hence if we give an instantaneous operation
for the Zone 2 , It may operate for an actual fault at the next line which should have cleared by the Distance relay and associated
CB at that remote station, To avoid such an indiscriminative tripping the Zone 2 is always with time delay typically say 400
sec
Q4) why zone 2 setting is not kept more than 150%?
Zone-2 setting in Distance Relay is kept at 150 % to avoid Overlap Problem. See the picture below.
As clear from the picture above, if the reach of Zone-2 of a relay R1 is extended too much, then it can overlap with the Zone2 of the relay R3. Under such a situation, there exists following conflict. If the fault is on line BC (and in Z2 of R3), relay R3
should get the first opportunity to clear the fault. Unfortunately, now both R1 and R3 compete to clear the fault. This means
that Z2 of the relay R1 has to be further slowed down. As Zone-2 protection already have a time delay, due to overlapping we
need to further introduce some time delay which will degrade the performance of Relay for Zone-2.
Hence, a conscious effort is made to avoid overlaps of Z2 of relay R1 and R3. Setting Zone-2 of R1 to maximum of 150% of
primary line impedance or primary line impedance plus 50% of smallest line impedance usually works out good compromise
without getting into Z2 overlap problem.
Q4) what about zone 3?
Backup protection for entire adjacent line is covered by zone 3 reach. It is normally set at least 1.2 times the impedance of
protected adjacent line. The set tripping time for zone 3 reach is typically several seconds
Q5) what is under reach and over reach of relay?
Under reach:
A distance relay is said to under reach when the fault impedance seen by relay is more than the relay setting impedance value
even though the fault point is within the protected zone of line. This means that reach of relay has decreased from the setting
value. If it is not able to detect the fault within its defined reach and detects only up to a lesser (nearer) point then it is said to
have under-reached.
Under Reach can be best illustrated by the figure below. Figure shows an uncompensated transmission line having an
impedance of Z and a distance relay.
A line to ground fault take place at point F. The impedance measured by distance relay is ZF and the setting of relay is Z (say).
In some cases, it may happen that line impedance ZF measured by relay due to fault at F may exceed the setting value Z i.e.
ZF > Z. What will happen then? The relay will not actuate to clear the fault even though point F is within the protected zone.
This scenario is called under reach of relay. Why it is called under reach?
Reach of distance relay is defined as line protection covered by relay in terms of line length or impedance. In the example, the
reach of relay is Z as it is expected to protect the entire line. But due to absence of residual compensation, the relay is not only
able to protect the whole line rather it is protecting only a part of line say 90% of line. This means the effective reach of relay
has decreased. This is why; we say relay is under reaching.
OVER REACH:
A distance relay is said to over reach when the fault impedance seen by relay less than the relay impedance setting.. It is when
a relay over reaches beyond its pre-set setting (the distance up to which it should protect the line) it is said to have over-reached
CAUSE OF OVER REACH
This phenomena is mostly observed in double circuit line. In double circuit line, when one of the line is taken out of service
and earthed ad both ends, then due to mutual coupling between the in-service line and out of service line, the impedance
measurement of distance relay of in-service line do not remain accurate.
Thus whenever one line of double circuit line is taken out and earthed for maintenance or other activity, distance relay setting
is reduced by this over reach percentage i.e. 16.4% by some authorities. In other words, distance relay setting is reduced to
85.6 %. After normalization of line, the setting is again changed to 100 %
TOPIC 42: DIRECTIONAL OVER CURRENT RELAY
Q1) why do we need directional over current relay?
Q2) what is Protection of Radial Feeders?
The main characteristic of a radial system is that power flow only in one direction, i.e. from the generator or the supply end
to the load end. It has the drawback that continuity of supply cannot be controlled at the load end in the occurrence of a fault.
In a radial system when the number of feeders is connected in series as shown in the figure. It is desired that the smallest
possible part of the system should be off. This is conveniently achieved by employing time graded protection. The over
current system should be adjusted in such a way that the longer the relay from the generating station the lesser the time of
operation.
When the fault occurs on the SS4, the relay OC5 should operate first and not any other i.e. the time require to operate the
relay OC4 must be less than the time required for relay OC3 and so on. This shows that the time setting required for these
relays must be properly graded. The minimum interval of time which can be allowed for the two adjacent circuit breaker
depends on its own clearance time, plus a small time for the safety margin.
With normal circuit breaker in use minimum, the discriminating time between adjustment breaker should be about 0.4
seconds. The time settings for relay OC1, OC2, OC3, OC4, and OC5 will be 0.2 seconds, 1.5 seconds, 1.5 seconds, 1.0
seconds, 0.5 second and instantaneous respectively. Along with the grading system, it is also essential that the time of
operation for the severe fault should be less. This can be done by using time limiting fuse in parallel with the trip coils.
Q3) Protection of Parallel Feeders?
The parallel connection of the supply is mainly used for the continuity of the supply and for sharing the load. When the fault
occurs on the protective feeder, the protective device will select and isolate the defective feeder while the other instantly
assume the increased load.
One of the simplest methods for the protection of the relay is the time graded overload relay with inverse time characteristic
at the sending end and instantaneous reverse power or directional relays at the receiving end as shown in the figure below.
When the heavy fault
F occur on any one of the lines, then the power is fed into fault from the sending end as well as from the receiving end of the
line. The direction of power flow will be reversed through the relay on D, which will be open.
The excess current is then restricted to B until its overload relay operates and trips the circuit breaker, thus completely
isolating the faulty feeder and supplying power through the healthy feeder. This method is only satisfactory when the fault is
heavy and reverse the power at D. Hence differential protection is also added along with the overloaded protection at both
the end of the line.
Q4) Protection of Ring Main System?
The ring main is a system of interconnection between a series of the power station by a different route. In the main ring
system, the direction of power can be changed at will, particularly when the interconnection is used.
The elementary diagram of such a system is shown in the figure below where G is the generating station, and A, B, C, and D
are substation. At the generating station, the power flow only in one direction and hence no time lag overload relays is used.
The time grade overload relay is placed at the end of the substation, and it will trip only when overload flows away from the
substation which they protect.
Going round the ring in the direction GABCD the relay on the further side of each station are set with decreasing time lags.
At generating station 2 seconds at station A, B, C and 1.5 seconds, 1.0 second, 0.5 second and instantaneous respectively.
Similarly going round the ring in the opposite direction the relay on the outgoing sides would be set as follows.
If the fault occurs at point F, the power F is fed into the fault through two paths ABF and DCF. The relay to operate is that
between substation B and fault point F and substation C and fault point F. Thus the fault on any section will cause the relay
on that section to operate, and the healthy section will be operating uninterruptedly.
Q5) Why transmission system in ring main and distribution system in radial form?
TOPIC 43: WHY INDUCTION MOTOR RUNS ON LAGGING POWER FACTOR
Induction motors are widely used in commercial and as well as residential application but have you ever wondered that why
they always operate on lagging power factor? Why not leading? To understand this concept, first look at some basics.
Power factor is given by cosine (Theta V – Theta I).
1) If the angle between voltage and current is zero degrees or both are in phase with each other than it is a purely
resistive circuit.
2) If current lags voltage by 90 degrees than it is a purely inductive circuit
3) If current leads voltage by 90 degrees than it is a purely capacitive circuit.
In induction motor we have windings over its stator and windings has its own resistance and inductance. Therefore
theoretically induction motor will always run at lagging power factor.
Now next question one might ponder is that then how any machines say synchronous machine would be able to operate at
any other power factor besides lagging?
Synchronous machines unlike induction machine have coils and circuitry on the rotor which can be directly controlled to
adjust the magnetic field. This control over the magnetic field makes it possible to operate the synchronous machine at any
power factor
What can be done to make the induction machine run at any other power factor?
However if capacitors are added in parallel to the machine terminals, the grid do not see the same impedance it once saw
when only induction machine was connected. This combined impedance of the motor and the capacitor is reflected to the
grid and it changes the phase angle of the current w.r.t. to voltage when it is drawn from the utility voltage source and hence
changes the net power factor measured by the utility meter. It should be noted here that the induction machine itself is still
operating at the lagging power factor in this case, but the utility company or the grid won't be able to sense it.
OR
If you take a look at the equivalent circuit of an induction motor you will find the rotor resistance to be R'/s and the rotor
reactance to be X' ( the parameters are referred to the primary )
Impedance Z= R'/s + jX'
So at starting slip =1, which means the rotor impedance will be Zstart = R'/1 + X' = R'+ jX'
When the machine is running the slip would be less than 1 which means s< 1
So at the running period, the impedance will be
Zrun= R'/s + jX'
Now since the slip is less than 1, the effective impedence has now increased.
Which means Zstart< Zrun
Since the starting impedence is less than the impedence when in running period.
The current drawn will be greater at the time of start. (I inversely proportional to Z
High inrush currents drawn by induction motor during starting can result in large dip in connected bus voltages. This dip in
bus voltages can impact the performance of other motors operating on the bus. Voltage dips during starting of large motors
can trip some of the motors operating on the same bus
TOPIC 44: SYNCHRONOUS VS INDUCTION PART 2
1) What will happen when the prime mover is removed from the synchronous generator at the time of running?
Here are the various possibilities for a synchronous generator connected to grid when Prime mover is accidently decoupled.
The mode depends on whether the field winding is excited.
1. Field Excitation is available (Field ON): Machine will work as a Synchronous Motor rotating in the same direction.
2. Field Excitation is failed (Field OFF)

Cylindrical rotor machine: work as Induction Motor running in same direction with speed less than
synchronous speed.
 Salient pole machine: work as Reluctance Motor running in same direction at synchronous speed.
Synchronous generator is connected to infinite bus bar and operated under steady state condition. If prime mover is
removed under running condition then machine will be act as synchronous motor and absorbed active power from bus
bar.
2) What is the difference between synchronous speed, rated speed and full load speed of an induction machine?
Synchronous speed: In an induction motor the synchronous speed is the speed at which the magnetic field rotates in the
stator and rotor periphery and it is the highest speed among all three. But, in an induction motor the rotor never runs on the
synchronous speed it runs below this speed.
Rated speed: When the induction motor is supplied by the ideal input values of voltage and current for which it is designed
for then, the speed at which the motor is running at these rated values of voltage and current is known as rated speed. The
induction motor gives maximum efficiency at rated speed.
Full Load Speed: it is speed of rotor at full load. It may or may not less than rated speed, according to load. If motor is
getting overload then its rpm will start reducing as motor is unable to generate required torque
3) How real power of a generator is controlled using prime mover while speed of rotor should be synchronous
speed?
It is true that the generator rotor has to maintain a constant speed as it is designed to produce a certain voltage at desired
frequency. However, the real power demand is variable and at any instant, it is the duty of the generator to match (supply) this
real power demand.
To increase real power supply to meet an increased demand, with a constant speed requirement, the only other parameter to be
varied is the toque (Torque times speed is power).
To realize torque increase, imagine a merry-go-round turned manually by an operator. When there are no kids in it, the operator
puts in minimal effort to push it clockwise, say 10 rounds in a minute. Imagine a few kids hop in to the merry-go-round, and
the operator still wants to maintain the 10 rounds/minute speed. The operator will have no choice but to push harder. You will
see him maintain the same speed, except he's putting a lot more effort, losing a few extra calories if he wants to keep the same
speed with heavier loads in his merry-go-round. What he did was apply greater force. And force times the perpendicular
distance from the point of his hand to the center of merry-go-round equals torque. So he basically increased torque input as
mentioned earlier. This obviously means he's consuming more energy, as the rate of doing work increased (More Torque times
speed equals more power).
In the generator case, the same should apply. In the man's case extra force (which translated to more torque) came in the form
of extra calories. The generator will have to do the same with increased fuel intake to increase prime mover torque. This could
be more rapid steam inlet to steam turbine, thereby burning more coal faster. This also satisfies law of conservation of energy,
because the real power demand increase must mean that the generating plant's fuel intake should increase.
This is achieved by using a device/ controller named AVR (Automatic Voltage Regulator). AVR will take a feedback from
the RPM of generator, system voltage and adjust inputs to prime mover to maintain power system voltage and frequency.
4) What is a prime mover for an electrical generator?
The actual meaning of prime mover is a primary source of power. A prime mover can be anything that provides mechanical
input to a machine.
It can even be you if you rotate the machine by yourself and give mechanical input you the machine.
The prime mover is the machine that turns the generator. This statement apples to both AC and DC machines. For example,
the AC synchronous generator at a power plant will be driven / turned / spun by the prime mover.
Examples of prime movers include:




water wheels
steam turbines
windmills
diesel engines
5) How does the prime mover of a generator rotate by the utilization of fuel?
That’s rather what they are designed to do.
Almost invariably they burn something and that causes gasses to expand, and that expansion moves pistons or turbine blades,
which we have coupled to a shaft in such a way that it rotates.
The chemical energy released by combustion raises the temperature of the gas, which by Boyle’s law tries to change volume.
In doing so it must do work against whatever is constraining it: the pistons or the turbine blades. The work done is transferred
to the rotating shaft, and hence to the generator.
6) If prime mover rotates in opposite direction to the generator then what will happen?
The phase sequence of the power generated will change. Phase difference between phases remain the same.
When a generator is connected to bus bar synchronized with other generators, a change in phase sequence of a generator
(among those connected to that bus bar) will cause the generator(whose phase sequence is changed) to fall out of step and
synchronization will be lost. Same phase sequence is an important criteria for a generator to achieve synchronization with
other generators.
As long as the rotor is being given mechanical and electrical input of the same magnitude, the generator power will not increase
or decrease.
Suppose R-Y-B is the phase sequence in normal operation then after rotating of prime mover in opposite direction phase
sequence would be R-B-Y. Because magnetic field of rotor poles will be firstly cut by R winding then B and then Y.
7) What will happen when I change the excitation in synchronous motor or generator?
Synchronous Motor or Generator can run in both form.
1. Over excitation or
2. Under excitation.
When run as alternator alone:
Over and under excitation will increase or decrease the terminal voltage.
When run as alternator connected with the grid or infinite BUS
1. Over excitation will deliver additional inductive VAR to the grid and will operate at lagging power factor
2. Under excitation will absorb reactive power from the grid and will operate at leading power factor.
When run as Synchronous Motor and connected with the system:
1. Over excitation will deliver additional leading VAR to the system and will operate at leading power factor
2. Under excitation will operate at lagging power factor and will increase inductive VAR of the system.
8) Can synchronous motor run at speed more than synchronous speed?
The main idea of calling a synchronous motor by this name is because it runs at synchronous speed. The characteristics of this
motor are:


It runs only at synchronous speed or not at all.
Only way to change its speed is by changing the supply frequency
 The principle of its operation is that the stator and rotor poles get locked together and hence rotor rotates at the same
speed as the stator field.

However, in case of an induction machine, it can be made to rotate at speeds greater than its synchronous speed. In
this case, the machine works as an induction generator.
The whole idea of calling a motor or generator a synchronous one is based on the fact that it runs on synchronous speed. If it
is not running at that speed then it is not a synchronous motor or generator.
9) What happens when DC excitation of a synchronous motor fails? Will it be rotating in synchronous speed?
Synchronous machine = induction machine + dc excitation
Suppose if any case your dc excitation fails than synchronous motor lost its synchronism and rotates a speed less than the
synchronous speed like induction motor.
10) Does synchronous generator rotate at synchronous speed? What happens to speed when alternator is loaded?
Synchronous alternator connected to load and load can vary at any time.
Synchronous alternator connected to input power thru governor who keeps speed of rotation constant and fixed under all
load conditions.
Change in speed will change the frequency.
Any significant change in frequency will affect badly the entire load system.
Governor take care of variation in speed due load variation.
11) What happens when synchronous motor operates on no load?
A synchronous motor running on no-load with leading power factor will act as synchronous condenser. This is not truly a noload operation since the motor, while behaving as a condenser is actually feeding a part of reactive load of the system to which
it is connected.
The same motor when operated with lagging power factor on no-load will draw a reactive current from the system depending
upon the system voltage.
12) What is field current and armature current of synchronous generator?
By definition field current is the one that produces magnetic field required by the machine, and armature current is the current
induced in the windings by the created mmf. Based on this, DC current on the rotor is field current, and induced AC current in
the stator is armature current.
13) What happens when generator loses synchronism?
When a generator loses synchronism, two things happen:
The rotor speed decreases/increases owing to the change in mechanical energy supplied by the prime mover.
The stator, which is connected to the grid, is essentially an inductor consisting of many coils. Hence, the changes in voltage
induced in various parts of the coil does not change instantaneously, in step with the rotor movement. Therefore, to maintain
the constant magnetic flux, it starts drawing power from the grid.
With the above changes to the stator and rotor, one can clearly deduce that the generator is now drawing power from the grid,
attempting to create a synchronism between stator rotating magnetic field and rotor i.e. Motor Action. Hence, in short,
whenever a synchronous generator loses its synchronism, it behaves like a synchronous motor load, consuming power from
the grid.
This is primarily why generators are isolated from the grid via circuit breakers as soon as they go out of synchronism.
14) Why the frequency of a synchronous generator is kept same at the time of synchronizing to the system?
There are lots of other generators running in the system. All those generators run at same frequency and potential to provide
hassle free power transmission…
In Simple words, AC power follows Sine wave which rises and drops 50times every second). If the generator is closed with
system without syncing Frequency and Voltage, there will be abnormalities occur in system or in the generator.
Closing breaker without frequency in sync may cause sudden variation in speed of generator. This sudden variation is due to
generators frequency suddenly moves from its own to frequency of the system. This impact could damage rotor shaft, bearings
or supporting structure. A high variation could instantly cause generator protect relays to open breaker or trip the generator.
Abnormalities may be Reverse flow of current from system to generator for fraction of second which in stabilizes voltage
across the bus bar resulting in blown fuses, Generator trip, breaker sparking, etc.
So it’s important to sync the generator with system to ensure smooth flow of current across the circuit.
15) What happen if after synchronization, the generator field supply is varied?
Assuming the generator is synchronized with the electrical grid and not directly controlling things like voltage and frequency
on the grid, the effect of raising or lowering the field supply voltage from the voltage regulator (aka ‘exciter’) would be to
raise or lower the VARs supplied by the generator to the grid. This can serve to buck or boost the system voltage, at least in
the local area.
Raising or lowering the voltage too far, and assuming too much VAR loading can significantly raise the temperature in the
stator windings. Typically the operators will be receiving (numerous) alarms well before that point, and there *should* be
protective relaying trips implemented to avoid serious damage. If the field voltage were to be lowered too far, it is possible to
lose the synchronous lock between the rotating magnetic fields in the rotor and the stator, causing what is referred to as ‘pole
slippage’, which usually results in massive localized current spikes and generally damage/destruction of the windings.
16) What would happen if the excitation of a synchronous generator is suddenly turned off while it remains connected
to the turbine?
The generator tries to maintain its current state. It operates as an induction generator. This means that it will draw reactive
power from the grid into which it is connected and operate at a speed above the synchronous value. As a rule of thumb,
whenever excitation fails in a synchronous machine, the machine loses synchronism. There is also a difference in power factor
of the machine. As a result large emf may get induced on rotor teeth’s which may cause damage due to overheating but
immediate stoppage of alternator is not needed until voltage collapse below a certain limit. Typically there are protection
schemes called 'Loss of excitation schemes' designed to trip the generator out of the system in such situations. Also, as an
alternator typically isn't designed to operate as an induction machine, the windings on these machines heat up and deform.
17) What is a loss of excitation in synchronous machine?
When field excitation to the synchronous generator is lost then synchronous generator operates as induction generator and
instead of delivering reactive power it absorbs the reactive power from the system as much as 2 to 4 times the generator's rated
load.
However the real power (MW output) delivered by the induction generator will remain almost the same as this is controlled
by the prime mover.
But loss of generator's reactive power in the system may cause instability to the system.
Also, as alternator is operating as induction generator, high currents are induced in to the rotor teeth and wedges and may
damage the rotor of the generator. However large alternators are designed to withstand this induced currents.
Immediate tripping of alternator in the case of loss of excitation is not necessary unless the terminal voltage of the generator
falls below desired limit due to voltage collapse.
Consider a Sync generator connected to a grid. When the excitation fails the power delivering capacity of machine suddenly
reduces. So the mechanical power input is greater than electrical power output. This imbalance will accelerate the rotor and
force it to run above sync speed. The circulating currents and then magnetic field are created. The machine will start to generate
power by induction.
TOPIC 45: LIMIT SWITCH
1)
What is a limit switch
Mechanical limit switches are contact sensing devices widely used for detecting the presence or position of objects in industrial
applications. As an object (or target) makes contact with the actuator of the switch, it eventually moves it to the “limit” where
the electrical contacts change state. Through this mechanical action, electrical contacts are either opened (in a normally closed
circuit) or closed (in a normally open circuit)
When installed in a machine system, a limit switch can usually start, stop, slow down, or accelerate operations, as well as
activate a forward or reverse process. Counting, detecting speed and hundreds of other application. In order to perform these
actions, limit switches are designed in a variety of shapes, sizes, and capacity ranges to accommodate differences in machine
systems and production processes.
Just like in given example when container hits the limit switch then it will be filled.
2)
Construction of limit switch
Actuator/Operating Head
The actuator is the part of the switch which physically comes in contact with the target. In some limit switches, the actuator
is attached to an operating head which translates a rotary, linear, or perpendicular motion to open or close the electrical
contacts of the switch.
Switch Body
The component containing the electrical contact mechanism.
Receptacle/Terminals
The component containing the terminal screws or screw/clamp assembly necessary for wiring purposes. While there are a
number of different styles of limit switches available in the market today, this manual will describe two classes of limit
switches — standard industrial oiltight and precision switches.
3)
Working of limit switch?
Working Principle:
It is just like a SPDT (Single Pole Double Throw) switch which has 3 contacts i.e COM (Common), NO, NC.
A Single Pole Double Throw (SPDT) switch is a switch that only has a single input and can connect to and switch between
2 outputs. This means it has one input terminal and two output terminals.
NO is the Normally Open contact (open if the plunger is free), NC is the Normally Closed contact (closed if the plunger
is free) and COM is the common.
NO normally will be open with respect to COM while NC will be shorted with COM. When the object hit the plunger, NO
will be shorted with the COM while NC will be open with respect to COM.
When plunger is not pressed then C and NC are connected and NO disconnected.
When plunger is pressed C and NO connects and NC disconnects.
4)
Limit Switches in the Home?
Open the refrigerator and the light comes on. Close the door and the light should go off. That small light switch is actually a
limit switch at work. It limits the time the light is on so it is not operating when the door is closed
An example of a limit switch is the switch that detects the open position of a car door, automatically activating the cabin light
when the door is opened
5)
What is difference between actuator and sensors?
Both are the energy transformer means.
In general way they characterize as
Sensor: Non-electrical quantity (input) → Electrical quantity (output)
Example- Pressure sensor, microphone, digital thermometer etc.
Sensors sense - that is to say they act as the eyes and ears of your device, and detect changes in the environment around them,
providing information for a processor, MCU or other system to react to.
Whereas Actuating means inducing a movement. A synonym to the word “actuator” is a “mover”. The prime function of the
actuator is to convert the signals of different form into mechanical action. Convert them into motion. Here are some
practical actuator applications you might recognize: Cell phones: When you switch it to vibrate, it's the mini-actuator
inside your phone moving rapidly back and forth that creates the buzzing sound that alerts you. In today's car you can find
over 100 actuators that help execute a variety of functions including, steering, braking, adjusting the side mirrors, repositioning
your seat and inciting the windshield wipers into action.
TOPIC 46: DCS
Q1) what is control system?
A control systems has two words Control and Systems. System is when a number of elements or components are connected
in a sequence to perform a specific function, the group thus formed is called as system. Examples: Human Beings, Planet,
animals.
A Control System is an arrangement of physical components connected in a proper manner to produce a desired output.
A control system is a system which provides the desired response by controlling the output. Everything is a control system.
Broadly a system incorporating a sensing element, a decision making element and a final control element with communication
links is a control system .For example : when you touch(process) hot vessel you hand(sensor skin) moves back (final control
by muscles ) where mind is the controller .
Similarly, our eyes close automatically when the amount of light that falls on to it increases. This is because our retina can
only take a certain amount of light inside. Your eyes sensed that it cannot further bear the light. So it closes. Therefore, all
parts of our body have inbuilt control mechanism.
Q2) Difference between centralized and distributed control system?
Distributed means scattered. Centralized means at one point. In every plant, there are different units and every area/ every unit
has its own process going on and for example if you have three areas in one plant and you put one controller there, one
centralized plc there, if that controller fails than all areas connected with it will be stopped. So to sort out that problem we have
distributed control system.
CENTRALIZED CONTROL SYSTEM:
DISTRIBUTED CONTROL SYSTEM:
Distributed control system is a control system in which the control is distributed throughout the system. It differs from the
centralized control system wherein a single controller at central location handles the control function, but in DCS each process
element or machine or group of machines is controlled by a dedicated controller. Like in this example every area has its own
controller. Suppose if area 1 goes down that it will not affect area 2. The main advantage of dividing control tasks for
distributed controllers is that if any part of DCS fails, the plant can continue to operate irrespective of failed section.
DCS consists of a large number of local controllers in various sections of plant control area and are connected via a high speed
communication network.
Q3) working of DCS system?
In this diagram we have different areas and each area has its own controller. Area 1 has its own controller. Area 2 has its own
controller. Area. These all controllers are on the network of fiber. Local communication is handled by a control network with
transmission over twisted pair, coaxial, or fiber optic cable and fiber is connected to particular switch there and that switch is
connected to LAN network and in that LAN network we have operating station, engineering station, and servers. Not only one
controller in one area but you have redundant controllers like in area 1 you have one main controller and one redundant
controller such that if main controller fails so redundant can take place. This is the beauty of DCS system. In case both case
than that particular area will shut down. There is not one controller controlling but multiple controller and they all are connected
in fiber ring and you have two cables here. One for main and one redundant.
Distributed individual automatic controllers are connected to field devices such as sensors and actuators. DCS uses set point
to control flow of process. It is controlling a process or a plant by a set point. Set point is the desired process value of the
system. DCS itself sense a set point for some pressure the pressure measured by transmitter is sent back to controller and now
controller will read it. When the measured variable reaches a certain point, the controller instructs a valve or actuation device
to open or close until the process reaches the desired set point.
Here pressure transmitter is set to 300psi. If pressure gets below then 300psi. Like 280 so it will send signal to controller.
Controller will take action and then will instruct a device to bring back pressure to its set point value
Q4) what are the substations/ components of DCS system?
There are four important components of DCS system.
1) Operator Station or HMI.
2) Engineering Station
3) Distributed Controller or Local Control Unit:
4) Communication System.
OPERATOR STATION or HMI:
This is used to operate, monitor and control plant parameters. It can be a PC or any other monitoring device on which operator
can view process parameter values and accordingly to take control action. We can see different process parameter coming from
field such as temperature, pressure. Operator cannot only see that but also can control that from operator station. For instance,
it is a DigiVis software tool that can run on a simple PC-environment in case ABB DCS.
Operating stations can be a single unit or multiple units where a single unit performs functions like parameter value display,
trend display, alarming, etc. while multiple units or PCs performs individual functions such as some PCs display parameters,
some for trend archives, some for data logging and acquiring, etc.
ENGINEERING STATION:
Engineering stations gives the flexibility to implement the system to meet project and process needs. This engineering station
offers powerful configuration tools that allow the user to perform engineering functions such as creating new loops, creating
various input and output points, modifying sequential and continuous control logic, configuring various distributed devices,
preparing documentation for each input/output device, etc. It can also use for creating new projects, new IO logics
Where we are programming all the logic in DCs. It can be a PC or any other computer that has dedicated engineering software
(for example, control builder F engineering station in case of ABB freelance distributed control system).
DISTRIBUTED CONTROLLER OR LOCAL CONTROL UNIT:
It can be placed near to field devices (sensors and actuators) or certain location where these field devices are connected via
communication link. It receives the instructions from the engineering station like set point and other parameters and directly
controls field devices. It collects the information from discrete field devices and sends this information to operating and
engineering stations.
In above figure AC 700F and AC 800Fcontrollers acts as communication interface between field devices and engineering
station. Most of the cases these act as local control for field instruments.
COMMUNICATION SYSTEM:
The communication medium plays a major role in the entire distributed control system. It interconnects the engineering station,
operating station, process station and smart devices with one another. It carries the information from one station to another.
The common communication protocols used in DCS include Ethernet, Profibus, Foundation Field Bus, DeviceNet, Modbus,
etc.
Communication media consists of transmission cables to transmit the data such as coaxial cables, copper wires, fiber optic
cables and sometimes it might be wireless. Communication protocols selected depends on the number of devices to be
connected to this network.
It is not mandatory to use one protocol for entire DCS, some levels can use one network whereas some levels use different
network. For instance, consider that field devices, distributed I/Os and process station are interconnected with Profibus while
the communication among engineering station, HMI and process station carried though Ethernet as shown in the figure below
.
Q5) What is the difference between PLC and DCS?
PLC are used to control the particular operation in a plant —but —DCS is used to control the entire Plant
DCS is used when PLC’s are not sufficient to control the entire automation process (more number of inputs and outputs)
PLC can handle IOs in 100s. Where DCS is having more no of IO handling capacity in 1000s. DCS is generally used in big
process industries like refinery, Oil Gas refinery, Food or Chemical industries. DCS are an advanced or a larger implementation
of the PLC
Q6) what is the difference between DCS and SCADA?
SCADA is a software which is used for monitoring process devices using PLC/DCS and any other controller, to display
information, log data and show alarms. DCS is like as computerized control system which is distributed in levels of control.
DCS control the process parameter by sending the signal to plant actuator, control valve, solenoid valve and many other
controlling equipment.
DCS and SCADA are monitoring and control mechanisms that are used in industrial installations to keep track and control
of the processes and equipment; to ensure that everything goes smoothly, and none of the equipment work outside the
specified limits. The most significant difference between the two is their general design. DCS, or Data Control System,
is process oriented, as it focuses more on the processes in each step of the operation.
DCS is process state driven, while SCADA is event driven. DCS does all its tasks in a sequential manner, and events are
not recorded until it is scanned by the station. In contrast, SCADA is event driven. It does not call scans on a regular
basis, but waits for an event or for a change in value in one component to trigger certain actions.
Hence, DCS does not keep a database of process parameter values as it always in connection with its data source, whereas
SCADA maintains a database to log the parameter values which can be further retrieved for operator display and this
makes the SCADA to present the last recorded values if the base station unable to get the new values from a remote
location.
In terms of applications, DCS is used for installations within a confined area, like a single plant or factory and for a complex
control processes. Some of the application areas of DCS include chemical plants, power generating stations, pharmaceutical
manufacturing, oil and gas industries, etc. On the other hand, SCADA is used for much larger geographical locations such as
water management systems, power transmission and distribution control, transport applications and small manufacturing and
process industries
DCS can be deployed for the supervision of a certain process, however if a number of DCS systems at different locations are
interconnected to a master station, they form a SCADA system.
TOPIC 47: PID CONTROLLER
Q1) what are controlled and uncontrolled systems?
Before explaining PID Controller, let's revise about Control System. There are two types of systems; open loop system and
close loop system. An open loop system is also known as an uncontrolled system and close loop system is known as
a controlled system. In open loop system, the output is not controlled because this system has no feedback and in a close loop
system, the output is controlled with the help of controller and this system requires one or more feedback paths. An open loop
system is very simple but not useful in industrial control applications because this system is uncontrolled. Close loop system
is complex but most useful for industrial application, because in this system output can be stable at a desired value, PID is an
example of Closed Loop System. Block diagram of this systems is as shown in below figure-1.
A close loop system is also known as feedback control system and this type of system is used to design automatically stable
system at desired output or reference. For this reason, it generates an error signal. Error signal e(t) is a difference between the
output y(t) and the reference signal u(t). When this error is zero that means desired output is achieved and in this condition
output is same as a reference signal.
For example, a dryer is running for a several times, which is pre-set value. When dryer is turned ON, timer starts and it will
run until timer ends and give output (dry cloth). This is a simple open loop system, where output is need not to control and not
require any feedback path. If in this system, we used a moisture sensor which provide feedback path and compare this with set
point and generates an error. Dryer runs until this error is zero. It means when moisture of cloth is same as set point, dryer will
stop working. In open loop system, dryer will run for fixed time irrespective of clothes are dry or wet. But in close loop
system, dryer will not run for fixed time, it will run until clothes are dry. This is the advantage of close loop system and use
of controller.
Q2) what is the working of PID controller?
The ultimate goal of a control system is to make a certain physical value (like temperature, position, velocity … etc.) reach a
desired value and to stay at that value even if it changes.
In any control systems, two pieces of information should be known, the desired value of the system AND the current value of
the system. We want the current value to reach the desired value which means we want to narrow the gap between these two
values. This gap is called the Error.
Imagine you are driving a car, trying to reach and maintain speed of 50 kilometers per hour.
- You watch the difference (error) between your speed and the desired speed.
- You press more or less on the gas pedal (control output).
The PID control has three parts (rules): P, I, D. Each contributes to the control output
1. P - proportional
The less you have, the more you push.
The farther you are from the desired speed, the more you press the gas pedal; the closer you are, the less you press
it. This works well, but has a small accuracy problem: When you get at the desired speed, according to this rule
you let off the gas pedal completely. The end result is that your car slows down and stays a little below the desired
speed of 50 (static error).
Proportional control is the main ingredient of any control. May be a little inaccurate.
In PID controller, P mode will make the system to how much the control output must increase for decreasing the error at
present but it still has a steady state error
2. I - integral
You wait a little, if no improvement you push a little more.
If you are stuck below the desired speed for a long time without progress, you push the gas pedal a little
further. If you still do not make it to the desired speed for some time, you again push the pedal a little further
down. Once you get to the desired speed you leave the pedal where it is.
Integral control gives you accuracy (zero static error) but you have to wait.
we can get rid of this steady state error by adding an integral(I) mode. But the problem with Integral mode was it will integrate
each and every error of the past but it will try to make steady state error zero after long time. Integral mode will cause some
oscillations in a system because it needs some time to integrate past errors like for positive/negative errors, error will integrate
and integrate and it will take some time to even after crossing the set point and it will take a desired action to reach the steady
state
3. D - derivative
You react to sudden changes.
A strong wind gust pushes your car. Suddenly your speed surges fast upward toward 50. Startled you release
the gas pedal. As the speed surge ends and the speed stabilizes, you return the pedal to where it was.
Derivative control manages sudden surges and may prevent overshooting your target speed.
Derivative mode as it will become PID controller. By adding Derivative (D) mode, it will try to analyze the future
behavior of the system situation and according to that it will try to make the pinpoint (output variable) equals to the set
point(SP) in less time so for analyzing the future situation it will take the rate of change in error with respect to time so
it won’t take much time to decrease the error and actually oscillations/overshoots occurs due to errors so indirectly it was
damping the oscillations of the system
response.
Q3) what is the applications of PID controller?
Proportional-Integral-Derivative (PID) controllers are used in most automatic process control applications in industry
today to regulate flow, temperature, pressure, level, and many other industrial process variables.
MANUAL CONTROL
Without a PID controller, manual control of water temperature is a tedious process. For example, to keep a constant temperature
of water discharged from an industrial gas-fired heater, an operator has to watch a temperature gauge and adjust a fuel gas
valve accordingly (Figure 1). If the water temperature becomes too high, the operator has to close the gas valve just enough to
bring the temperature back to the desired value. If the water gets too cold, he has to open the gas valve.
The control task done by the operator is called feedback control, because the operator changes the firing rate based on feedback
from the process via the temperature gauge. The operator, valve, process, and temperature gauge form a control loop. Any
change the operator makes to the gas valve affects the temperature, which is fed back to the operator, thereby closing the loop.
AUTOMATIC CONTROL
To automate temperature control with a PID controller, the following are required:



Install an electronic temperature measurement device
Automate the valve by adding an actuator (and perhaps a positioner) so it can be driven electronically
Install a controller and connect it to the temperature measurement device and automated control valve
The operator sets the PID controller’s set point (SP) to the desired temperature, and the controller’s output (CO) sets the
position of the control valve. The temperature measurement, called the process variable (PV), is then transmitted to the PID
controller, which compares it to the set point and calculates the difference, or error (E), between the two signals. Based on the
error and the controller’s tuning constants, the controller calculates the appropriate controller output to set the control valve at
the correct position to keep the temperature at the set point (Figure 2). If the temperature rises above its set point, the controller
will reduce the valve position and vice versa.
Each of the controller’s three modes reacts differently to the error. The amount of response produced by each control mode is
adjustable by changing the controller’s tuning settings.
CONTROL MODES:
PROPORTIONAL CONTROL MODE
The proportional control mode changes the controller output in proportion to the error. If the error increases, the control action
increases proportionally.
The adjustable setting for proportional control is called the Controller Gain (Kc). A higher controller gain increases the amount
of proportional control action for a given error. If the controller gain is set too high, the control loop will begin oscillating and
become unstable. If set too low, the control loop will not respond adequately to disturbances or set point changes.
For most controllers, adjusting the controller gain setting influences the amount of response in the integral and derivative
control modes.
THE PROPORTIONAL-ONLY CONTROLLER
A PID controller can be configured to produce only a proportional action by turning off the integral and derivative modes.
Proportional controllers are simple to understand and easy to tune: the controller output is simply the control error times the
controller gain, plus a bias. The bias is needed so the controller can maintain a non-zero output while the error is zero (process
variable at set point). The drawback is offset, which is a sustained error that cannot be eliminated by proportional control alone.
Under proportional-only control, the offset will remain present until the operator manually changes the bias on the controller’s
output to remove the offset. This is known as a manual reset of the controller.
INTEGRAL CONTROL MODE
figure 3. (left) The non-interactive PID controller algorithm; (right) the parallel PID controller algorithm
The need for manual reset led to the development of automatic reset, known as the integral control mode. The function of the
integral control mode is to increment or decrement the controller’s output over time to reduce the error, as long as there is any
error present (process variable not at set point). Given enough time, the integral action will drive the controller output until the
error is zero.
If the error is large, the integral mode will increment/decrement the controller output at a fast rate; if the error is small, the
changes will be slow. For a given error, the speed of the integral action is set by the controller’s integral time setting (Ti). If
the integral time is set too long, the controller will be sluggish; if it is set too short, the control loop will oscillate and become
unstable.
Most controllers use integral time in minutes as the unit of measure for integral control. Some use integral time in seconds,
and a few controllers use integral gain (Ki) in repeats per minute.
DERIVATIVE CONTROL MODE
Derivative control is rarely used in controlling processes, though it is often used in motion control. It is very sensitive to
measurement noise, it makes trial-and-error tuning more difficult, and it is not absolutely required for process control.
However, using a controller’s derivative mode can make certain types of control loops – temperature control, for example –
respond faster than with PI control alone.
The derivative control mode produces an output based on the rate of change of the error. It produces more control action if the
error changes at a faster rate; if there is no change in the error, the derivative action is zero. This mode has an adjustable setting
called Derivative Time (Td). The larger the derivative time setting, the more derivative action is produced. If the derivative
time is set too long, however, oscillations will occur and the control loop will be unstable. A Td setting of zero effectively
turns off the derivative mode. Two units of measure are used for the derivative setting of a controller: minutes and second
TOPIC 48: SOLENOID VALVE WORKING
Q1) what is solenoid valve?
Valves are mechanical devices designed to control the flow of liquid and gases. Many valves are manually operated.
Electrically operated valves are known as solenoid valves.
Some industries require the use of liquid or gas to complete a job, such as the medical and dentistry fields. Despite the
differences in the fields that operate this type of equipment, the fact remains that the equipment needs to be able to start and
stop liquid or gas as needed. That's where solenoid valves come into play.
Solenoid valves help to control the flow of liquid or gas. These valves are incorporated into the equipment so that the equipment
can be used safely and efficiently. What a solenoid valve does is use a plunger to open or close the valve, either allowing the
liquid to flow through or sealing it off without any leaks. This is an extremely important process in the automation of fluid and
gas control, and there are different types of solenoid valves that does the same job in different ways.
Q2) how solenoid valve works?
A solenoid is a coil of insulated wire wound on a rod-shaped form made of solid iron, solid steel, or powdered iron which
produces magnetic field when electric current passed through it.
Working: When electric current starts flowing through the wire, it produces magnetic field inside the iron core. This magnetic
field produced is depends on turns of the coil. More the number of turns of coil, more is the magnetic field produced within
the wire
A solenoid valve is the combination of a basic solenoid and mechanical valve. So a solenoid valve has two parts namelyElectrical solenoid, mechanical valve.
Solenoid converts electrical energy to mechanical energy and this energy is used to operate a mechanical valve that is to open,
close or to adjust in a position.
Image of – Working procedure of a typical Solenoid valve:
Q3) what are the limitations of solenoid valve?
The main limitation of the solenoid is its short stroke, which is usually under an inch.
Still, there are many applications for short-stroke linear motion; examples are activating electric car-door locks, opening and
closing valves, and triggering mechanical latches.
Most applications use the solenoid as a on or off device—that is, the coil is either completely energized or switched off.
However, variable-position control is possible by varying the input voltage.
Q4) what are the problems in solenoid valve?
TOPIC 49: DIRECT VS INDIRECT FIRED BURNER
Q1) what are burners and what is the purpose of using them?
At a basic level, burners are devices used to mix fuel and air (or oxygen) to achieve controlled combustion while producing a
specific flame and heat-release pattern. The heat generated is used to control the temperature within a combustion chamber to
the process set points. Combustion in industrial burners is a critical operation in the chemical process industries (CPI) for
supplying thermal energy for heat transfer, fluid heating, steam generation, distillation, endothermic chemical reactions, metal
melting and others. Burners are mechanical devices utilized for mixing proper quantities of fuel and air, and also for
maintaining a stable flame inside fired equipment.
Q2) what are DIRECT FIRED burners?
Direct fired heaters are similar to a gas grill where the flame comes into direct contact with the air. In a direct fired heater, the
gas is fed directly to the burner while the airstream provides the needed oxygen for combustion. These heaters provide heat
by forcing air through a gas flame. Air is forced through the burner baffle where it mixes with the gas. Air heaters that are
direct fired are utilized in applications where the end product will not be affected by the by-products of combustion.
Q3) what are the benefits of DIRECT FIRED burners?


.
SMALLER SIZE – A direct fired burner can produce more heat in a smaller envelope compared to an indirect fired burner.
This results in an overall smaller equipment footprint in most cases
EFFICIENCY: As nearly 100% of the fuel is being converted to heat, fuel consumption and operating costs are reduced. These
heaters are 100% combustion efficient with an overall thermal efficiency of 92% (8% heat loss due to water formation during
combustion).
Q4) what are the disadvantages of DIRECT FIRED burners?



VENTILATION REQUIREMENTS – Although these heaters operate within code prescribed safety limits, due to
combustion products entering the airstream, proper ventilation is required to avoid buildup of gases such as carbon
monoxide.
REHEATING CONCERNS – Due to the fresh air requirement to maintain proper combustion, if building air is to be
circulated, a minimum of 20% outside air may be introduced
Direct-fired heaters are not appropriate for use in tightly sealed spaces or near flammable materials because of the exhaust
and fumes produced. The heated space must have some type of air exchange or an exterior window or door that you can
open
Q5) what are INDIRECT FIRED burners?
In an indirect fired heater, the burner is fired into a heat exchanger. Rather than sending air through a gas flame, the air is
heated as it passes over an enclosed heating element. This keeps the air cleaner and reduces the amount of CO2 released by
the heater. Air is heated by passing over the heat exchanger, allowing the combustion by-products to remain within the heat
exchanger which is then exhausted through a flu. An everyday example of an indirect fired heater would be a gas furnace with
a chimney
Q6) what are the benefits of INDIRECT FIRED burners?



Indirect fired heaters are better for areas where people will be living or working. Because they do not add much CO2 into
the air. Even areas with poor ventilation like small office areas and basements can make use of indirect fired heaters
without worrying about a change in air quality.
No products of combustion are introduced into the workspace – As opposed to direct fired heaters, indirect fired heaters
are the preferred choice in areas such as office spaces, event arenas, specific manufacturing facilities, schools and
pharmaceutical applications.
Reheating Ability – Unlike direct fired heaters, 100% of the air may be recirculated through an indirect fired heater as no
combustion by-products are introduced into the airstream by the heating process
Q7) what are the disadvantages of INDIRECT FIRED burners?




Larger Equipment Size - Due to the heat exchanger required, equipment size is larger than a direct fired unit with the
same BTU output.
Temperature control of an indirect fired heater is not as precise as a direct fired heater as it often “under or overshoots”
the desired temperature by a few degrees. This is due to the heater reaching a set cutoff temperature and then shutting
down the burner until it reaches a minimum temperature before firing the burner again. This may be unacceptable for
applications requiring tight temperature ranges.
Lower Efficiency – Due to heat loss through the flue and inefficiencies in the heat exchanger, indirect fired heaters are
approximately 80% efficient. This results in higher operating costs compared to a direct fired heater.
Cost – Higher equipment costs due to the inclusion of the heat exchanger and higher price of the air handler compared to
direct fired heater
TOPIC 50: MOTOR TROUBLESHOOTING GUIDE
TOPIC 51: TYPES OF SENSORS USED IN INDUSTRY
Proximity means something close. A proximity sensor is anything which detects changes in environment in its surroundings.
The main function of proximity sensors is the detection of the presence or absence of objects in an area. There are different
technologies used to achieve this goal, like using electromagnetic fields, light, and sound
Types of proximity sensor are
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

capacitive proximity sensor
inductive proximity sensor
photoelectric proximity sensor
magnetic proximity sensor
Other sensors used in industry are:
Temperature sensor (PT-100, Thermocouple, PT-1000)
Pressure switch
Level sensor
Beam sensor
Limit switch
APPLICATION OF PROXIMITY SENSOR:
MOBILE PHONES: when you bring your phone to ears for calling, the screen turns of automatically and as soon as you bring
phone away from your ears or and surface the light turns on that is because of the proximity sensors inside your cell phone
COMMON SENSORS IN MOBILE PHONE:
Basic sensors include
1.
2.
3.
4.
5.
Ambient light sensor: to adjust screen brightness and control flash during imaging automatically.
Proximity sensor: to detect proximity to ears to lockout screen preventing accidental clicks during calls.
Accelerometer and gyroscope: to detect orientation of phone and rotate screen accordingly.
Pedometer - sensor used for counting the number of steps that the user has taken.
Heart rate sensor - it is made to measure one's pulse, and it does that by detecting the minute pulsations of the blood
vessels inside one's finger.
6. Fingerprint sensor - used as an extra layer of security – as a substitute for a lock screen password.
7. Gesture Sensor - recognizes hand movements by detecting infrared rays that are reflected from the user’s palm.
TOPIC 52: ENERGY CRISES IN PAKISTAN
There is a direct relationship between economic development and power consumption
Energy is essential for the maintenance and development of human life as well as for economic activities. Most of developing
countries plans well in advance to ensure not only sufficient energy for the present but also for the future requirements.
Today, Pakistan is facing a looming energy crisis with shortfall that varies from 5000-7000MW. Due to that shortfall, long
load shadings at one hand have adversely affected individual and social life. While on the other hand it has resulted into shut
down of a number of industrial units. It had not only affected the attitude of the people and forced them to come on the roads
as protest but also plays a destructive role by shattering the confidence of the investors and the industrialists.
Pakistan’s power generation is based on fossil fuels (64.12%), hydro-electric power (33.30%), nuclear plant power (2.5%),
and renewable sources (0.41%).
SOLUTIONS:
1) REDUCTION OF IMPORTED FUEL:
Almost half of the country’s total energy is generated by expensive imported thermal fuel. Reliance on oil and gas has led to
high electricity prices for consumers. We are producing over 50 percent of power using the most expensive furnace oil as
fuel.
2) COAL POTENTIAL IN PAKISTAN
Pakistan has the 5th largest coal reserve in the World, amounting to approximately 185.175 billion Tones. Thar has largest
reserve in the country that is approximately
75.5 billion Tones. Pakistan can generate more than 100,000 MW of electricity for next 30 years if it uses all coal available to
it. At present Pakistan generates only 0.79% of its total electricity from coal. Coal contributes approximately 39% of the total
global primary energy demand. Share of coal in total electricity produced in different countries is
Estimated Thar coal reserves are around 850 Trillion Cubic feet, which are equal to 400 Barrel of furnace oil, Equal to
the oil reserves of SAUDIA ARABIA, which can generate Electricity for almost forty years.
Coal contributes to less than 1% of Pakistan’s generation, even though the Thar mines contain the world’s third-largest coal
reserves. Even ‘green’ countries like Germany are in the process of installing coal-powered plants. In generating electricity,
coal clearly remains the choice of first resort. Pakistan may install imported coal power plants initially, and after exploitation
of THAR coal, we may erect the coal power plants designed on our own Thar coal.
Coal based power has been conspicuous by its almost total absence from Pakistan’s power generation despite the country
having one of the largest coal reservoirs in the world
While most electricity in the world is generated from coal, Pakistan has been relying on oil to produce power despite
the fact that the country has large coal reserves among other natural resources and its nuclear power plant that started
work in 1972 is entirely self-sufficient and doesn’t owe the government any money.
While the oil, gas and coal reserves in the world are fast depleting and will be gone by 2100 or 2200, nuclear power is
constant,” he said before pointing out that mostly coal was used across the world for power production but Pakistan used oil,
which was very expensive, to meet its electricity needs. Coming to coal energy, he said that 50,000 megawatts could be
produced by just Thar coal alone. About gas, he said, it was regretful that more gas reserves had not been explored.
2) INFRASTRUCTURE AND THEFT:
About 25% of power is lost through inefficient power distribution networks, poor infrastructure, mismanagement and theft of
electricity and this needed to be fixed. The public sector power projects are operating a very low efficiency. Sensible solution
to the crisis is to privatize and deregulate this sector.
3) NUCLEAR POWER PLANT:
The Chashma nuclear power projects units C-1, C-2 and C-3 have been successfully contributing to the national grid with an
excellent performance since 2000, 2011 and 2016 respectively. Chashma Nuclear Power Plant Unit-4 (C-4) would be
operational and connected to the country’s power grid with tremendous effort from Pakistan Atomic Energy Commission
(PAEC) and China. Currently, Pakistan’s four nuclear power plants — KANUPP, C-1, C-2 and C-3 — are generating a total
of 1,030MW of power, which will be enhanced with the inauguration of the C-4 Chashma plant.
Elaborating, he said: “When you organise a dinner party at home, you have a main dish, maybe biryani or pulao, along with
several add-ons or side dishes including a curry, salad, etc. The main dish is like nuclear power for you and the rest are the
other energy sources such as oil, gas, coal, hydro-power. But right now it is like serving dinner that has been bought from an
expensive restaurant, which is what you are doing by making power from oil.”
About the different safety measures taken to control the spread of radiation in case of an accident, he said that anyone living
right next to a nuclear power plant would receive 10 micro Sievert radiation a year, which was equal to what one faced on a
flight from say Islamabad to Karachi. “It is that safe,” he emphasized.
The cost and benefit analysis of the alternate sources with that nuclear power shows that they are quite expensive and majorly
they are dependent on the activity of the weather such as wind and solar so they cannot be available for 24/7. Nuclear power
reactors are expensive to build but they are relatively cheap to operate. Nuclear power plants operate reliably and have a
continuous output of power. This is in contrast to other alternative energies like solar and wind, which depend on the activity
of the weather. Keeping in view the current economic situation prevailing in Pakistan; nuclear power is a viable option. Nuclear
power generation appears a good alternative for sustainable and inexpensive energy. Pakistan is blessed with nuclear
technology and also with seasoned professionals in this field. Construction of more nuclear power plants can help relieve the
energy crisis and also in the economic development.
4) HYDRO-ELECTRIC POWER POTENTIAL
Pakistan has a huge potential to produce electric power from hydro-electric power plants. In table 5 presents a view of electric
power generation with power plants whose feasibility study has been completed or is under process. Construction of all these
plants gives almost 55,000 MW. This easily meets the electrical energy requirement of Pakistan for next 20-25 years. So
government should make new projects like KALA BAGH DAM which is very important for our country. If government make
only kala bagh dam and bhasha dam then we can easily solve the problem of electricity in Pakistan. Pakistan has the fifth
largest water system in the world where you can have small hydro plants, instead of having huge ones, at several places. And
these plants can be manufactured locally
4) LINE LOSSES MECHANISM
The second important thing is to minimize the line losses and it can be done such that the government should establish a
team including the police force. The task of this team will be to shutting down the markets and wedding halls which will
open till late night because a lot of energy is misused in the high resistance lights which they use in late hours. The second
task of this force is to catch those peoples which are including in the theft of electricity. If government reduce these things so
we can save up to 10 % which is almost 300 MW of energy.
5) SOLAR AND WIND:
Frequent load shedding, high electricity bills, power blackouts and never ending UPS battery issues! All of these probably
sound familiar to an average Pakistani. In the past few years, one viable alternative to counter power and energy problems has
emerged in the form of solar panels. While still not very common, solar panels are increasingly becoming popular across the
country, both in urban areas and in rural regions which have access to very little or no electricity at all
Number of projects at the macro level across the country are underway like the Quaid-e-Azam solar park in Bahawalpur, which
will be completed later this year, covers an area equivalent to 500 soccer fields and will generate 1000MW of electricity.
The solar energy technologies have not been exploited on a large scale for a number of reasons such as, high cost, lack of
motivation and inadequate demonstration of effective use of the technology. Thar in Sindh and entire Balochistan province is
considered ideal for utilization of solar energy
Light is the only requirement for these houses located in remote areas of the province and the electric requirement for each
house is 100 watt at maximum. Extension of grid lines for such small power requirements would certainly be very
uneconomical and local power generation could be the best solution. In case, diesel generators are used, transportation of fuel
to such remote areas and maintenance is again costly proposition therefore solar energy seems an attractive option for these
areas.
Baluchistan province is particularly rich in solar energy. It has the highest annual mean sunshine duration in the world.
Rick-E, an electronic, solar-powered rickshaw, developed by WRL technologies
Pakistan has huge potential for alternative energy generation, an estimated 2.9 million megawatt (MW) f solar energy and
340,000 MW of wind, according to Pakistan Alternative Energy Board as well as some 100,000 MW of hydropower. But
around 70 per cent of its electricity is generated through dirty fossil fuels – primarily oil and gas
Pakistan producing more than 1,000MW of clean energy
As up to 49 per cent of electricity produced in Pakistan is used in the domestic sector, the government should be providing
more incentives for households to switch to solar which could in turn reduce the country’s reliance on fossils and reduce its
carbon footprint, In Pakistan to the cost of solar panels has dropped significantly in the past few years but sales have not
picked up.“The biggest issue is that the government doesn’t support the sector,”
With huge investment flowing in from China under the China-Pakistan Economic Corridor, there are plans to add more than
10,000 MW to the national grid. Work is currently underway on 17 priority projects.
6) SMART METERING:
Install smart meters to curb electricity theft. Losses due to outright theft and unpaid bills approach nearly half of all
electricity generated in Pakistan. What business can ever run like this?
At their current price of Rs10,000 apiece, smart meters can return the investment made in them in about a year On the other
hand, the energy crisis costs Pakistan about four percent of its GDP – smart meters alone can make a major difference.
7) REDUCTION OF CIRCULAR DEBT:
One of the foremost reasons for Pakistan’s Energy crises is the Circular Debt Issue that needs immediate remedies Circular
Debt is the amount of cash shortfall within the Central Power Purchasing Agency (CPPA) that it is unable to pay to the
power supply companies the issue of circular debt is related to cash flows in the power sector, which are affected by the
performance of the distribution companies (Discos) in terms of their collections and line losses as well as the decisions
relating to tariff determination by the regulator (Nepra) and subsidy budgeting.
Distribution companies are facing line losses due to multiple reasons, but electricity theft and non-payments by the
consumers are burning issues
8) TRASH TO POWER:
Ever-increasing prices of fossil fuels have promoted waste-to-energy technology, which is fast becoming an increasingly
important option for alternate power generation the world over. It has the potential to contribute largely to total energy mix in
Pakistan as well, for on-grid as well as off-grid applications.
About 3,000 MW combined gross electricity can be generated using agricultural residue and municipal solid waste. There is,
however, a need for the government to ensure timely implementation of all the waste-to-energy projects currently in pipeline
to encourage additional investments in this area.
Pakistan is blessed with large quantity of biomass that has a potential to produce 6000 MW of electricity. Our companies are
using corncob, rice husk, wheat straw, cotton plant sticks and other agriculture residue, solid municipal waste, slippers, sandals,
and used tyres to generate energy.
Denmark now regards garbage as a clean alternative fuel rather than a smelly, unsightly problem Denmark now has 29 such
plants
9) IMPROVING POWER GENERATING CAPABILITY
WAPDA and IPPs Thermal power plants are running 50 % of plant factor, unfortunately of their total plant capacity. It
means that only 50 % of the total power is in workable condition. The plant factors of thermal power plants are about 75 to
80 % worldwide. So it means that if the government works on this side we add up to 20 to 30 % of power in our system. This
is a lot of thing for a country like Pakistan to save up to that percentage because it will minimize the present shortage to
substantial extent. In place of setting up new power plant if the government improve the existing power plants it will be far
better
TOPIC 53: ELECTRICAL STANDARDS
Q1) what is NEC?
The National Electrical Code (NEC), or NFPA 70, is a regionally adoptable standard for the safe installation of electrical
wiring and equipment in the United States. The NEC is developed by NFPA's Committee. A licensed electrician will have
spent years of apprenticeship studying and practicing the NEC requirements prior to obtaining his or her license.
The NEC is composed of an introduction, nine chapters, annexes A through J, and the index. The first four chapters cover
definitions and rules for installations (voltages, connections, markings, etc.), circuits and circuit protection, methods and
materials for wiring (wiring devices, conductors, cables, etc.), and general-purpose equipment (cords, receptacles, switches,
heaters, etc.). The next three chapters deal with special occupancies (high risk to multiple persons), special equipment (signs,
machinery, etc.) and special conditions (emergency systems, alarms, etc.). Chapter 8 is specific to additional requirements for
communications systems (telephone, radio/TV, etc.) and chapter 9 is composed of tables regarding conductor, cable and
conduit properties. It’s a 1000 page book easily available on internet
Q2) what is IEEE?
The Institute of Electrical and Electronics Engineers (IEEE) is a professional association with its corporate office in New
York founded in 1963. The Institute of Electrical and Electronics Engineers (IEEE) is a world technical professional society
with presence in 150 countries. It is solely toward innovating, educating and standardizing the electrical and electronic
development industry. IEEE primarily innovates new electronic products and services, designs the standards that govern
them and imparts, publishes and promotes industry knowledge through publications, conferences and partnering with academic
institutes. The prime areas of focus for IEEE are electrical, electronics, computer engineering, computer science, information
technology and most of their related disciplines.
Pakistan is a member of IEEE. It’s even written on their website
Q3) what is IEC?
The International Electro technical Commission (IEC) is a similar organization to the IEEE and primarily covers Europe and
many other countries around the world.
The International Electro technical Commission is an international standards organization[4][5] that prepares and publishes
International Standards for all electrical, electronic and related technologies. IEC standards cover a vast range of technologies
from power generation, transmission and distribution to home appliances and office equipment, semiconductors, fiber optics,
batteries, solar energy, nanotechnology and marine energy as well as many others.
Currently, 82 countries are members while another 82 participate in the Affiliate Country Program, which is not a form of
membership but is designed to help industrializing countries get involved with the IEC. Originally located in London,
Pakistan is a full time member of IEC
Q4) what is NEMA?
The National Electrical Manufacturers Association (NEMA) is the largest trade association of electrical equipment
manufacturers in the United States. It was founded in 1926 and maintains its headquarters in Rosslyn, Virginia, just outside
Washington, D.C. the National Electrical Manufacturers Association (NEMA) is an organization developed to form the
technical standards for the manufacturing of electrical equipment and medical imaging equipment. The National Electrical
Manufacturers Association (NEMA) devised a rating system for enclosures, connectors and other equipment that is exposed
to liquids, rain, ice, corrosion and contaminates such as dust. You will often see written on equipment’s “Nema 1, Nema 2,
Nema 3, Nema 3s, Nema 3sx. These are NEMA ratings define the types of environments in which an electrical enclosure can
be used.
Nema 1: are enclosure Protects against dust, light, and indirect splashing but is not dust-tight; primarily prevents contact
with live parts; used indoors and under normal atmospheric conditions
TOPIC 54: PURPOSE OF LOAD FLOW, SHORT CIRCUIT AND TRANSIENT
ANALYSIS
Q1) what is LOAD FLOW analysis?
A load flow study calculates the voltage drop on each feeder, the voltage at each bus, and the power flow and losses in all
branch and feeder circuits. Load flow studies determine if system voltages remain within specified limits under normal or
emergency operating conditions, and whether equipment such as transformers and conductors are overloaded
The most important information obtained from the load flow analysis is the voltage profile of the system. If voltage varies
greatly over the system, large reactive flows will result. This, in turn, will lead to increased real power losses and, in extreme
cases, an increased likelihood of voltage collapse. When a particular bus has an unacceptably low voltage, the usual practice
is to install capacitor banks in order to provide reactive compensation to the load. Load flow studies are used to determine how
much reactive compensation should be applied at a bus, to bring its voltage up to an appropriate level. If new lines (or additional
transformers) are to be installed, to reinforce the system, a power flow study will show how it will relieve overloads on adjacent
lines. An inefficient or unbalanced load can also cause unpredictable behavior in your localized power grid, increasing the risk
of equipment damage and unplanned outage
Q2) what is SHORT CIRCUIT analysis?
A Short Circuit analysis is used to determine the magnitude of short circuit current the system is capable of producing and
compares that magnitude with the interrupting rating of the overcurrent protective devices (OCPD).
It's very important to understand the meaning of the term "short-circuit fault." Basically, a short-circuit fault in a power system
is an abnormal condition that involves one or more phases unintentionally coming in contact with ground or each other. Thus,
short-circuit protection is necessary to protect personnel and apparatus from the destructive effects of the resulting excessive
current flow, which is caused by the relatively low impedance of the short-circuit fault connection.
Q3) what are Sources of fault current?
Where does fault current come from? Basically, it comes from rotating electric machinery, usually in the form of synchronous
generators, synchronous motors and condensers, induction machines, and electric utility systems. The magnitude of fault
current from these sources is limited by the impedance of the machine itself as well as the impedance between the machine
and the fault itself.
* Contact-parting (interrupting) network.
* Approximately 30 cycle network.
These networks only differ from one another by the assignments of constant reactances for the machines.
First-cycle (momentary) network. This network is used to calculate the first-cycle (momentary) symmetrical rms current. Here,
the rotating machine sources of short-circuit current are represented, for the most part, by their sub transient reactance’s,
according to the entries in the first column of Tables 4-1 and 4-2 of the 1993 edition of the IEEE Red Book (or Tables 24 and
25 of the 1986 edition).
Contact-parting (interrupting) network. This network is used to calculate the contact-parting (interrupting) symmetrical rms
current for circuit breaker minimum contact-parting times of 1.5 to 4 cycles after the inception of the short-circuit fault. Here,
the rotating machine sources of short-circuit current are represented by different constant reactances than the first-cycle
(momentary) network, according to the entries in the second column of Tables 4-1 and 4-2 of the 1993 edition of the IEEE
Red Book (or Tables 24 and 25 of the 1986 edition).
Approximately 30 cycle network. This network is often a minimum-source representation to investigate whether minimum
short-circuit currents are sufficient to operate current-actuated relays. Minimum-source networks might apply at night or when
production lines are down for any reason. Some of the source circuit breakers may be open and all motor circuits may be off.
In-plant generators are represented with transient reactance or a larger reactance that is related to the magnitude of decaying
generator short-circuit current at the desired calculation time.
Q4) what is the concept of Sub transient, Transient & Steady State?
The concept of Subtransient, Transient and Steady State arises in case of fault in an Alternator. Let us assume a sudden short
circuit in three phase of alternator. The fault current will flow in all the three phases of alternator and its waveform will be as
shown in figure below.
When the alternator is short-circuited, the currents in all the three-phases rise rapidly to a high value of about 10 to 18 times
of full load current, during the first quarter cycle. The flux crossing the air gap is large during a first couple of cycles. The
reactance during these first two or three cycle is least and the short circuit current is high. This reactance is called subtransient
reactance and is denoted by X". The first few cycles come under sub-transient state.
After a first few cycles, the decrement in the r.m.s. value of short circuit current is less rapid than the decrements during the
first few cycles. This state is called the Transient State and the reactance in this state is called transient reactance X'. The circuit
breaker contacts separate in the transient state.
Finally the transient dies out and the current reaches a steady sinusoidal state called the Steady State. The reactance in this
state is called steady state reactance Xd. Since the short circuit current of the alternator lags behind the voltage by 90 degree,
the reactance involved are direct axis reactance.
As clear from the figure above, the d.c. components in the three phases are different; hence the waveforms of the three phases
are not identical. If voltage of phase, say, Y, is maximum at the instant of short circuit, the DC component of short circuit
current is zero. Hence the waveform is symmetrical as shown in figure below.
The currents and reactance are given by the following expressions,
Where
I = Steady state current, r.m.s. value
I' = Transient current, r.m.s. value
I'' = Sub-transient current, r.m.s. value
Ea = Induced e.m.f. per phase
Xd = Direct axis synchronous reactance
Xd' = Direct axis transient reactance
Xd'' = Direct axis sub-transient reactance
As the short circuit occurs, the short-circuit current attains high value. The circuit breaker contact starts separating after the
operation of the protective relay. The contacts of the circuit breaker separate during 'transient state.' The r.m.s. value of the
current at the instant of the contact separation is called the breaking current of the circuit-breaker and is expressed in kA.
If a circuit-breaker closes on existing fault, the current would increase to a high value during the first, half cycle. The highest
peak value of the current is reached during the peak of the first current loop. This peak value is called making current of the
circuit breaker and is expressed in kA. This is the reason making current of Circuit Breaker is higher than the Breaking Current
By definition, the short-circuit current is limited only by the circuit inductance. The current in an inductor cannot change
instantaneously from the initial value (zero) to the steady state value (in the figure, -1.414A, the peak value of a symmetrical
current of 1.00A). To achieve a current balance at the instant of short circuit initiation, we consider that the short-circuit current
consists of an ac component (the symmetrical component) and a dc component
“A symmetrical fault is a balanced fault with the sinusoidal waves being equal about their axes, and represents a steady-state
condition.
An asymmetrical fault displays a DC offset, transient in nature and decaying to the steady state of a symmetrical fault after a
period of time…”
“Direct current offset…occurs as a result of two natural laws:
1. Current cannot change instantaneously in an inductance and
2. Current must lag the applied voltage by the natural power-factor…”
Secondly, the generator is a large inductor. The current in an inductor cannot change instantaneously.”
Q5) what is TRANSIENT ANALYSIS?
In circuit analysis we study the circuit completely also we study its behavior during different states. Such as Steady and
Transient. In steady state a system behaves normally everything is fine with the circuit but if any fault occurs or any input is
given to the circuit suddenly or if any input is removed from the circuit then for a very small time the circuit goes in transient
state. Basically if any change occurs in the circuit then it goes in transient mode. Generally transients last for very short duration
but it is very important to study that small duration of time. In that small instant of time current or voltage may rise or drop to
a certain value if that happens then our circuit must sustain that conditions also so we perform transient analysis on the system.
Being an Electrical Engineer I would suggest one example consider a motor as a small circuit when we switch on the supply
for a very short duration the motor takes very high current which may damage the motor but before designing any such machine
transient analysis is done everything’s are calculated such as amount of current that motor will take any most important time
for which it will take all such things for any system are defined after performing transient analysis on the system.
Suppose you are traveling in a bus in constant speed. And suddenly the bus stops or change the speed. This sudden change is
called transient. A transient means interruption
TOPIC 55: SOLAR PV SYSTEM SIZING
1. Determine power consumption demands
The first step in designing a solar PV system is to find out the total power and energy consumption of all loads that need to
be supplied by the solar PV system as follows:
1.1 Calculate total Watt-hours per day for each appliance used.
Add the Watt-hours needed for all appliances together to get the total Watt-hours per day which
must be delivered to the appliances.
1.2 Calculate total Watt-hours per day needed from the PV modules.
Multiply the total appliances Watt-hours per day times 1.3 (the energy lost in the system) to get
the total Watt-hours per day which must be provided by the panels.
2. Size the PV modules
Different size of PV modules will produce different amount of power. To find out the sizing of PV module, the total peak
watt produced needs. The peak watt (Wp) produced depends on size of the PV module and climate of site location. We have
to consider “panel generation factor” which is different in each site location. For Thailand, the panel generation factor is
3.43. To determine the sizing of PV modules, calculate as follows:
2.1 Calculate the total Watt-peak rating needed for PV modules
Divide the total Watt-hours per day needed from the PV modules (from item 1.2) by 3.43 to get
the total Watt-peak rating needed for the PV panels needed to operate the appliances.
2.2 Calculate the number of PV panels for the system
Divide the answer obtained in item 2.1 by the rated output Watt-peak of the PV modules available
to you. Increase any fractional part of result to the next highest full number and that will be the
number of PV modules required.
Result of the calculation is the minimum number of PV panels. If more PV modules are installed, the system will perform
better and battery life will be improved. If fewer PV modules are used, the system may not work at all during cloudy periods
and battery life will be shortened.
3. Inverter sizing
An inverter is used in the system where AC power output is needed. The input rating of the inverter should never be lower
than the total watt of appliances. The inverter must have the same nominal voltage as your battery.
For stand-alone systems, the inverter must be large enough to handle the total amount of Watts you will be using at one
time. The inverter size should be 25-30% bigger than total Watts of appliances. In case of appliance type is motor or
compressor then inverter size should be minimum 3 times the capacity of those appliances and must be added to the inverter
capacity to handle surge current during starting.
For grid tie systems or grid connected systems, the input rating of the inverter should be same as PV array rating to allow
for safe and efficient operation.
4. Battery sizing
The battery type recommended for using in solar PV system is deep cycle battery. Deep cycle battery is specifically
designed for to be discharged to low energy level and rapid recharged or cycle charged and discharged day after day for
years. The battery should be large enough to store sufficient energy to operate the appliances at night and cloudy days. To
find out the size of battery, calculate as follows:
4.1 Calculate total Watt-hours per day used by appliances.
4.2 Divide the total Watt-hours per day used by 0.85 for battery loss.
4.3 Divide the answer obtained in item 4.2 by 0.6 for depth of discharge.
4.4 Divide the answer obtained in item 4.3 by the nominal battery voltage.
4.5 Multiply the answer obtained in item 4.4 with days of autonomy (the number of days that you
need the system to operate when there is no power produced by PV panels) to get the required
Ampere-hour capacity of deep-cycle battery.
Battery Capacity (Ah) = Total Watt-hours per day used by appliances x Days of autonomy
(0.85 x 0.6 x nominal battery voltage)
5. Solar charge controller sizing
The solar charge controller is typically rated against Amperage and Voltage capacities. Select the solar charge controller to
match the voltage of PV array and batteries and then identify which type of solar charge controller is right for your
application. Make sure that solar charge controller has enough capacity to handle the current from PV array.
For the series charge controller type, the sizing of controller depends on the total PV input current which is delivered to the
controller and also depends on PV panel configuration (series or parallel configuration).
According to standard practice, the sizing of solar charge controller is to take the short circuit current (Isc) of the PV array,
and multiply it by 1.3
Solar charge controller rating = Total short circuit current of PV array x 1.3
Remark: For MPPT charge controller sizing will be different. (See Basics of MPPT Charge Controller)
Example: A house has the following electrical appliance usage:
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One 18 Watt fluorescent lamp with electronic ballast used 4 hours per day.
One 60 Watt fan used for 2 hours per day.
One 75 Watt refrigerator that runs 24 hours per day with compressor run 12 hours and off 12 hours.
The system will be powered by 12 Vdc, 110 Wp PV module.
1. Determine power consumption demands
Total appliance use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 24 x 0.5 hours)
= 1,092 Wh/day
Total PV panels energy needed
= 1,092 x 1.3
= 1,419.6 Wh/day.
2. Size the PV panel
2.1 Total Wp of PV panel capacity
needed
2.2 Number of PV panels needed
= 1,419.6 / 3.4
= 413.9 Wp
= 413.9 / 110
= 3.76 modules
Actual requirement = 4 modules
So this system should be powered by at least 4 modules of 110 Wp PV module.
3. Inverter sizing
Total Watt of all appliances = 18 + 60 + 75 = 153 W
For safety, the inverter should be considered 25-30% bigger size.
The inverter size should be about 190 W or greater.
4. Battery sizing
Total appliances use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)
Nominal battery voltage = 12 V
Days of autonomy = 3 days
Battery capacity = [(18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)] x 3
(0.85 x 0.6 x 12)
Total Ampere-hours required 535.29 Ah
So the battery should be rated 12 V 600 Ah for 3 day autonomy.
5. Solar charge controller sizing
PV module specification
Pm = 110 Wp
Vm = 16.7 Vdc
Im = 6.6 A
Voc = 20.7 A
Isc = 7.5 A
Solar charge controller rating = (4 strings x 7.5 A) x 1.3 = 39 A
So the solar charge controller should be rated 40 A at 12 V or greater.
TOPIC 56: COGENERATION POWER PLANT
A conventional power plant makes electricity by a fairly inefficient process. A fossil fuel such as oil, coal, or natural gas is
burned in a giant furnace to release heat energy. The heat is used to boil water and make steam, the steam drives a turbine, the
turbine drives a generator, and the generator makes electricity. The trouble with this is that energy is wasted in every step of
the process—sometimes quite spectacularly. Instead of letting heat escape uselessly up cooling towers, why not simply pipe it
as hot water to homes and offices instead? That's essentially the idea behind CHP: to capture the heat that would normally be
wasted in electricity generation and supply it to local buildings as well. Cogeneration or combined heat and power (CHP)
is the use of a heat engine[1] or power station to generate electricity and useful heat at the same time
Cogeneration is more thermally efficient use of fuel than producing process heat alone because in electricity production some
energy must be rejected as waste heat, but in cogeneration some of this thermal energy is put to good use.
Cogeneration is the production of two types of energy at a single plant. Most commonly, a plant produces both electricity and
steam or hot water
Cogeneration improves energy efficiency from 30 percent energy capture to almost 90 percent, and it reduces carbon emissions.
By utilizing the exhaust energy from gas turbines, useful steam can be generated in a heat exchanger which can then be used
in any number of applications, all with no additional fuel consumption. As a result, the overall efficiency of CHP systems can
exceed 80 percent, making CHP one of the most energy-efficient methods of power generation.
Why? With traditional power generation, electricity and thermal energy are produced separately using two different processes
and fuel sources-- conventional fossil fuels are used to generate the electricity and, in most cases, the heat produced as a
byproduct to this process is lost to the atmosphere. Then an on-site boiler or furnace is used to generate heat.
Cogeneration solutions use a single fuel in a combustion engine, like a gas turbine or gas engine, to generate electricity. The
heat that is created as a result of the process is captured and recycled to provide hot water or steam for other uses--like heating
or cooling for the facility. In addition to eliminating waste and increasing energy production efficiency, cogeneration solutions
have many advantages
TOPIC 57: T & D LOSSES
Q1) what is transmission & distribution losses?
Power generated in power stations pass through large & complex networks like transformers, overhead lines, cables & other
equipment’s and reaches at the end users. It is fact that the Unit of electric energy generated by Power Station does not match
with the units distributed to the consumers. Some percentage of the units is lost in the Distribution network. This difference
in the generated & distributed units is known as Transmission and Distribution loss.
Transmission and Distribution loss are the amounts that are not paid for by users
Distribution Sector considered as the weakest link in the entire power sector. Transmission Losses is approximate 17% while
Distribution Losses is approximate 50%.

There are two types of Transmission and Distribution Losses
1. Technical Losses
2. Non-Technical Losses (Commercial Losses)
There is no difference between a transmission line and a distribution line except for the voltage level and power handling
capability. Transmission lines are usually capable of transmitting large quantities of electric energy over great distances.
They operate at high voltages. Distribution lines carry limited quantities of power over shorter distances.
Q2) what are Technical Losses in transmission line?
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



The technical losses are due to energy dissipated in the conductors, equipment used for transmission Line,
Transformer, sub- transmission Line and distribution Line and magnetic losses in transformers.
The major amount of losses in a power system is in primary and secondary distribution lines. While transmission and
sub-transmission lines account for only about 30% of the total losses. Therefore the primary and secondary
distribution systems must be properly planned to ensure within limits.
The unexpected load increase was reflected in the increase of technical losses above the normal level
Losses are inherent to the distribution of electricity and cannot be eliminated.
There are two Type of Technical Losses.
Technical losses are due to energy dissipated in the conductors and equipment used for transmission, transformation, sub
transmission and distribution of power. Technical losses on distribution systems are primarily due to heat dissipation resulting
from current passing through conductors and from magnetic losses in transformers. Losses are inherent to the distribution of
electricity and cannot be eliminated. The major part of this loss is heat dissipation or I2R loss in the distribution conductors.
Since this loss depends upon the value of current, it is the maximum during peak load. Other causes of the technical loss are
low power factor, phase imbalance, improper joints, and extraneous factors like tree touching etc. This loss difference between
in the transformer output and the sum of all invalid consumption.
Technical losses relate to transmission and transformation inefficiencies and poor planning and design of distribution network.
Variable technical losses are those that occur in transmission lines, cables and copper part of transformation, and vary in
amount of electricity transmitted.
(a) Permanent / Fixed Technical losses:
Fixed losses do not vary according to current. These losses take the form of heat and noise and occur as long as a transformer
is energized.
Between 1/4 and 1/3 of technical losses on distribution networks are fixed losses. Fixed losses on a network can be influenced
in the ways set out below.
Corona Losses.
Leakage Current Losses.
Dielectric Losses.
Open-circuit Losses.
Losses caused by continuous load of measuring elements
Losses caused by continuous load of control elements.
(b) Variable Technical losses
Variable losses vary with the amount of electricity distributed and are, more precisely, proportional to the square of the current.
Consequently, a 1% increase in current leads to an increase in losses of more than 1%.
Between 2/3 and 3/4 of technical (or physical) losses on distribution networks are variable Losses.
By increasing the cross sectional area of lines and cables for a given load, losses will fall. This leads to a direct trade-off
between cost of losses and cost of capital expenditure. It has been suggested that optimal average utilization rate on a
distribution network that considers the cost of losses in its design could be as low as 30 per cent.
joule losses in lines in each voltage level
impedance losses
Losses caused by contact resistance.
Main Reasons for Technical Losses:
(1) Lengthy Distribution lines:
In practically 11 KV and 415 volts lines, in rural areas are extended over long distances to feed loads scattered over large areas.
Thus the primary and secondary distributions lines in rural areas are largely radial laid usually extend over long distances. This
results in high line resistance and therefore high I2R losses in the line.
Haphazard growths of sub-transmission and distribution system in to new areas.
Large scale rural electrification through long 11kV and LT lines.
(2) Installation of Distribution transformers away from load centers:
Distribution Transformers are not located at Load center on the Secondary Distribution System.
In most of case Distribution Transformers are not located centrally with respect to consumers. Consequently, the farthest
consumers obtain an extremity low voltage even though a good voltage levels maintained at the transformers secondary. This
again leads to higher line losses. (The reason for the line losses increasing as a result of decreased voltage at the consumers
end Therefore in order to reduce the voltage drop in the line to the farthest consumers, the distribution transformer should be
located at the load center to keep voltage drop within permissible limits.
(3) Low Power Factor of Primary and secondary distribution system:
In most LT distribution circuits normally the Power Factor ranges from 0.65 to 0.75. A low Power Factor contributes towards
high distribution losses.
For a given load, if the Power Factor is low, the current drawn in high And the losses proportional to square of the current
will be more. Thus, line losses owing to the poor PF can be reduced by improving the Power Factor. This can be done by
application of shunt capacitors.
Shunt capacitors can be connected either in secondary side (11 KV side) of the 132 KV/11 KV power transformers or at various
point of Distribution Line.
A more appropriate manner of improving this PF of the distribution system and thereby reduce the line losses is to connect
capacitors across the terminals of the consumers having inductive loads.
By connecting the capacitors across individual loads, the line loss is reduced from 4 to 9% depending upon the extent of PF
improvement.
(4) Bad Workmanship:
Bad Workmanship contributes significantly role towards increasing distribution losses.
Joints are a source of power loss. Therefore the number of joints should be kept to a minimum. Proper jointing techniques
should be used to ensure firm connections.
Connections to the transformer bushing-stem, drop out fuse, isolator, and LT switch etc. should be periodically inspected and
proper pressure maintained to avoid sparking and heating of contacts.
Replacement of deteriorated wires and services should also be made timely to avoid any cause of leaking and loss of power.
(5) Load Factor Effect on Losses:
Power consumption of Customer varies throughout the day and over seasons. Residential customers generally draw their
highest power demand in the evening hours. Same commercial customer load generally peak in the early afternoon. Because
current level (hence, load) is the primary driver in distribution power losses, keeping power consumption more level throughout
the day will lower peak power loss and overall energy losses. Load variation is Called load factor and It varies from 0 to 1.
Load Factor=Average load in a specified time period / peak load during that time period.
For example, for 30 days month (720 hours) peak Load of the feeder is 10 MW. If the feeder supplied a total energy of 5,000
MWH, the load factor for that month is (5,000 MWh)/ (10MW x 720) =0.69.
Lower power and energy losses are reduced by raising the load factor, which, evens out feeder demand variation throughout
the feeder.
(6) Other Reasons for Technical Losses:
Unequal load distribution among three phases in L.T system causing high neutral currents.
leaking and loss of power
Over loading of lines.
Abnormal operating conditions at which power and distribution transformers are operated
Low voltages at consumer terminals causing higher drawl of currents by inductive loads.
Poor quality of equipment used in agricultural pumping in rural areas, cooler air-conditioners and industrial loads in urban
areas.
Distribution transformers use copper conductor windings to induce a magnetic field into a grain-oriented silicon steel core. So,
transformers have both load and noload core losses.
(2) Non-Technical (Commercial Losses):
Non-technical losses are at 16.6%, and related to meter reading, defective meter and error in meter reading, billing of customer
energy consumption, lack of administration, financial constraints, and estimating unmetered supply of energy as well as energy
thefts.
Main Reasons for Non-Technical Losses:
(1) Power Theft:
Theft of power is energy delivered to customers that is not measured by the energy meter for the customer. Customer tempers
the meter by mechanical jerks, placement of powerful magnets or disturbing the disc rotation with foreign matters, stopping
the meters by remote control. In Pakistan, Theft of Electricity is very common, which contribute a lot to the line losses. Some
Distribution companies line losses are near 40% due to the Theft of Electricity, which needs proper control on the Distribution
system. Corruption is also included in this huge loss of Electricity.
(2) Metering Inaccuracies:
Losses due to metering inaccuracies are defined as the difference between the amount of energy actually delivered through the
meters and the amount registered by the meters.
All energy meters have some level of error which requires that standards be established. Measurement Canada, formerly
Industry Canada, is responsible for regulating energy meter accuracy.
Statutory requirements5 are for meters to be within an accuracy range of +2.5% and – 3.5%. Old technology meters normally
started life with negligible errors, but as their mechanisms aged they slowed down resulting
In under-recording. Modern electronic meters do not under-record with age in this way.
Consequently, with the introduction of electronic meters, there should have been a progressive reduction in meter errors.
Increasing the rate of replacement of mechanical meters should accelerate this process
(3) Error in Meter Reading:
Proper Calibrated Meter should be used to measure Electrical Energy. Defective Energy Meter should be replaced immediately.
The reason for defective meter are Burning of meters, Burn out Terminal Box of Meter due to heavy load, improper C.T.ratio
and reducing the recording, Improper testing and calibration of meters.
(4) Billing Problems:
Faulty and untimely serving Bill should be main part of non-Technical Losses.
Normal Complain regarding Billing are Not Receipt of Bill, Late Receipt of Bill, Receiving wrong Bill, Wrong Meter Reading,
Wrong Tariff, wrong Calculations.
HOW TO REDUCE TECHNICAL LOSSES:
(1) Converting LV Line to HV Line:
Many Distribution pockets of Low Voltage (430V) in Town are surrounded by higher voltage feeders. At this lower voltage,
more conductor current flows for the same power delivered, resulting in higher I2R losses.
Converting old LV (430V) feeders to higher voltage the Investment Cost is high and often not economically justifiable but If
parts of the LV (430V) Primary feeders are in relatively good condition, installing multiple step-down power transformers at
the periphery of the 430 volt area will reduce copper losses by injecting load current at more points (i.e., reducing overall
conductor current and the distance traveled by the current to serve the load).
(2) Large Commercial / Industrial Consumer get direct Line from Feeder:
Design the distribution network system in such a way that if it is Possible than large consumer gets direct Power Line from
feeder.
(3) Adopting Arial Bundle Conductor (ABC):
Where LT Line are not totally avoidable use Arial Bundle Conductor to minimize faults in Lines, to avoid direct theft from
Line (Tampering of Line).
(4) Utilize Feeder on its Average Capacity:
By overloading of Distribution Feeder Distribution Losses will be increase.
The higher the load on a power line, the higher its variable losses. It has been suggested that the optimal average utilization
rate of distribution network cables should be as low as 30% if the cost of losses is taken into account.
(5) Replacements of Old Conductor/Cables:
By using the higher the cross-section area of Conductor / cables the losses will be lower but the same time cost will be high so
by forecasting the future Load an optimum balance between investment cost and network losses should be maintained.
(6) Feeder Renovation / Improvement Program:
Re conductoring of Transmission and Distribution Line according to Load.
Identification of the weakest areas in the distribution system and strengthening /improving them.
Reducing the length of LT lines by relocation of distribution sub stations or installations of additional new distribution
transformers.
Installation of lower capacity distribution transformers at each consumer premises instead of cluster formation and substitution
of distribution transformers with those having lower no load losses such as amorphous core transformers.
Installation of shunt capacitors for improvement of power factor.
Installation of single-phase transformers to feed domestic and nondomestic load in rural areas.
Providing of small 25kVA distribution transformers with a distribution box attached to its body, having provision for
installation of meters, MCCB and capacitor.
Lying of direct insulated service line to each agriculture consumer from distribution transformers
Due to Feeder Renovation Program T&D loss may be reduced from 60-70 % to 15-20 %.
(7)
Strictly Follow Preventive Maintenance Program:
Required to adopt Preventive Maintenance Program of Line to reduce Losses due to Faulty / Leakage Line Parts.
Required to tights of Joints, Wire to reduce leakage current.
HOW TO REDUCE NON-TECHNICAL LOSSES:
(1) Making mapping / Data of Distribution Line:
Mapping of complete primary and secondary distribution system with all parameters such as conductor size, line lengths etc.
Compilation of data regarding existing loads, operating conditions, forecast of expected loads etc.
Preparation of long-term plans for phased strengthening and improvement of the distribution systems along with transmission
system.
(2) Implementation of energy audits schemes:
It should be obligatory for all big industries and utilities to carry out Energy Audits of their system.
Further time bound action for initiating studies for realistic assessment of the total T&D Losses into technical and non-technical
losses has also to be drawn by utilities for identifying high loss areas to initiate remedial measures to reduce the same.
The realistic assessment of T&D Loss of a utility greatly depends on the chosen sample size which in turn has a bearing on the
level of confidence desired and the tolerance limit of variation in results.
In view of this it is very essential to fix a limit of the sample size for realistic quick estimates of losses.
(3) Mitigating power theft by Power theft checking Drives:
Theft of electric power is a major problem faced by all electric utilities, DISCOS in Pakistan. It is necessary to make strict rule
by State Government regarding Power theft. Electricity Act 1910 &Wapda Act 1958 may been amended to make theft of
energy and its abatement as a cognizable offense with deterrent punishment of up to 3 years imprisonment and Fines.
The impact of theft is not limited to loss of revenue, it also affects power quality resulting in Load Shedding, low voltage and
voltage dips.
Required to install proper seal management at Meter terminal Box, at CT/PT terminal to prevent power theft. Identify Power
theft area and required to expedite power theft checking drives.
Installation of medium voltage distribution (MVD) networks in theft-prone areas, with direct connection of each consumer to
the low voltage terminal of the supply transformer.
All existing un metered services should be immediately stopped.
(4)
Replacement of Faulty/Sluggish Energy Meter:
It is necessary to replacement of Faulty or sluggish Meter by Distribution Agency to reduce un metered Electrical energy.
Required to test Meter periodically for testing of accuracy of meter. Replacement of old erroneous electromechanical meters
with accurate Electro static Meter (Micro presser base) for accurate measurement of energy consumption.Now Smart Meters
may be installed Categorically.
Use of Meter boxes and seals them properly to ensure that the meters are properly sealed and cannot be tampered.
(5) Bill Collection facility:
Increase Bill’s Payment Cells, Increasing drop Box facility in all Area for Payment Collection. All Discos do not have proper
market orient systems.
E-Payment facility gives more relief to Customer for bill Payment and Supply agency will get Payment regularly and speedily
from Customer.
Effectively disconnect the connection of defaulter Customer who does not pay the Bill rather than give them chance to pay the
bill. In Pakistan, Most of the Elite people having Industrial Units do not pay the bill in collaboration with the staff of Discos.
Small consumers do not pay bills due to high cost of Electricity due to their financial constraints. Those areas where there is
less collection, Load shedding should be increased there and relief be given to the consumers who are paying the Bill regularly.
Corruption within DISCOS should be eliminated by severe punishments. Some Political figures are also involved in the Nonpayment of Bills by influencing and Threatening the Staff of Discos, therefore proper protection should be provided to the
Honest Staff/officers.
(6) Watchdog effect on users.
Users must aware that the distribution Agency can monitor consumption at its convenience. This allows the company fast
detection of any abnormal consumption due to tampering or by-passing of a meter and enables the company to take corrective
action.
The result is consumer discipline. This has been shown to be extremely effective with all categories of large and medium
consumers having a history of stealing electricity. They stop stealing once they become aware that the utility has the means to
detect and record it.
These measures can significantly increase the revenues of utilities with high non-technical losses.
High-Voltage Direct Current (HVDC) Transmission An emerging trend being considered is high-voltage direct current
(HVDC) lines because of some of the advantages in efficiency. According to an ABB study, HVDC lines provide 25 percent
lower line losses, two to five times the capacity of AC lines at similar voltages, and the ability to precisely control the flow of
power. Historically, the costs have been too high for most transmission operators to consider HVDC as an option, except in a
few long-distance applications. However, with technological improvements and more economical options becoming available,
HVDC may be considered more feasible in the near future.
Improvements in T&D technology include several means, such as enhanced power factor at generation, use of high-tech
material in distribution lines, installation of smart grid through automation, laying underground distribution lines, adjusting
measurement system and placing modern electric meters.
Do capacitive and inductive reactance’s cause losses in transmission lines?
Power loss cannot occur due to the capacitance or inductance present in the transmission line; it can only occur due to the
resistance in the transmission line. The capacitance and inductance present in the line however can trap energy in their
electric fields and magnetic fields.
Elaborating a little more: let us assume that the transmission line has no resistance. That means no power loss. But still the
receiving end power would be less than the sending end power. This difference is because of the energy trapped in the
capacitance and inductance of the transmission line. This trapped energy is coarsely analogous to the minimum balance we
have to maintain in our bank account. The minimum balance cannot be withdrawn and therefore cannot be spent.
TOPIC 58: REDUCE YOUR ELECTRICITY BILL
Energy efficiency is often called alternate energy or an alternate source of energy. Why is that? Five years from now, if we are
able to cool the same amount of food using 20 per cent less electricity, we could say that we gained 20 per cent more electricity
or power capacity, because the 20 per cent savings could then be used by other people, equipment or appliances requiring
electricity.
Globally, energy efficiency is a big component of energy conservation. It allows us to have the same quality of service (lighting,
cooling, heating, entertainment, productivity, mobility etcetera) or comfort level for a smaller amount of energy as technology
advances with time. However, in Pakistan, when people hear the phrase “energy conservation”, they think of turning off the
lights when leaving the room, or not using their air conditioners. These measures are also very important to remember.
However, the biggest challenge we have faced in the Karachi Energy Conservation Awareness campaign are people who say
"Hum kahan bijli zaya karte hai" or "We don't waste electricity". Pakistani consumers don't realise the criminal wastage of
energy from using old or inefficient equipment in a bid to ’save’ money
How much gains have new appliances really made? The biggest example is fridges. Our parents, aunts and uncles still use
fridges that are 20 years old. However, an energy efficient fridge manufactured in 2013 uses 75 per cent less electricity than
the one manufactured 15 years ago.
For a standard household-size, 14 cubic feet combi fridge-freezer means a difference of 1000 kWh per year, or nearly 15,000
rupees per year in electricity bills. By upgrading to a new energy-efficient fridge, you could get your money back in 2-3 years.
And the rest of the savings will go in your pocket.
Regrettably, we do not have access to the most energy-efficient appliances in Pakistan. Even if the world’s most energy
efficient brands like Bosch, Siemens, Electrolux and Daikin are available, they are overpriced, taking into account the energy
savings. However, as consumers, we have to make the best possible decision with the choices we have.
Lighting
Stop thinking of how many Watts of lighting you need! Start thinking of how many Lumens of light you need, since that is the
actual unit for light. For standard lighting (living rooms, bed rooms, corridors etc), 15 lumens/ ft2 is enough. For task lighting
(factories, workshops, etc) you may need up to 50 lumens/ ft2. Office and academic spaces fall somewhere in the middle.
Once you know how many lumens you need, find the type of lighting method that provides you that amount using the least
amount of electricity. The number to look out for is the Efficacy of the bulb, that is, the amount of lumens it provides per Watt
of electricity. This number can be found on the packaging.
The table below shows the example of a small company housed inside a 1000 sq. yd double-storied house.
As you can see there is really no need to pay five times more for incandescent bulbs in your office or household. In fact, for
offices and factories, LFLs (new T5 and T8 tube lights) are the preferred method of lighting, while for your house, use LFLs
or CFLs (energy saver bulbs).
The best part about CFLs, LFL and LEDs is that they not only save electricity in lighting, but they also save money on air
conditioning. An old incandescent 100W bulb would have raised the temperature of a 4x4x4m3 room by five degrees every
hour, heat that your AC would have needed to remove. The corresponding CFL bulb uses just 18W and raises the temperature
of the same room by less than one degree an hour.
• Switch off electrical appliances when not in use
• Measure the size of the room you wish to install an AC in and then ascertain exactly what size of AC you need. A bigger
AC uses more electricity and fails to serve its purpose
• Use energy savers or LFLs (tube lights). They have a longer life and are more efficient
• Perform regular maintenance on your household appliances so that they do not end up consuming more power because of
wear and tear.
• Paint your roof white. This will keep your house cooler as the white colour reflects at least 50% of the sun’s heat
Switch to Light Emitting Diodes (LEDs) or Compact Fluorescent Lamps (CFls)
Switch to Inverter Air Conditioners
Inverter Air Conditioners cost more but they are extremely energy efficient and consume about 40%-60% less electricity than
normal ones This is primarily
because an inverter AC is able to vary the speed of the compressor and does not have to stop and start again to maintain the
set temperature. It is also able to reach the desired temperature much quicker than a normal AC would and is also not susceptible
to temperature fluctuations.
Prices of inverter AC’s vary according to brand for e.g. a locally assembled 1.5 ton Orient AC would cost about PKR 84,000
while a standard Orient of the same size would be in the range of PKR 45,000 to PKR 48,000.An imported brand like LG
would cost approximately 120,000 for a 1.5 ton inverter AC while a standard LG of the same size would be in the range of
PKR 78,000 to PKR 82,000.
Switch off unnecessary lights, fans and air conditioners
TOPIC 59: DIFFERENT TYPE OF MAINTENANCE
Preventive Maintenance: Preventative maintenance is a precautionary approach. It involves regularly performed, planned
tasks that are scheduled based on either time passed or meter triggers. This is done to reduce the possibility of asset failure.
Periodically checking, cleaning and maintaining equipment increases an asset’s life span, making it more reliable. These
scheduled tasks also reduce downtime, directly impacting overall productivity and company profitability.
Predictive Maintenance: Predictive maintenance is the process of anticipating a fault before it happens. Predictive
maintenance is a more active approach. By monitoring key parameters of your system, predictive maintenance seeks, as its
name implies, to predict where failure may occur. In this way, the developing issue can be rectified before it causes any
damage, loss of product, or emergency shutdowns.
Proper predictive maintenance can catch early signs of leaks, system failure, and other issues that, under less proactive
maintenance plans, could go unnoticed for some time. A typical predictive maintenance plan for an air compressor includes
regular inspections, a range of external sensors, and lubrication oil analyzation to track levels of contamination.
Breakdown Maintenance: Breakdown maintenance is maintenance performed on equipment that has broken down and is
unusable.
EXAMPLE:
Imagine you purchased a new economy car three months ago. To get around, you alternate between riding a bike, walking,
and driving your new car. So in the three months since you’ve owned the car, you’ve driven about 1,000 miles on it. However,
the car’s manual says to get the oil replaced every three months or 3,000 miles. Since it’s been three months, you dutifully take
your car to the shop for a costly oil change that is supposed to keep your car running in great condition. This is an example of
preventive maintenance.
Now imagine, you own a luxury car. Maybe it’s a BMW that is equipped with condition-based service indicators for your
engine oil and air filters among other parts. You drive the car six months, put around 5,000 miles on it, and then an alert comes
on that says you have 500 miles left before you must change the oil. This is an example of predictive maintenance. It prevents
breakdowns and gives you service reminders that are a reflection of how much you actually drive your car with enough advance
warning to repair the issue before the machine fails.
TOPIC 60: DELTA VS STAR
1. What is the difference between a star and a delta connection? What happens when it is star connected or delta
connected? Are the circuits designed in our homes star or delta connected?
First of all i want to clear you that Star & Delta Connection are only possible in 3 phase system, So in our domestic system it
is not possible, because generally all house hold electrical equipment are designed with single phase supply.
Next,
Difference between Star & Delta.
Star Connected System:
Star connection is used where we require Neutral terminal to obtain Phase voltage.
In a star connected system VL=√3Vph, mean Phase voltage is root 3 times less than line voltage.
In a star Connected system IL=I phase.
Star connected system require less insulation level.
Star Connected system is used where low starting current is required.
Delta Connected System:
In a Delta Connected system Line Voltage is equal to Phase Voltage.
While phase current is √3 times less than Line current.
Insulation level is high because line voltage = Phase Voltage.
is generally used where high starting Torque is required
2. What type of connection is used at transformers used at generation side and distribution side (start, delta)? Why?
Generator side: The transformers are generally step up transformers with LV winding (i.e. primary) connected in Delta and
HV winding (i.e. secondary) connected in Star.
-The advantages of using transformer's primary in delta are:
1) The line current gets divided by √3 and hence the cross-sectional area of the conductor to be used in each of the three
phases of the primary winding will be reduced. Thus saving in copper.
2) Delta connection provides a path for the third order harmonic current and hence no distortion because of it.
-The advantage of using transformer's secondary in star is:
1) On the secondary side, the line voltage is high and thus using star connection, the phase voltage will be 1/√3 time’s line
voltage. Thus the cost of insulation is saved. Also, the cost of insulation throughout the transmission line is reduced.
Distribution Side: The transformers in distribution side are step down transformers. The HV winding is connected to source
and LV winding is connected to load. Here the LV winding (i.e. secondary) is always star connected to provide a neutral
point to consumers by which 3-phase can be converted to 1-phase. The HV winding (i.e. primary) may be delta or star based
upon the KVA rating of transformers and economical aspects of insulation and cross-sectional area of conductor to be used.
3) Is Distribution transformer Delta Star connected to step down voltage?
At distribution substations the 3-phase transformer used to step down the voltage is connected in star delta.( because line to
line voltage in star is root 3 times the phase voltage whereas the line to line voltage in delta is equal to phase voltage.).
Whereas the transformer near your house is deliberately chosen as DELTA STAR to provide 3 phase 4 wire service to
provide the neutral path for unbalanced loads
TOPIC 61: AC VS DC
Q1) why transmission is done in AC? Why not in DC?
1. AC can be generated at high voltages, but DC cannot be generated at high voltages because sparking starts at the
commutator at high voltage, due to which commutator gets damaged.
2 High voltages AC generators are much simpler and cheaper than DC generators of the same range. It is because in AC
generators there i no commutator which is costly part and is damaged.
Q2) TESLA VS EDISON:
Starting in the late 1880s, Edison developed a cost-effective means of generating DC electricity. However, there was a
problem. There was no way back then to convert the DC voltage to higher or lower values. To be safe for use in homes and
factories, the DC generators were designed to produce electricity at low voltages. The downside was that this meant the losses
during transmission from the generator to the consumer were high. Edison judged that to be an acceptable compromise, but it
limited the distance between the generator and consumers to less than a kilo metre or two.
Tesla believed that alternating current (or AC) was the solution to this problem. Alternating current reverses direction a certain
number of times per second -- 60 in the U.S. -- and can be converted to different voltages relatively easily using a transformer.
Edison, not wanting to lose the royalties he was earning from his direct current patents, began a campaign to discredit
alternating current. He spread misinformation saying that alternating current was more dangerous, even going so far as to
publicly electrocute stray animals using alternating current to prove his point.
Today our electricity is still predominantly powered by alternating current, but computers, LEDs, solar cells and electric
vehicles all run on DC power. And methods are now available for converting direct current to higher and lower voltages. Since
direct current is more stable, companies are finding ways of using high voltage direct current (HVDC) to transport electricity
long distances with less electricity loss.
So it appears the War of the Currents may not be over yet. But instead of continuing in a heated AC vs. DC battle, it looks like
the two currents will end up working parallel to each other in a sort of hybrid armistice.
And none of that would be possible without the genius of both Tesla and Edison.
Q3) what is the difference between AC and DC current?
In an AC, or alternating current, electrical current periodically reverses direction. With a DC, or direct current, the current
flows only in one direction.
AC can be generated using a device called an alternator. This device is a special type of electrical generator designed
to produce alternating current.
AC can come in a number of forms, as long as the voltage and current are alternating. The most common type of AC is
the sine wave.
DC:
Direct Current (DC), which is a constant stream of electrons in one direction. DC provides a constant voltage or current.
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DC can be generated in a number of ways:
An AC generator equipped with a device called a commutator can produce direct current
Use of a device called a rectifier that converts AC to DC
Batteries provide DC, which is generated from a chemical reaction inside of the battery
Everything that runs off of a battery, plugs in to the wall with an AC adapter, or uses a USB cable for power relies
on DC. Examples of DC electronics include: cellphones, radio, TV, computer or any electronic gadgets.
TOPIC 62: POWER SYSTEM PROTECTION
Q1) what is relay?
Relay
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A relay is an Electrical Switch which is used to open or close a circuit with electromechanical relay or electronically.
It is generally used to switch lower current.
It changes their states when energies, from No (Normally Open) to Close or from NC (Normally Close) to Open.
There are two types of relay
1. Electromechanical Relays: In electromechanical relays, contacts are opened or closed by a magnetic force.
2. Solid State Relays: With solid-state relays (SSR), there are no contacts and switching is totally electronic.
Q2) what is over current protection?
Overcurrent relay is a sensing relay, which operates when the current increases beyond the operating value of the relay.
Depending upon the time of operation, overcurrent relays may be categorized as instantaneous over current relay, inverse time
overcurrent relay, definite time overcurrent relay, inverse definite time overcurrent relay, very inverse overcurrent relay and
extremely inverse overcurrent relay.
Q3) what is impedance or distance protection?
An impedance relay works by measuring the voltage and current present at the line terminal where the relay is located. The
line impedance is calculated by taking the ratio of the voltage to the current: Z=V/IZ=V/I. This measured impedance is then
compared to the characteristic impedance of the line. When a fault occurs on the protected portion of the line, the voltage dips
while the current increases. The impedance ZZ is thereby reduced while the fault is present. When the measured impedance
falls below a predetermined setting based on the characteristic line impedance, the relay issues a trip command to open the
circuit breaker protecting line and this action clears the fault.
Q4) what is differential protection?
Differential relay: - A two-winding relay that operates when the difference between the currents / Voltage in the two windings
reaches a predetermined value is called differential relays.
There are three fundamental systems of differential or balanced protection:
I. current differential relay
II. voltage differential relay
III. Biased beam relay or percentage differential relay .
Uses:
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Transformer Protection.
Generator Protection.
Bus-bar Protection.
Motor Protection.
Q5) what are the protection system used in transformers?
The protection used in transformers areBucholz relay- For tripping in case transformer has internal fault.
WTI alarm and trip- Winding temperature indicator. It gives alarm and trips when temperature of the transformer winding
increases beyond specified limit.
OTI alarm
- Oil temperature indicator. It gives alarm when temperature of the oil increases beyond specified limit.
PRV- Pressure Relieve Valve.- To release pressure from the tank.
MOG- Magnetic Oil Gauge- It gives alarm when the oil level falls below the specified limit in the conservator tank.
Q9) what is protection of bus bar?
When the fault occurs on the bus bars whole of the supply is interrupted, and all the healthy feeders are disconnected. The
majority of the faults is single phase in nature, and these faults are temporary. The bus zone fault occurs because of various
reasons likes failure of support insulators, failure of circuit breakers, foreign object accidentally falling across the bus bar,
etc., For removing the bus fault, all the circuits connecting to the faulty section needs to be open.
The most commonly used schemes for bus zone protection are:
Backup protection
Differential Overcurrent Protection
a. Circulating current protection
b. Voltage Overvoltage Protection
3. Frame leakage protection.
1.
2.
9b) Define – Operating Time of a relay
Operating Time of a relay is defined as the time interval between the occurrence of fault and closure of relay contact.
10. Define – Resetting Time of a relay
Resetting Time of a relay is defined as the time which elapses between the instant when the actuating quantity becomes less
than the reset value to the instant when the relay contact returns to its normal position.
11. What is meant by time setting multiplier in protective relaying?
The operating time of the relay depends upon the distance between the moving contact and the fixed contact of the relay. The
distance between the contacts is adjusted by the movement of the disc back stop which is controlled by rotating a knurled
moulded wheel at the base of the graduated time multiplier scale. This is known as time multiplier setting
12) What is meant by time-graded system of protection?
Time-graded system of protection is a non-unit type of protection which is used when a time-lag can be permitted and
instantaneous operation is not necessary. This is also called a backup protection to the primary main protection.
13) What are the main safety devices used for a transformer protection?
The main safety devices used for a transformer protection are
1. Buchholz relay
2. Pressure relief value
3. HRC fuses
4. Horn gaps
5. R-C surge suppressors
6. Surge arresters
7. Percentage differential protection
8. Over fluxing and over voltage protection
9. Over current protection
10. Earth fault protection
11. Thermal overload relay
14) What are the various types of transformer faults?
The various types of transformer faults are
1. Incipient fault
2. Internal fault
3. Phase-to-phase and phase-to-ground fault
4. Saturation of magnetic circuit
5. Earth fault
6. Through fault
7. Overloading
8. High voltage surges due to lightning
15) What is an over fluxing protection in a transformer?
Power transformers are designed to withstand the ratio of its applied voltage with respect to frequency (V/f) continuously,
where V is the normal highest rms voltage applied and fn is its frequency. High V/f can occur in a transformer if full excitation
is applied to generator before reaching its full synchronous speed. Over fluxing protection in a transformer blocks increasing
excitation current of a generator before it reaches its full speed.
16) What are the uses of Buchholz relay?
The uses of Buchholz relay are
1. To protect the transformer from incipient faults below the oil level, which results in the decomposition of oil
2. To give advance warning and prevent the transformer from short circuit condition and subsequent damage
17) What are the various faults to which a turbo alternator is likely to be subjected?
The various faults to which a turbo alternator is likely to be subjected are
1. Thermal overloading
2. Unbalanced loading
3. Stator winding faults
4. Field winding faults
5. Over voltages
6. Loss of synchronism
7. Over speeding
8. Vibration
9. Excessive bearing temperature
10. Wrong synchronization
18) What is the importance of bus bar protection?
The busbar protection is important because of the following reasons
1. Fault level at busbar is very high
2. The stability of the system is affected by fault in bus zone.
3. The fault in a busbar causes discontinuation of power to a large portion of the system.
17) What are the types of protections used for bus-bars?
The types of protections used for bus-bars are
1. Over current protection
2. Differential protection
3. Earth fault protection
4. Over voltage protection
5. Surge voltage protection
18) What is meant by a power swing?
During switching of lines, larger loads or generators, surges of real power and reactive power flow through the transmission
lines causing oscillations in the voltage and current waves. These oscillations are called power swings
19) what is difference between unit and non-unit protection?
Unit type schemes protect a specific area of the system, i.e., a transformer, transmission line, generator or bus bar. In these
schemes, the effects of any disturbance or operating condition outside the area of interest are totally ignored and the
protection must be designed to be stable above the maximum possible fault current that could flow through the protected
area.
The non-unit schemes, while also intended to protect specific areas, have no fixed boundaries. As well as protecting their
own designated areas, the protective zones can overlap into other areas. While this can be very beneficial for backup
purposes, there can be a tendency for too great an area to be isolated if a fault is detected by different non unit schemes.
20) Define – Energizing Quantity
Energizing quantity is defined as the current, voltage or frequency which is used to operate the relay under abnormal condition.
21) Transformer protection:
It is common practice to provide Buchholz relay protection to all 0.5 MVA and above transformers. While for all small
size distribution transformers, only high voltage fuses are used as main protective device. Differential protection should be
provided in the transformers rated above 5 MVA.
A transformer generally suffers from following types of transformer fault1. Over current due to overloads and external short circuits,
2. Terminal faults,
3. Winding faults,
4. Incipient faults.
All the above mentioned transformer faults cause mechanical and thermal stresses inside the transformer winding and its
connecting terminals. Thermal stresses lead to overheating which ultimately affect the insulation system of transformer.
Deterioration of insulation leads to winding faults. Sometime failure of transformer cooling system, leads to overheating of
transformer. So the transformer protection schemes are very much required.
The general winding faults in transformer are either earth faults or inter-turns faults. Phase to phase winding faults in a
transformer is rare. The phase faults in an electrical transformer may be occurred due to bushing flash over and faults in tap
changer equipment. Whatever may be the faults, the transformer must be isolated instantly during fault otherwise major
breakdown may occur in the electrical power system.
Incipient faults are internal faults which constitute no immediate hazard. But it these faults are over looked and not taken care
of, these may lead to major faults. The faults in this group are mainly inter-lamination short circuit due to insulation failure
between core lamination, lowering the oil level due to oil leakage, blockage of oil flow paths. All these faults lead to
overheating. So transformer protection scheme is required for incipient transformer faults also. The earth fault, very nearer to
neutral point of transformer star winding may also be considered as an incipient fault.
The following discusses protection devices typically delivered as a part of the power transformer delivery.
1. Buchholz (Gas) Relay
2. Pressure Relay
3. Oil Level Monitor Device
4. Winding Thermometer
Remember that a differential relay is basically an instantaneous overcurrent relay that operates on the difference of current
flowing into and out of the protected zone.
TOPIC 63: MACHINE DESIGN
Q1) what are the factors involved while designing a machine?
When the designer designs the elements of the machine or the complete machine, they have to consider several important
parameters. Here are some of the important factors to be considered while doing machine design
Cost: Cost has always been the major factor of consideration while designing the machine elements or machine and in this
age of competition it has become more important. The best machine design is the one which helps get the finished product
with all the major functionalities and highest possible quality at the lowest possible cost
2) High output and efficiency: Earlier machines used to be very heavy and consume lots of power. Now the trend is of full
functional machines consuming low power and giving high output
3) Strength: The machine elements or the machine should be strong enough to sustain all the forces it is designed for so that
it is not damaged or permanently deformed during its life time
1)
Q2) what is electrical circuit?
An electrical circuit is a path or line through which an electrical current flows.
Q3) what is a magnetic circuit?
The magnetic circuit is the path of magnetic flux. The mmf of the circuit creates flux in the path against the reluctance of the
path. The equation which relates flux, mmf and the reluctance is given by, Flux = mmf/reluctance.
Q4) what is eddy current? How it is produced and why lamination is done?
Eddy current is a useless current whose path is not fixed and most of the times it flows in core's material through entire area
and leads to loss. When metal piece is held in changing magnetic field or a metal piece is moved in steady magnetic field,
Eddy currents induce in metal. Metal resistance is very small therefore, large current flows and metal piece heats up instantly.
Laminations are provided to reduce eddy currents. In a transformer core eddy currents are produced which heats up the core
and also causes losses by distorting the main current. If the core were a single piece its area would be high, and hence low
resistance which causes a larger current to flow increasing the losses. The eddy current loss is proportional to the square of the
diameter of the core. Larger the diameter, more the eddy current loss
Q5) why iron core is thin & laminated in transformer
The iron core transformer has higher permeability thus it is applied in transformer in place of air core in modern transformer.
But a solid iron core has some own disadvantage due to some losses. Thus to reduce the losses solid core is not used in
transformer. Thin & laminated iron core is stacked up to form the complete core. This individual core are electrically separated
from each other as this thin coating layer is an insulating material but let passes the magnetic flux.
Q6) Write any two similarities between magnetic and electric circuits.
1. In electric circuit the emf circulates current in a closed path. Similarly in a magnetic circuit the mmf creates the flux in a
closed path.
2. In electric circuit the flow of current is opposed by resistance of the circuit. Similarly in magnetic circuit the creation of flux
is opposed by reluctance of the circuit.
Q7) Write any two essential differences between magnetic and electric circuit.
1. When the current flows in electric circuit the energy is spent continuously, whereas in magnetic circuit the energy in
needed only to create the flux but not to maintain it.
Q8) what is demagnetization?
A suitably intense magnetic field applied in a direction opposite to that of the existing magnetization will serve to reduce or
destroy that magnetization. Demagnetization is the process by which a magnet loses its magnetic properties.
Q9) what is di electric?
A material, as rubber, glass, etc., or a medium, as a vacuum, gas, etc. that does not conduct electricity but can sustain an electric
field: dielectrics are used in capacitors, between adjacent wires in a cable, etc.
Q10) why thickness of insulation depends on voltage?
The dielectric strength of a material is expressed as Volts/meter and gives the maximum electric field strength that it can
withstand intrinsically without breaking down. Therefore, the thicker the layer of insulator, the higher is the breakdown voltage.
Insulating materials generally have atoms with tightly bound electrons thus restricting electron flow. But, practically speaking,
at sufficiently high voltages electron flow occurs in insulators. Thus with a high enough applied voltage, electrons can be freed
from the atoms of insulating materials, resulting in current through that material.
Q11) what is difference between rotational friction and sliding friction?
Rolling friction is the resistance to motion experienced by a body when it rolls upon another. It is much less than sliding
friction for same pair of bodies. When one body rolls upon another, there is theoretically no sliding or slip between them. And
if both are perfectly rigid, there is no surface of contact
TOPIC 64: INVERTER
Electronic inverters can be used to produce this kind of smoothly varying AC output from a DC input. They use electronic
components called inductors and capacitors to make the output current rise and fall
Inverters can also be used with transformers to change a certain DC input voltage into a completely different AC output
voltage (either higher or lower) but the output power must always be less than the input power: it follows from
the conservation of energy that an inverter and transformer can't give out more power than they take in and some energy is
bound to be lost as heat as electricity flows through the various electrical and electronic components. In practice, the
efficiency of an inverter is often over 90 percent.
Circuit:
WORKING:
When S1 and S2 are closed, the voltage across the load, V0 would be positive (the same polarity as assumed in the above
circuit) and after a while, S1 and S2 are opened, and S3 and S4 are closed. Now trace the path of the current and you will
find that the current through the load has reversed. The polarities of the voltage across the load are now reversed (opposite to
the polarity assumed across the load). The ON duration of each set of inverters can be varied by varying the firing angle of
the switches.
Here, the output is a square waveform. When passed through filters to eliminate harmonics, you get a sinusoidal waveform
OUTPUT WAVEFORM
An inverter can produce a square wave, modified sine wave, pulsed sine wave, pulse width modulated wave (PWM) or sine
wave depending on circuit design. The two dominant commercialized waveform types of inverters are modified sine wave
and sine wave.
SQUARE WAVE
This is one of the simplest waveforms an inverter design can produce and is best suited to low-sensitivity applications such
as lighting and heating. Square wave output can produce "humming" when connected to audio equipment and is generally
unsuitable for sensitive electronics.
APPLICATIONS
UNINTERRUPTIBLE POWER SUPPLIES
An uninterruptible power supply (UPS) uses batteries and an inverter to supply AC power when mains power is not
available. When mains power is restored, a rectifier supplies DC power to recharge the batteries.
ELECTRIC MOTOR SPEED CONTROL
Inverter circuits designed to produce a variable output voltage range are often used within motor speed controllers
POWER GRID
Grid-tied inverters are designed to feed into the electric power distribution system.
HVDC POWER TRANSMISSION
With HVDC power transmission, AC power is rectified and high voltage DC power is transmitted to another location. At the
receiving location, an inverter in a static inverter plant converts the power back to AC. The inverter must be synchronized
with grid frequency and phase and minimize harmonic generation.
WHY IS IT NOT SUITABLE TO USE A SQUARE WAVE POWER INVERTER IN A HOUSE AS COMPARED TO SINE WAVE
INVERTER?
Harmonics. Sine wave inverter has lesser harmonics compared to a square wave inverter. Apart from the fundamental
frequency, square wave inverters have odd frequency components. These harmonics causes machine flux to be saturated,
thus leading to poor performance of the machine, sometimes even damaging them. Since most of the household appliances
have transformers (Refrigerator-Laptop charger) or induction machines (electrical fans), it is imperative that they be run with
sine wave inverters.
DIFFERENCE BETWEEN CONVERTER AND INVERTER
The main difference between converters and inverters is what they do to the voltage. An inverter changes a DC voltage into
an AC voltage and either increases or decreases it into the appropriate level. In comparison, a converter changes the voltage
level but does not change its type; so an AC voltage would still be AC and a DC voltage would still be DC.
In general, a converter is a transformer to "convert" the voltage from one value to another. The voltage may typically convert
from 120 VAC to 220 VAC or vice versa.
DIFFERENCE BETWEEN UPS AND INVERTER:
UPS – In UPS, the AC from the main power is transformed to DC. This DC is continuously charging the battery. The DC
output from the battery is fed to a sine waveform inverter that converts it into AC and supplies it to the equipment.
The energy is regularly drawn from battery, and hence there no gap during the outage on the main. The battery stops getting
charged now, but the UPS continues its power supply till the battery gets completely discharged.
Inverter – The main power AC is supplied to the inverter, and it is transformed into DC simultaneously, which keeps
charging the battery continuously. There is a sensor and relay structure that always monitors the ON or OFF status of the
main.
As soon as there is a power failure, the relay actuator activates the inverter switch. Every other action is similar to the UPS,
but because of the sensor and relay process, there is a delay while activating the switch.
WORKING OF DC INVERTER AC:
DC inverter air conditioners make use of a variable frequency drive to regulate the motor's speed, thereby controlling the
speed of compressor too.
This variable frequency drive includes a rectifier that converts the incoming Alternating Current (AC) to Direct Current
(DC). It then utilizes Pulse Width Modulation (PWM) of the DC in the inverter to generate AC of desired frequency. The
alternating current is used to drive an induction motor or a brushless motor.
Since the frequency of the alternating current and the speed of induction motor are directly proportional to each other, the
compressors in an inverter window air conditioner unit operate at various speeds.
The current ambient air temperature is then sampled by using a microcontroller and then the compressor's speed is adjusted
appropriately.
The fixed speed compressor in a standard air conditioner runs at 100% capacity when it is started, but an inverter unit starts
at a low level and then progressively enhances its capacity, depending on the requirement to heat up the room or cool it
down. Fixed speed compressors start or stop automatically in order to sustain the desired temperature, but an inverter unit
regulates the capacity of the compressor instead.
TOPIC 65: VOLTAGE FLUCTUATION
Voltage fluctuations can be described as repetitive or random variations of the voltage envelope due to sudden changes in the
real and reactive power drawn by a load
CAUSES:
Voltage fluctuations are caused when loads draw currents having significant sudden or periodic variations. The fluctuating
current that is drawn from the supply causes additional voltage drops in the power system leading to fluctuations in the supply
voltage. Loads that exhibit continuous rapid variations are thus the most likely cause of voltage fluctuations. Examples of loads
that may produce voltage fluctuations in the supply include
• Arc furnaces
• Arc welders
• Installations with frequent motor starts (air conditioner units, fans)
• Motor drives with cyclic operation (mine hoists, rolling mills)
• Equipment with excessive motor speed changes (wood chippers, car shredders)
Often rapid fluctuations in load currents are attributed to motor starting operations where the motor current is usually between
3-5 times the rated current for a short period of time. If a number of motors are starting at similar times, or the same motor
repeatedly starts and stops, the frequency of the voltage changes may produce voltage fluctuation
It will mainly due to over load in the transformer from which power is taken to your house.
VOLTAGE SWELLS
Voltage increases above the nominal value can be destructive. In many cases, voltage swells will cause fuses or circuit breakers
to pop, but generally you can't rely alone on protection from MCBs (miniature circuit breakers) as they may not trip (disconnect
the circuit) before damage is done. Voltage surges can occur because of instability in the electricity network.
VOLTAGE SURGES
Unlike voltage swells, surges are sharp and can be several thousand volts for very short periods - these are particularly
destructive. Surges are usually caused by lightning, but the source could be some large distance from your home. The voltages
are often ‘induced’ - that is, they are not the result of a direct strike, but of a nearby
TOPIC 66: PROTECTION VS MEASURING CT
Current Transformers (CT) are used for current metering and protection in high voltage network systems. They transform the
high current on the high voltage side into low current (1 or 5 A) adequate to be processed in measuring and protection
instruments (secondary equipment, such as relays and recorders).
Protection CT's are meant to protect an electrical installation in case of overcurrent or short circuit and their operating current
range is above nominal current In
A metering CT carries the full load current and are designed near the saturation point. Whereas the basic purpose of the
protection CT is to provide protection in case of any fault. A fault current is 10 times more than the usual full load current.
Hence the protection CT carries the fault current and are designed much below the saturation point.
The protection CT has thicker core than metering CT.
1) Normally the difference between protection CTs and metering CTs is the core construction and specifically, the size (cross
sectional area) used. Protection cores are generally larger in cross sectional area than metering cores. The larger cross-section
gives a lower nominal operating flux density, meaning that the protection cores can handle a higher current level before
saturating.
TOPIC 67: LOAD FACTOR/ DEMAND FACTOR/ DIVERSITY FACTOR
Q1) What is load factor, diversity factor, demand, maximum demand, load curve?
Load Factor
It is ratio of average load to maximum demand during certain period of time (e.g. day/month/year) is called load factor.
Following formula is used to calculate Load Factor.
Load factor = Average Demand/Maximum Demand
Since average load is always less than maximum demand, hence load factor is always less than unity.
Demand Factor
The ratio of actual maximum demand on the system to the total rated load connected with the system is called demand
factor. It is always less than unity. Following formula is used to calculate demand factor.
Demand Factor = Maximum Demand/Connected Load
Diversity Factor
The ratio of sum of individual maximum demand of all the consumers supplied by it to maximum demand of power station is
called diversity factor. Following formula is used to calculate diversity factor.
Diversity Factor = (Sum of Individual Max. Demand)/(Max. Demand of Power Station)
TOPIC 68: INTRODUCTION TO POWER SYSTEM PROTECTION
Q1) what is power system protection?
Power-system protection is a branch of electrical power engineering that deals with the protection of electrical power systems
from faults through the isolation of faulted parts from the rest of the electrical network. The objective of a protection scheme
is to keep the power system stable by isolating only the components that are under fault, whilst leaving as much of the network
as possible still in operation. Failures may occur in each part, such as insulation failure, fallen or broken transmission
lines, incorrect operation of circuit breakers, short circuits and open circuits
Q2) what are faults in power system?
Fault is an abnormal condition which occurs in our system due to which equipment failure or damage is possible. During fault
conditions, system parameters such as current and voltages deviates from their normal operating range.
Q3) what are series and shunt faults?
SHUNT FAULTS are simple short circuit and they occurs when phases comes in contact with each other due to failure of
insulation. During short circuit condition a short path is developed having a low impedance and high amount of current flows.
Failure of insulation can occur due to continuous over voltages, lightning strike or aging of insulation. Voltages directly affect
insulation and the more the voltage, the more should be the insulation.
SERIES FAULTS are open circuit. These faults occur due to the failure of one or more conductors. The most common causes
of these faults include joint failures of cables and overhead lines, and failure of one or more phase of circuit breaker and also
due to melting of a fuse or conductor in one or more phases.
Consider that a transmission line is working with a balanced load before the occurrence of open circuit fault. If one of the
phase gets melted, the actual loading of the alternator is reduced and this cause to raise the acceleration of the alternator,
thereby it runs at a speed slightly greater than synchronous speed. This over speed causes over voltages in other transmission
lines.
Q4) what are interconnected power system? Its advantages and disadvantages?
Interconnected power systems are those in which multiple EHV lines are joined together or in which many generating sources
are present.
Major advantage of power system interconnection is their reliability i.e. if any of the source gets disconnected then supply can
be continued from other sources. Economical operation can be ensured by sharing of load among the stations is arranged in
such a way that more efficient stations work continuously throughout the year at a high load factor and the less efficient plants
work for peak load hours only.
Disadvantage of interconnected power system is that fault level of the system increases as more & more power sources are
connected. In case of any fault at any point, heavy fault current flow which are fed from the sources, causing heavy voltage
dip in the system
Similarly cascaded failure is another problem in interconnected power system
Q5) what are the desirable attributes of protection relays?
Relays should be designed on the basis of RSSSE principle where:
R= Reliability
S= Selectivity
S= Speed
S= Sensitivity
E= Economy
1) Reliability: Reliability or dependability means that relay should only operate under fault condition and not in case of
normal operating condition. There are many situations which can trick relay such as:
When motor starts, it draws inrush current which is much higher than the rated current but remember that it is not a fault
condition. It is actually the way motor starts. It’s a normal operating condition of motor. So, relay should be able to distinguish
between normal operating conditions and fault condition.
2) Selectivity: Relay should know where the fault has occurred and should only trip the breaker close to the fault. It should
not unnecessarily trip other circuit breakers in the system
3) Sensitivity: Relays should be able to detect all kinds of faults in the system. It should detect the smallest possible fault
current. The smaller the current that it can detect, the more sensitive it is
4) Speed: To maximize safety, and minimize equipment damage and system instability, a fault should be cleared
as quickly as possible. This implies that relay should quickly arrive at a decision and circuit breaker operation
should be fast enough.
5) Economy: Relays should be made economical.
Q6) what is a relay?
Formally, a relay is a logical element which processes the inputs (mostly voltages and currents) from the system/apparatus
and issues a trip decision if a fault within the relay's jurisdiction is detected. To monitor the health of the apparatus, relay
senses current through a current transformer (CT), voltage through a voltage transformer (VT). VT is also known as
Potential Transformer (PT).
A relay R1 is used to protect the transmission line under fault F1.The relay element analyzes these inputs and decides whether
(a) there is a abnormality or a fault and (b) if yes, whether it is within jurisdiction of the relay. The jurisdiction of relay R1 is
restricted to bus B where the transmission line terminates. If the fault is in it's jurisdiction, relay sends a tripping signal to
circuit breaker (CB) which opens the circuit. A real life analogy of the jurisdiction of the relay can be thought by considering
transmission lines as highways on which traffic (current/power) flows. If there is an obstruction to the regular flow due to fault
F1 or F2, the traffic police (relay R1) can sense both F1 and F2 obstructions because of resulting abnormality in traffic (power
flow). If the obstruction is on road AB, it is in the jurisdiction of traffic police at R1; else if it is at F2, it is in the jurisdiction
of R2. R1 should act for fault F2, if and only if, R2 fails to act. We say that relay R1 backs up relay R2. Standard way to obtain
backup action is to use time discrimination i.e., delay operation of relay R1 in case of doubt to provide R2 first chance to clear
the fault.
Q7 What is a circuit breaker?
A Circuit Breaker (CB) is basically a switch used to interrupt the flow of current. It opens on relay command. The relay
command initiates mechanical separation of the contacts. It is a complex element because it has to handle large voltages (few
to hundreds of kV's) and currents (in kA's). Interrupting capacity of the circuit breaker is therefore expressed in MVA.
Power systems under fault behave more like inductive circuits. X/R ratio of lines is usually much greater than unity. For 400
kV lines, it can be higher than 10 and it increases with voltage rating. From the fundamentals of circuit analysis, we know
that current in an inductive circuit (with finite resistance) cannot change instantaneously. The abrupt change in current, if it
happens due to switch opening, will result in infinite di/dt and hence will induce infinite voltage. Even with finite di/dt, the
induced voltages will be quite high. The high induced voltage developed across the CB will ionize the dielectric between its
terminals. This results in arcing. When the current in CB goes through the natural zero, the arc can be extinguished
(quenched). However, if the interrupting medium has not regained its dielectric properties then the arc can be restruck. The
arcing currents reduce with passage of time and after a few cycles the current is finally interrupted.
Usually CB opening time lies in the 2-6 cycles range. CBs are categorized by the interrupting medium used. Minimum oil,
air blast, vacuum arc and SF6 CBs are some of the common examples. CB opening mechanism requires much larger power
input than what logical element relay can provide. Hence, when relay issues a trip command, it closes a switch that energizes
the CB opening mechanism powered by a separate dc source (station battery). The arc struck in a CB produces large amount
of heat which also has to be dissipated.
Q8) What are the different relays that employed for protection of apparatus and transmission lines?
Answer: The relays that are usually employed for protection of transmission lines include









Over current relay
Directional relay
Distance relay
Under Voltage relay
Under-frequency relay
Thermal relay
Differential relay
Phase sequence relays
pilot relays
TOPIC 68: LOADSHEDDING IN PAKISTAN.
Pakistan did not have the problem of load shedding in the early 2000s as long as I remember. Were people more pious those
days? Or were Pakistanis less corrupt those days? Or was it because we were a better nation back then?
The biggest cause of load shedding in Pakistan is wrongly proportioned Energy mix. In simpler terms it means that WE ARE
GENERATING VERY EXPENSIVE ELECTRICITY. More than 31% of our electricity is produced using furnace oil.
Total installed generation capacity of Pakistan is ~ 25000 MW [1] where as this year the peak demand was around 20300
MW.[2] This shows that there is enough capacity to generate even surplus electrical power to meet the load demand. However
we all witnessed that there was an acute power shortage that kept fluctuating between 5000 MW to 7000 MW[3] during this
year’s peak load season. Further note that Pakistan is also planning to add another 5770 MW[4]into the system by mid 2018 by
both conventional and alternate energy means. Government of Pakistan (GOP ) and other DISCOs are taking all possible
measures to deal with this issue but you know it is a two-way street. Active consumer participation is also required to put the
Genie back in the bottle.
Ever heard of Circular Debt? IPPs buy furnace oil and other form of fossil fuel from GOP to produce power at their own.
DISCOs according to the certain predefined pacts purchase this generated power to augment their power pool and supply
power from this pool to the costumers. For this purchased power DISCOs have to pay back to IPPs who in turn would pay
back the GOP. So they all are dependent on DISCOs to recover their money from their costumers and pay it to IPPs who would
pay back the GOP in order to keep the wheel spinning.
But what if DISCOs are not recovering what they had invested? Here comes the problem and Genie comes out of the bottle
and wheel stops spinning. This all then becomes liability of GOP who would then shrink fuel supply to IPPs (in order to reduce
circular debt) which will produce less power and as a result of this we will face power outage. Hence a good business from
DISCOs and their increased profitability due to improved recovery ratio and reduced losses we can keep the wheel spinning
and enjoy the perks of uninterrupted power supply.
Unfortunately DISCOs here are not doing enough business due to variable on ground realities, this situation makes it difficult
to keep on producing power up to the required limit as a result we have to face controllable load shedding.
Yes, it can be controlled if every common man think about saving energy. System in our country needs to be changed and it is
not responsibility of only government and electricity suppliers but people needs to co-operate with them.
For example: You will see during day time street lights are “ON”, not only this, but people in little cities and villages they do
not pay bills and always steal electricity in short “Kunda” system. Now will they ever think of saving electricity?
That is the reason there is lot of load-shedding in villages even in cities because WAPDA and NTDC thinks that they cannot
control people from stealing electricity therefore it is better to do load-shedding.
On the other side, if you take example of Clifton area of Karachi, there is almost no load-shedding? Why? Because people are
paying bills and revenue generation in posh areas is very high.
Furthermore, if our government do proper investment in electricity sector such as installation of solar and wind plants all over
the country can support the generation of electricity too.
The generation cost each KWH of electricity produced using Furnace oil is more than Rs. 10.7
The other 26% is produced using gas at more than Rs. 6 per KWH.
Expensive generation increases the rate of electricity. When consumers get high bills they are less inclined to pay. This reduces
recovery of electricity bills. The power companies cannot recover enough money to keep purchasing electricity from the
IPPs(independent power producing companies). When IPPs don’t get money, they can’t purchase oil to generate electricity.
This reduces power generation.
Ever heard of the word circular debt. Well, this is the money owed by the government to the IPPs or oil suppliers. Whenever
the government pays off the circular debt, IPPs produce electricity and load shedding is either reduced or completely
eliminated.
As long as we keep generating expensive electricity, the problem of circular debt wont end.
Solution:
The most obvious solution is to use cheaper electricity. Hydel energy is the cheapest. Each unit of electricity generated
by Tarbela dam costs merely Rs. 1.5 only.





Build more dams.
Use nuclear energy.
Make use of local coal ( instead of imported coal)
Utilize renewable energy.
And lastly, Conserve energy. We can save upto 1000 MW by merely being more conservative in the use of
electricity. Don’t waste it. Whether it is you or your school, college, university, academy , office or home;
whether it is you or someone else who pays the bill, use electricity miserly. 1000 MW saved Is equivalent to 1000
MW generated
TOPIC 69: MAGNETIC CONTACTOR
A contactor is an electrically controlled switch used for switching an electrical power circuit, similar to a relay except with
higher current ratings.
Contactors are used to control electric motors, lighting, heating, capacitor banks, thermal evaporators, and other electrical
loads.
We can also said that a contactor is type of electrical relay which we use for power electric motor to start or
stop. Contactor make easy to controlled 3 phase induction motors. We know that 3 phase motor required three phase supply
to run, 3 phase supply is two dangerous which we cannot handle with a circuit breakers for induction motor. Contactor make
easy to handle the 3 phase motor controlling. The parts of the magnetic contactor are coil, iron core, 3 contacts points and
NC, NO points.
WORKING OF MAGNETIC CONTACTOR:
The function of a contactor or motor starter is simple, when the AC supply goes to the contactor coil, the coil make a strong
magnetic field and field pulls the iron core to the coil and make an electrical contacts.
DIFFERENCE BETWEEN MAGNETIC CONTACTORS AND RELAYS:
Contactors and Relays perform the same task of switching a circuit. If you see from the application point of view, you would
have seen contactors placed in control panels of industrial motors or other heavy loads. Whereas, relays are used in low
voltage applications such as switching a LED or tube-light or even actuating a circuit breaker.
Both are electromagnetic switches and operate under similar principles. The difference comes if we see from the application
perspective. Contactors are used for high voltage switching purposes whereas relays are used for low voltage switching.
TOPIC 70: RLCB VS ELCB
Difference between RCCB and ELCB: They both are same. ELCB is the old name and often refers to voltage operated
devices that are no longer available. RCCB or RCD is the new name that specifies current operated (hence the new name
to distinguish from voltage operated).

The new RCCB is best because it will detect any earth fault. The voltage type only detects earth faults that flow back
through the main earth wire so this is why they stopped being used.
 The easy way to tell an old voltage operated trip is to look for the main earth wire connected through it.
 RCCB will only have the line and neutral connections.
 ELCB is working based on Earth leakage current. But RCCB is not having sensing or connectivity of Earth, because
fundamentally Phase current is equal to the neutral current in single phase. That's why RCCB can trip when the both
currents are deferent and it withstand up to both the currents are same. Both the neutral and phase currents are
different that means current is flowing through the Earth.
 RCD does not necessarily require an earth connection itself (it monitors only the live and neutral). In addition, it
detects current flows to earth even in equipment without an earth of its own.
This means that an RCD will continue to give shock protection in equipment that has a faulty earth. It is these
properties that have made the RCD more popular than its rivals. For example, earth-leakage circuit breakers (ELCBs)
were widely used about ten years ago. These devices measured the voltage on the earth conductor; if this voltage was
not zero this indicated a current leakage to earth. The problem is that ELCBs need a sound earth connection, as does the
equipment it
TOPIC 71: LOAD CELL, CHARGE CONTROLLER, THERMAL OVERLOAD RELAY
1) Load cell: A load cell is a transducer which converts force into a measurable electrical output. Although there are many
varieties of load cells, strain gage based load cells are the most commonly used type
2) Charge controller: A charge controller, charge regulator or battery regulator limits the rate at which electric current is
added to or drawn from electric batteries. It prevents overcharging and may protect against overvoltage, which can reduce
battery performance or lifespan, and may pose a safety risk. Charge controller is found in dotting machine at PSL. It’s of 48v
and 200 amperes.
4) Thermal Overload relay: Thermal overload relays are protective devices. They are designed to cut power if the motor
draws too much current for an extended period of time. To accomplish this, thermal overload relays contain a normally closed
(NC) relay. When excessive current flows through the motor circuit, the relay opens due to increased motor temperature, relay
temperature, or sensed overload current, depending on the relay type.
Thermal overload relays are similar to circuit breakers in construction and use, but most circuit breakers differ in that they
interrupt the circuit if overload occurs even for an instant. Thermal overload relays are conversely designed to measure a
motor's heating profile; therefore, overload must occur for an extended period before the circuit is interrupted.
The basic working principle of thermal relay is that, when a bimetallic strip is heated up by a heating coil carrying over
current
of
the
system,
it
bends
and
makes
normally
open
contacts.
TOPIC 72: ELECTRICITY SECTOR OF PAKISTAN
Electricity in Pakistan is generated, transmitted, distributed, and retail supplied by two vertically integrated public sector
utilities: Water and Power Development Authority (WAPDA) for all of Pakistan (except Karachi), and the Karachi Electric (KElectric) for the city of Karachi and its surrounding areas. There are around 42 independent power producers (IPPs) that
contribute significantly in electricity generation in Pakistan.
Installed capacity:
Electricity – total installed capacity: 25,100 MW (2015)[10]
Electricity – Sources (2014)
Fossil fuel – 14,635 MW – 64.2% of total (oil-35.2% + gas-29%)
hydro – 6,611 MW – 29% of total
nuclear – 1,322 MW – 5.8% of total
average demand-17,000 MW
shortfall-between 5,000 MW and 6,000 MW
There are four major power producers in country: WAPDA, K-Electric, IPPs and Pakistan Atomic Energy
Commission (PAEC).
K-Electric
Korangi Power Complex, Combined Cycle Power Plant (KPC) 247 MW
Korangi Gas Turbine Power Station, Korangi (KGTPS) 100 MW
Gas Turbine Power Station, SITE (STGTPS) 100 MW .GE JENBACHER
Thermal Power Station, Bin Qasim (BQPS-I) 1260 MW
Combined Cycle Power Plant (BQPS-II) 560 MW
Combined Cycle Power Plant (BQPS-III) Construction Initiated 900 MW
K-Electric total generation capacity is 2,267 MW
It also has Power Purchase Agreements for 1,064 MW from various Independent Power Producers, the Water and Power
Development Authority, the Karachi Nuclear Power Plant and through imports.
KE now generates almost 50.3% of the electricity it distributes through its own systems.
Future Projects OF KE
900 MW BQPS III Power Project
KE has initiated a 900 MW project for an RLNG-based power plant at our Bin Qasim site, at an estimated cost of USD 1
billion. This will supplement the power needs of Karachi and add value to the economy through better and sustainable power
to business and industry. The project, known as BQPS III, also includes an upgrade to associated transmission infrastructure,
comprising the construction of four grids. BQPS III will be comprised of two 450 MW units, The machines operate in a range
of 58.8% – 60.7% in a combined cycle mode. The first phase of 450 MW is expected to be commissioned by mid-2018 and
the target for the second 450 MW phase is the end of 2019.
(350 x 2) MW DPKPGL Coal Power Project
K-Electric is developing a (350 x 2) MW coal power project in Port Qasim. In this regard, a Joint Development Agreement
has been signed with China Datang Overseas Investment Co. Ltd (CDTO) and China Machinery Engineering Corporation
(CMEC), wherein CDTO is taking 51% equity share in the project; 25% equity is being contributed by CMEC and 24% by
KE. Additionally, a local project company, ‘Datang Pakistan Karachi Power Generation Limited’, has been formed.
52 MW FFBL Coal Power Plant
K-Electric has entered into a strategic partnership with Fauji Fertilizer Bin Qasim Limited (FFBL) for procuring 52 MW of
coal-based power. A Power Purchase Agreement between KE and FPCL (FFBL Power Company Limited) was signed in April
2017, followed by Take & Pay Tariff approved by NEPRA. The first coal power plant constructed on CFB technology was
commissioned in May 2017.
Sindh Nooriabad Power Company (Gas)
Sind Nooriabad Power Company (Pvt.) Limited (SNPC) and Sind Nooriabad Power Company (Pvt.) Limited Phase-II (SNPCII) are developing 50 x 2 (100) MW gas-fired power plants at Nooriabad, Sindh. Both these companies are sponsored by the
Government of Sindh (GoS) and Technomen Kinetics (Pvt.) Limited. In this regard, KE has initialed Power Purchase
Agreements with SNPC and SNPC-II.
Engro Powergen (RLNG)
KE and Engro Powergen are in the process of developing a 450 MW RLNG-fired power plant at Port Qasim, wherein KE will
be the power off-taker. A new special purpose company “Kolachi Portgen (Pvt.) Ltd.” (KPL) has been established by Engro
Powergen to undertake this project
Renewable Power
KE is engaged in seeking potential partners and sponsors to promote technological development, construction, operation and
maintenance of solar and wind power plants within its franchise area. KE has signed its first renewable project, a 50 MW solar
power plant, to be commissioned at Gharo for supply of power to KE. KE is also conducting a system study to determine its
renewable energy mix and expand its renewable portfolio, mainly in wind and solar.
Embedded Generation
KE has already started development of embedded generation plants, up to a capacity of 500 MW. These projects will mainly
be developed on dual fuel technology at the mouth of the load centres, such that energy can directly be consumed in the area
through the 132 kV network. This will reduce pressure on KE’s EHT network, enabling KE to expand/enhance the EHT
network in due course. KE is working with Western Electric Limited to set up a 300 MW embedded power plant at Northern
Bypass with COD expected by mid-2019. KE is also working with Orient Power for a 200 MW embedded power plant at
Baldia under an IPP Structure and its COD is expected by end 2019. KE will also have an option of 24% equity in Orient
Kolachi (Baldia).
Captive Power Plants
As a fast-track solution to bridge the demand-supply gap in the power sector, KE purchases surplus power from various small
and large scale captive power plants set up by industrial-scale users. Recent development includes Lotte Chemicals Pakistan
Limited that would be supplying 11-14 MWs to KE and its COD is expected within FY 2018.
KE has succeeded in reducing line losses and recoveries in the low and medium loss areas. It focuses on continuous process
improvements and several pilot projects are being explored.
Annual T&D losses as at March 2016 (rolling average) stood at 23.3%, a decrease of 12.6% over the last six years.
Initiatives
Smart Grid Initiative
This will establish KE as a role model for utilities in the country and the region. The initial phase involves remotely managed
smart meters at customers’ premises and transformers backed by IT systems. The ability to accurately monitor energy flows
(allowing for loss identification, remote billing and remote disconnection) and the network health will result in reduced energy
losses, improved recoveries and increase productivity.
Aerial Bundled Cables
Aerial Bundled Cables are being installed on High Loss Transformers to control theft. The pilot project has yielded encouraging
returns, and the programme has successfully rolled out on 945 Pole-Mounted Transformers. KE has successfully converted
high loss areas to low loss through the ABC initiative.
Mobile Meter Reading
Mobile Meters digitally record meter readings, which are transmitted wirelessly from the field for quick processing; pictures
of faulty meters are uploaded through customised software.
New Connection
KE has eased the new connection process for its customers:
Auto SMS Generation: For keeping consumers updated about their case status
Smart Check List: Comprehensible process awareness for consumers’ convenience
Web Based Automation: This enables online registration
Easy Payment Facility: Customers can pay through Telenor’s EasyPaisa
Mobile New Connection Van: Low cost meters have been introduced for consumers in less privileged areas.
System Improvement Plan
This is an initiative for remote monitoring of the 11KV network parameters, reducing the time it takes to identify and restore
faults.
IBC on Wheels
KE has brought quality customer service to its consumers’ doorsteps The project is the first of its kind and eases the customer
experience for payment and support, as the vehicles are equipped with all the technical tools and trained staff.
To make a complaint, please call (021)118
TP 1000:
KE has made significant investments to expand its transmission network, which will include the installation of new grid stations
and transmission lines. A comprehensive and integrated scope of TP 1000 is conceived to meet Karachi’s growing load
demand, which will enhance the extra high tension (EHT) transmission network, increase efficiency for dispatch, and increase
the reliability, stability and capacity of the transmission network and grids. TP 1000 will potentially increase the capability of
serving an additional 1,000 MVA through the EHT network.
TOPIC 73: INSULATION MATS
Title: Insulating mats maintenance and use.
Created By:
Maintenance department
Issue Date:
Electrical standards
Placement of
insulation mats
Approved
by:
1
Current
Version:
1. Insulation matting should be provided where the voltage to ground exceeds 150v ( NFPA70, 250.174)
2. A minimum clearance of 3 feet for electrical equipment operating at 600 volts or less should be
provided (NFPA 70 110.26)
3. Operating temperature of insulating mats should be between -40°C and +55°C ( IEC 61111)
1. Near HT and LT Panels
2. In front of switchboard
3. At electrically sensitive machinery.
Precautions in use
Periodic inspection
and testing
1. Mat should be dry, without holes and should not contain conducting materials.
2. Care should be exercised to ensure that dust, metal chips which may collect on insulating material are
removed at once.
3. Do not spill liquid on the mats.
4. Do not walk over mats with dirty shoes.
5. If the mats are dirty they should be cleaned with soap and water and then dried in a manner that will
not cause the temperature of the mats to exceed 55 °C.
6. If electrical insulating matting come in contact with conductive grease, it should be cleaned as soon
as possible with a suitable solvent
Once in 12 months. The tests consist of visual inspection, and then a working voltage dielectric test
subjecting to condition of insulation mats.
TOPIC 74: BASICS OF ELECTRICAL SAFETY
SAFETY CHAPTER 1
Measure twice . . . cut once. Measure once . . . cut twice.
The NEC in 90.1(A) makes it pretty clear. It states that the purpose of this Code is the practical safeguarding of persons and
property from hazards arising from the use of electricity.*
Do not work on live circuits! Always de-energize the system before working on it! There is no compromise when it comes
to safety! Many injuries and deaths have occurred when individuals worked on live equipment. The question is always: “Would
the injury or death have occurred had the
Power been shut off?” The answer is “No!”
Electricity is all around us, just waiting for the opportunity to get out of control. Repeat these words: Safety First . . . Safety
Last . . . Safety Always! Safety is not a joke!
Lockout/tag out (sometimes called LOTO) is the physical restraint of all hazardous energy sources that supply power to a piece
of equipment. It simply means putting a padlock on the switch and applying
a warning tag on the switch.
What about Low-Voltage Systems?
Although circuits of less than 50 volts generally are considered harmless, don’t get too smug when working on so-called low
voltage. Low-voltage circuits are not necessarily low hazard. A slight tingle might cause a reflex. A capacitor that is discharging
can give you quite a jolt, causing you to jump or pull back.
Think of a 12-volt car battery. If you drop a wrench across the battery terminals, you will immediately see a tremendous and
dangerous arc flash. It is the current that is the harmful component of an electrical circuit. Voltage pushes the current through
the circuit. If you’re not careful, you might become part of the circuit.
IT’S THE LAW:
A direct quote from OSHA 1910.333(a)(1) states that “Live parts to which an employee may be exposed shall be de-energized
before the employee works on or near them, unless the employer can demonstrate
That de-energizing introduces additional or increased hazards or is infeasible due to equipment design or operational
limitations. Live parts that operate at less than 50 volts to ground need not be de-energized if there will be no increased
exposure to electrical burns or to explosion due to electric arcs.
PPE’S
Working on electrical equipment while wearing rings and other jewelry is not acceptable. OSHA
states that “Conductive articles of jewelry and clothing (such as watch bands, bracelets, rings, key chains, necklaces, metalized
aprons, cloth with conductive thread, or metal headgear) may not be worn if they might contact exposed energized parts.
However, such articles may be worn if they are nonconductive by covering, wrapping, or other insulating means.” OSHA
1910.132(f)(1)
When turning a standard disconnect switch “ON,” don’t stand in front of the switch. Instead, stand to one side. For example,
if the handle of the switch is on the right, then stand to the right of the switch,
Using your left hand to operate the handle of the switch, and turn your head away from the switch. That way, if an arc flash
occurs when you turn the disconnect switch “ON,” you will not be standing in front of the switch. You will not have the
switch’s door fly into your face, and the molten metal particles resulting from the arc flash will fly past you.
Classifying Electrical Injuries
OSHA recognizes the four main types of electrical injuries as:
• Electrical shock (touching “live” line-to-line or line-to ground conductors)
• Electrocution (death due to severe electrical shock)
• Burns (from an arc flash)
• Falls (an electrical shock might cause you to lose your balance, pull back, jump, or fall off a ladder)
The NEC is published by the National Fire Protection Association and is referred to as NFPA 70. The NEC was first published
in 1897. It is revised every 3 years so as to be as up to date as possible.
How does one know if a product is safe to use?
Nationally Recognized Testing Laboratories (NRTL) have the knowledge, wherewithal, and test equipment to test and evaluate
products for safety. A NRTL will perform tests on a product based on a specific nationally recognized safety standard. After
the product has been tested and found to comply with the safety standard, the product is considered to be free from reasonably
foreseeable risk of fire, electric shock, and related hazards.
The following laboratories do a considerable amount of testing and listing of electrical equipment:
Underwriters Laboratories, Inc.
(UL), founded in 1894, is a highly qualified, nationally recognized testing laboratory with several testing laboratories These
tests determine whether the product can perform safely under normal and abnormal conditions to meet published standards.
UL Marking. The UL marking is required to be on the product! The UL marking will always consist of
four elements—UL in a circle, the word “LISTED” in capital letters, the product identity, and a unique alphanumeric control
or issue number. If the product is too small, or has a shape or is made of a material
that will not accept the UL Mark on the product itself, the marking is permitted on the smallest unit carton or container that
the product comes in. Marking on the carton or box is nice but does not ensure that the product is UL listed! The letters will
always be staggered: UL. They will not be side by side: UL
TOPIC 75: BASICS OF POWER SYSTEM
Q1) suppose a consumer consumes 1000 watt load per hour daily for one month. What will be the total energy bill of
the consumer if the per unit rate is 9
ANS) 1 unit = 1 kwh
Total watt hour consumed in one month = 1000*24*30= 720000watthour
We want to convert it into units hence
Total unit consumed = 720000/1000
( 1 kwh=1000wh)
Total unit consumed= 720 units
Cost per unit = 9
Bill = 720 *9 = 6,480 rupees
Q2) Why AC needs more insulation then DC?
ANS) lets say we have 220v dc, its means that its peak value is 220v where as if we say "220v ac", it is actually its RMS
value and its peak value will be
peak value=RMS/0.707
which will be 311v ac. Hence 220 V DC= 311 V Ac. Therefore Ac needs more insulation then dc
Q3) What is the basic function of switch gear?
ANS) You turn on the fan or light in your bedroom by switches and guess what similar thing is what switch gear does. It
simply switches power to different places..
In switch gear, the terminals where incoming power cable is connected is called an incomer. The incomer will be supplying
power to internal bus bar of switch gear. The same power is distributed to different parts of a factory or a building by
switches connected to switch gear and these switches are called feeders.
In simple wordings, it is a device which receives power and distribute it through switches
Q4) What do we get charge for? Current? Voltage? Power?
ANS) We get charge for energy (kwh) which is the product of power and time.
Q5) How much energy is consumed by a 5hp, 3 phase motor in 1 hour?
1 hp= 746 watt
Power= 5hp= 3730 watt
Time= 1 hour
Energy = power* time
Energy= 3.730kwh i.e roughly 4 units
Q6) why there are only 3 phases? Why not 6 or 9 phases?
ANS) Because we don’t need 6 or 9 phases. 3 phases are sufficient enough to handle our load whether it is an industrial or
residential load. 6 phases means we would need 6 wires for our system. It will increase system complexity and as well as the
cost of the system.
Q7) Classification of power cables?
ANS) Power cables are manufactured on the basis of the voltage which they will be used for. Their classification is as
follows:
1)
Low tension cable up to 1kv
2)
High tension cable upto 11kv
3)
Super tension cable from (22-33)kv
4)
Extra high tension(EHT) cable upto 66kv
5)
Oil filled and gas filled cables for above 66kv.
Q8) Why 3 phases are 120 degrees apart from each other? Why not 100 degrees or 200 degrees apart?
ANS) Because we have to set coils at equal distances such that they complete 360 degrees. We can also used 6 coils which
would be 60 degrees apart from each other such that they complete 360 degrees( (6*60=360 degrees). But the more the coils,
the more will be the cost.
Q9) What is phase sequence and type of phase sequence?
ANS) The order in which phasor reaches their maximum value is called as phase sequence. Motor’s rotation can be change
by changing its phase sequence.
POSITIVE PHASE SEQUENCE: When rotor is rotated in anticlockwise direction than phase sequence is positive and
phase difference is 120 degrees.
NEGATIVE PHASE SEQUENCE: When rotor is rotated in clockwise direction than phase sequence is negative and phase
difference is 120 degrees.
ZERO PHASE SEQUENCE: When the phase difference is 0 degrees than it is called as zero phase sequence.
Q10) Why transmission line are connected in parallel?
ANS) If a fault occurs on one line so transmission can be continued by other two lines i.e. supply of electricity won’t stop
hence to improve the reliability of the system, transmission lines are connected in parallel.