Transmission Lines
In communications and electronic engineering, a transmission line is a specialized cable
designed to carry alternating current of radio frequency, that is, currents with a frequency high
enough that their wave nature must be taken into account. Transmission lines are used for
purposes such as connecting radio and receivers with their antennas, distributing cable
television signals, and computer network connections.
Purpose of Transmission Lines
• To experimentally determine the speed of signal propagation and the characteristic
impedance of transmission lines.
• To interpret these measurements in terms of the capacitance and inductance per unit
length of the cable.
• To observe termination effects in cables and interpret them in terms of virtual running
waves.
• To observe clipping, capacitive charging and resonance in cables and interpret the
observations in terms of the previously measured characteristics.
The transmission line has a single purpose for both the transmitter and the antenna. This
purpose is to transfer the energy output of the transmitter to the antenna with the least possible
power loss. How well this is done depends on the special physical and electrical characteristics
(impedance and resistance) of the transmission line.
Choice of Voltage and Frequency
When next you switch on the electric light or television in your house, think for a moment of all
the effort and work that went into generating electricity and sending it to your home.
Power stations all over South Africa are linked by overhead transmission lines. (Transmission is a
word from the verb “to transmit” which means to send from one place to another.) The
transmission lines are supported by towers called pylons and transport the electricity by means of
thick aluminum and copper wires. The network of transmission lines is called the National Grid.
Voltage
In order for the electricity to be transmitted safely and efficiently over long distances, it must be
at a high voltage (pressure) and a low current (flow). This is because if the current is too high, the
lines would heat up too much and even melt. If the voltage were too low, hardly any energy
would be carried.
The generators in the power stations produce electricity at ±20 000 volts (20kV). This voltage is
raised by transformers before it is sent out. The high voltage transmission system in Eskom
comprises a 132 000, 275 000, 400 000 and 765 000 volt system. These very high voltages are
necessary to “push” the required flow of electricity efficiently through the long distance lines.
From the high voltage network, the electricity is transformed down, for example, to 11 000 volts
for local distribution and then to 240/220 V for domestic use.
Frequency
A further point to understand is that we generate and transmit Alternating Current (AC).
Eskom’s generators are synchronized to the National Grid at a frequency of 50 Hertz (Hz).
We could also say that frequency is the speed at which an electric current flows. This “speed” is
measured in a sine wave as illustrated in the following diagram.
The standard frequency in South Africa is 50 Hertz, whereas in some other countries like the
USA, the frequency is 60 Hz.
There is a definite relationship between the rotational speed of a generator and frequency. The
rotor of a generator is in effect a huge electro-magnet with magnetic poles. Most generators
within the Eskom system are classed as two poles generators with one North Pole and one South
Pole, i.e. a pair. The typical rotational speed of these generators is 3 000 rpm. When a generator
has one pair of poles and the rotational speed of 3 000 rpm is divided by 60 seconds, a frequency
of 50Hz is obtained.
A slower rotating generator would need more pairs of magnetic poles in order to achieve the same
50 Hz frequency.
Types of Transmission Lines
1.
2.
3.
4.
5.
There are the following types of transmission lines.
Balanced two wire
Co-axial Cable
Wave guide
Micro strip
Fiber Optic
1. Balance Two Wire Line
As shown in the given diagram the two wire transmission lines consists of the figures A,B and C. each
construction for the given figure is given as below.
1. In this type of construction for two wire transmission lines the insulated spacers are used in order to
maintain the distance between the transmission lines or between the two conducting wire equally
throughout.
2.
In this type of transmission line the two conducting wires are kept parallel to each other with the help of
plastic material. This is used throughout between the conducting wires.
3.
In this type of transmission line the rubber piping is used in circular rectangular or square shape. The two
conducting wires are kept inside the rubber at opposite sides of the piping. These conducting wires urn
throughout the construction and remains parallel to each other.
Merits
1.
2.
3.
4.
There are the following merits of balance two wire lines.
The cost of two wire transmission line is very low as compared to other types of lines.
To design the open two line transmission line is quite simple and easy too.
Open two wire lines are capable of handling high power.
Demerits
5.
6.
7.
8.
9.
The external interference of the signal in open two wire lines is more as compared to other types of
transmission lines.
Due to external interference the output at the load end of two wire transmission line will be noisy.
To use the two wire transmission lines in the twisty paths is quite difficult.
Formula for finding out the characteristics impedance of open two wire transmission line is given below.
It cannot be used on very high frequencies because it will generate skin effect.
Impedance
Zo = 276 log10 2D/d
Here d stands for the diameter of the wire.
D stands for the distance between two wires from its aerials.
Zo stands for characteristics impedance
2. Co-axial Cable
As shown in the given diagram the co-axial cable consists of inner conducting wire made of copper, over
this conducting wire the coating of polyethylene or taplon material is carried out. Then it is enclosed in the
braded wire in the shape of mash. The outer surface of this wire is enclosed in a plastic jacket.
Merits
1.
2.
3.
4.
5.
There are the following merits of co-axial cable.
As the outer conductor (braded wire) is grounded, therefore the possibility of external interference is
minimized. The output of the load end will be less noised.
The co-axial cable is used for high frequencies transmission.
This type of transmission cables can be easily used if the path of energy from source to load is twisty or
complicated.
Co-axial cable occupies less space as compared to two wire lines.
The conductor which carries the energy from source to load is protected from dust, rust etc. due to proper
insulation.
Demerits
1.
2.
3.
4.
There are the following demerits of the co-axial cable used as transmission line.
This type of transmission line is costly with respect to two wire lines.
Designing of co-axial cable is difficult as compared to two wire lines.
This type of transmission lines handles low power transmissions.
Formula for the impedance is
Impedance
Zo = 233 log10 D/d
Where Zo stands for characteristics impedance.
D stands for internal wire diameter of the braded wire.
d stands for diameter of the inner conductor.
3. Micro strip Line
A microstrip is simply a copper track running on a side of the PCB while the other side is plain groun
plane. The formula will give you the charactersistic impedance of the track, as well as the effective
dielectric constant based on the geo-metric parameters. The table provides usual values for 1.6 and 0.8 mm
thick PCBs as well as for the standard FR4 substrate or the most advanced Rogers R04003.
Merits
1.
2.
3.
4.
5.
6.
7.
8.
There are the following merits of the micro strip line.
Very high frequency.
Small size
Low weight.
Losses are minimum.
This type of transmission line is used for very high frequency.
Micro strip lines are used in integrated circuits where distance between load and source is very short.
As the path of energy is made of very good conductor like gold, therefore the losses of energy are
minimum possible.
The weight of micro strip line is low.
Demerits
There are the following demerits of micro strip line.
The cost of micro strip is very high as compared to co-axial and two wire line.
The micro strip line cannot be used as a transmission line when the distance between source and load is
long.
3. This type of transmission line cannot be used in twisty paths between source and load.
1.
2.
Ferranti Effect


A long transmission line draws a substantial quantity of charging current. If such a line is open
circuited or very lightly loaded at the receiving end, Receiving end voltage being greater than
sending end voltage in a transmission line is known as Ferranti effect. All electrical loads are
inductive in nature and hence they consume lot of reactive power from the transmission lines.
Hence there is voltage drop in the lines. Capacitors which supply reactive power are
connected parallel to the transmission lines at the receiving end so as to compensate the
reactive power consumed by the inductive loads.
As the inductive load increases more of the capacitors are connected parallel via electronic
switching. Thus reactive power consumed by inductive loads is supplied by the capacitors
thereby reducing the consumption of reactive power from transmission line. However when
the inductive loads are switched off the capacitors may still be in ON condition. The reactive
power supplied by the capacitors adds on to the transmission lines due to the absence of
inductance. As a result voltage at the receiving end or consumer end increases and is more
than the voltage at the supply end. This is known as Ferranti effect.
Transmission Line Corona
Electric transmission lines can generate a small amount of sound energy as a result of corona.
Corona is a phenomenon associated with all transmission lines. Under certain conditions, the
localized electric field near energized components and conductors can produce a tiny electric
discharge or corona that causes the surrounding air molecules to ionize, or undergo a slight
localized change of electric charge.
Utility companies try to reduce the amount of corona because in addition to the low levels of
noise that result, corona is a power loss, and in extreme cases, it can damage system components
over time.
Corona occurs on all types of transmission lines, but it becomes more noticeable at higher
voltages (345 kV and higher). Under fair weather conditions, the audible noise from corona is
minor and rarely noticed. During wet and humid conditions, water drops collect on the conductors
and increase corona activity. Under these conditions, a crackling or humming sound may be heard
in the immediate vicinity of the line.
Corona results in a power loss, so our industry has been studying this effect for over 50 years.
Power losses like corona result in operating inefficiencies and increase the cost of service for all
ratepayers; a major concern in transmission line design is the reduction of losses. Steps that VT
Transco has taken to minimize these line losses and corona activity include:
1. Bundling – on our 345 kV lines, we have installed multiple conductors per phase. This is a
common way of increasing the effective diameter of the conductor, which in turn results in less
resistance, which in turn reduces losses.
2. Elimination of sharp points- electric charges tend to form on sharp points; therefore when
practicable we strive to eliminate sharp points on transmission line components.
3. Corona rings – On certain new 345 kV structures, we are now installing corona rings. These
rings have smooth round surfaces which are designed to distribute charge across a wider area,
thereby reducing the electric field and the resulting corona discharges.
Skin Effects
Skin effect is a tendency for alternating current ( AC ) to flow mostly near the outer
surface of a solid electrical conductor, such as metal wire, at frequencies above the audio
range. The effect becomes more and more apparent as the frequency increases.
The main problem with skin effect is that it increases the effective resistance of a wire for
AC at moderate to high frequencies, compared with the resistance of the same wire at
direct current ( DC ) and low AC frequencies. The effect is most pronounced in radiofrequency ( RF ) systems, especially antenna s and transmission lines. But it can also
affect the performance of high-fidelity sound equipment by causing attenuation in the
treble range.
Skin effect can be reduced by using stranded rather than solid wire. This increases the
effective surface area of the wire for a given wire gauge . Tinned wire should be avoided
because tin has higher resistance than copper. In large RF antenna arrays, hollow tubing
can be used in place of solid rods with little or no loss of efficiency; in this respect, skin
effect is an asset.
Skin effect occurs with brief pulses of current , for the same reasons it occurs at high AC
frequencies. This can save lives. If you are caught in a thundershower, you can take
refuge in a car or other metal vehicle and be relatively safe even if you suffer a direct hit.
The skin effect causes virtually all of the current to flow on the outside of the vehicle as it
passes from cloud to ground.
Sag and Tension Calculations
The energized conductors of transmission and distribution lines must be placed to totally
eliminate the possibility of injury to people.
Overhead conductors, however, elongate with time, temperature, and tension, thereby changing
their original positions after installation. Despite the effects of weather and loading on a line,
the conductors must remain at safe distances from buildings, objects, and people or vehicles
passing beneath the line at all times.
To ensure this safety, the shape of the terrain along the right-of-way, the height and lateral
position of the conductor support points, and the position of the conductor between support
points under all wind, ice, and temperature conditions must be known.
Sag and tension calculations for conductor earth wire are done for the river crossing by following
steps :
i. Determination of Equivalent Span :
Based on anchor spans L1 & L3 and crossing span L2, the equivalent span for river crossing
portion is determined by the following formula :
3
3
3
Eq. Span =
1 + L2 + L3 / L1 + L2 + L3 (Refer Figure 10-II)
ii.
Sag and tension calculation for conductor/earth wire for above equivalent span is done
from the following formula :
T2 (T - K + aEa (q2-q1)) = W2 L2 aE / 24 x q2
where,
T = Tension at temperature q2 (kg)
K= Constant
a = Area of conductor/Earth wire (mm2)
E = Modulus of Elasticity kg/mm2
a = Linear Coeft. of expansion (per degree celcius)
W = weight of conductor (kg/m)
L = Equivalent span (m)
q = Wind load factor = P2 + W2 / W2 = 1 (At no wind condition)
q1 = Initial condition temperature
iii.
Calculation for conductor
a. Initial condition for conductor is taken as 32°C and No wind and T0 tension under these
conditions is taken as 22% of ultimate tensile strength of conductor.
b.From the above value of T0, we calculate the constant 'K which is fixed for all further sagtension calculations.
c. Considering the calculated value of K, the tension at 0°C under No-wind & full wind and 32°C
full wind and 75°C No-wind is determined from the above formula by hit & trial method.
d.Sag at various tension :
= WLA2 / 8T where LA is actual span
iv. Calculation for Earth wire
a. The earth wire sag at 0°C and no-wind should be 90% of the Conductor sag at 0°C and No-wind.
Value of tension at 0° and no wind is determined by the following formula
T = {WL2 / 0.9 x Sag of Conductor at 00N/W}
for equivalent span
where L = Equivalent span in meters.
b.As in the case of conductor, the tensions at 0°C (No-wind & full wind Condition), 32"C full
wind conditions and 75°C no-wind condition are determined.
c. Sag = [WLA2 / 8TA] where LA = Actual span.
Use of Dampers
A damper is a valve or plate that stops or regulates the flow of air inside a duct, chimney,VAV box, air
handler, or other air handling equipment. A damper may be used to cut off central air conditioning (heating
or cooling) to an unused room, or to regulate it for room-by-room temperature and climate control. Its
operation can be manual or automatic. Manual dampers are turned by a handle on the outside of a duct.
Automatic dampers are used to regulate airflow constantly and are operated
by electric or pneumatic motors, in turn controlled by a thermostat or building automation system.
Automatic or motorized dampers may also be controlled by a solenoid, and the degree of air-flow
calibrated, perhaps according to signals from the thermostat going to the actuator of the damper in order to
modulate the flow of air-conditioned air in order to effect climate control. [1]
In a chimney flue, a damper closes off the flue to keep the weather (and birds and other animals) out and
warm or cool air in. This is usually done in the summer, but also sometimes in the winter between uses. In
some cases, the damper may also be partly closed to help control the rate of combustion. The damper may
be accessible only by reaching up into the fireplace by hand or with a woodpoker, or sometimes by a lever
or knob that sticks down or out. On a woodburning stove or similar device, it is usually a handle on the vent
duct as in an air conditioning system. Forgetting to open a damper before beginning a fire can cause
serious smoke damage to the interior of a home, if not a house fire.
Conductor Insulator
A Conductor insulator is a material whose internal electric charges do not flow freely, and which
therefore does not conduct an electric current, under the influence of an electric field. A perfect insulator
does not exist, but some materials such as glass, paper andTeflon, which have high resistivity, are very
good electrical insulators. A much larger class of materials, even though they may have lower
bulk resistivity, are still good enough to insulate electrical wiring and cables. Examples include rubberlike polymers and most plastics. Such materials can serve as practical and safe insulators for low to
moderate voltages (hundreds, or even thousands, of volts).
Insulators are used in electrical equipment to support and separate electrical conductorswithout allowing
current through themselves. An insulating material used in bulk to wrap electrical cables or other
equipment is called insulation. The term insulator is also used more specifically to refer to insulating
supports used to attach electric power distribution ortransmission lines to utility poles and transmission
towers.
Types of Insulators
Various types of insulators are briefly described below.
1. Pin Type Insulators: The Pin type insulator is designed to be mounted on a pin which in
turn is installed on the cross-arm of the pole.
For lower voltages generally one piece type of insulators is used. For high-voltage transmission
lines larger, stronger pin type insulators are used. The high voltage pin type insulators differ in
construction from low voltage type in that they consist of two or three pieces of porcelain
cemented together. These pieces form what we call petti-coats. These are designed to shed rain
and sleet easily. These are available for use up to 50,000 volts.
2. Suspension Type Insulators. With the increase in operating voltage, the insulation required
increases. Transmission lines use extremely high voltages, 220 kv, for example. AT these
voltages the pin type insulators become bulky, cumbersome and costly. Besides, the pin which
must hold it would have to be in ordinarily long and large. In order to meet the problem of
insulators for these high voltages, the suspension insulator was developed.
The suspension insulator hangs from the cross arm, as opposed to the pin insulator which sits
on the top of it. The line conductor is attached to its lower end. Because there is no pin
problem, we can put any distance between the suspension insulator and the conductor just by
adding more insulators to the “string”. The entire unit of suspension insulators is called a
string. How many insulators the string consists of depends upon the working voltage, the
weather conditions, the type of transmission construction, and the size of insulator used. It is
worthnoting that in a string of suspension insulators one or more insulators can be replaced
without replacing the whole string.
3. Strain Insulators. Sometimes a line is to withstand great strain, for instance at a dead end or
at a corner or on sharp turns. In such a circumstance for LT (low tension) lines shackle
insulators are used but H T (high tension) transmission lines strain insulators consisting of an
assembly of suspension type insulators are used. Because of its peculiarly important job, a
strain insulator must have considerable strength as well as the necessary electrical properties.
Two or more strings of suspension insulators are used in parallel when the tension is
exceedingly high. The discs of strain insulators are employed in a vertical plane where as the
suspension insulators are used in horizontal plane.
4. Shackle Insulators. The shackle or spool-type insulator, which is easily identified by its
shape, is usually used on it lines. Both the low voltage conductors and the house service wires
are attached to the shackle insulator.
5. Egg or Stay Insulators. Such insulators are of egg shape and used in guy cables, where it is
necessary to insulate the lower part of the guy cable from the pole for the safety of the people
on the ground. These are provided at a height of about 3 m from the ground level.
Voltage Distribution
For overhead lines, operating on higher voltage, use of number of discs connected in series is
made. The whole unit formed by connecting a number of discs in series is known as string of
insulators. The insulators consist of metal fittings and metal fitting of each unit has a
capacitance relative to metal fitting of next unit. The capacitance due to two metal fittings on
either side of an insulator is known as mutual capacitance. Further there is also a capacitance
between metal fitting of each unit and the earth or tower. The capacitance formed is known as
shunt capacitance.
The voltage impressed on a string of suspension insulators does not distribute itself uniformly
across the individual discs owing to the proximity for supporting structure (which is at earth
potential). The line unit (unit nearest th conductor) shall have to withstand the maximum
percentage of voltage, the figure progressively decreasing as the unit nearest the tower is
approached. The inequality of voltage distribution between individual units is all the more
pronounced with a larger number of insulator units and also depends on the ratio of capacity of
insulator to capacity to earth. The ratio of flash over voltage of n units to n times of flash-over
voltage of one unit is called the string efficiency.
i.e. String efficiency = (Flash-over voltage of the string)/(n ×Flash-over voltage of one unit
) ×100= E/ne ×100
When the insulators are wet mutual capacitance increases while shunt capacitance remains
constant (except for the unit nearest to the cross-arm) so the ratio of shunt capacitance to mutual
capacitance decreases, more uniform potential distribution is obtained and the string efficiency
improves.
String Efficiency
The ratio of voltage across the whole string to the product of number of discs and the voltage across the
disc nearest to the conductor is known as string efficiency i.e.,
where n = number of discs in the string.
String efficiency is an important consideration since it decides the potential distribution along the string.
The greater the string efficiency, the more uniform is the voltage distribution. Thus 100% string efficiency
is an ideal case for which the volatge across each disc will be exactly the same. Although it is impossible to
achieve 100% string efficiency, yet efforts should be made to improve it as close to this value as possible.
Methods of Improving String Efficiency
The maximum voltage appears across the insulator nearest to the line conductor and decreases
progressively as the crossarm is approached. If the insulation of the highest stressed insulator (i.e.
nearest to conductor) breaks down or flash over takes place, the breakdown of other units will
take place in succession. This necessitates to equalise the potential across the various units of the
string i.e. to improve the string efficiency.
The various methods for this purpose are :
1. By using longer cross-arms. The value of string efficiency depends upon the value of K
i.e., ratio of shunt capacitance to mutual capacitance. The lesser the value of K, the
greater is the string efficiency and more uniform is the voltage distribution. The value of
K
can be decreased by reducing the shunt capacitance. In order to reduce shunt capacitance,
the distance of conductor from tower must be increased i.e., longer cross-arms should be
used. However, limitations of cost and strength of tower do not allow the use of very long
cross-arms. In practice, K = 0·1 is the limit that can be achieved by this method.
2. By grading the insulators. In this method, insulators of different dimensions are so
chosen that each has a different capacitance. The insulators are capacitance graded i.e.
they are assembled in the string in such a way that the top unit has the minimum
capacitance, increasing progressively as the bottom unit (i.e., nearest to conductor) is
reached. Since voltage is inversely proportional to capacitance, this method tends to
equalise the potential distribution across the units in the string. This method has the
disadvantage that a large number of different-sized insulators are required. However,
good results can be obtained by using standard insulators for most of the string and larger
units for that near to the line conductor.
3. By using a guard ring. The potential across each unit in a string can be equalised by
using a guard ring which is a metal ring electrically connected to the conductor and
surrounding the bottom insulator. The guard ring introduces capacitance between metal
fittings and the line conductor. The guard ring is contoured in such a way that shunt
capacitance currents i1, i2 etc. are equal to metal fitting line capacitance currents i′1, i′2
etc. The result is that same charging current I flows through each unit of string.
Consequently, there will be uniform potential distribution across the units.
Insulator Testing
According to the British Standard, the electrical insulator must undergo the following tests
1. Flashover tests of insulator, 2. Performance tests and 3. Routine tests
Let's have a discussion one by one,
Flashover Test
There are mainly three types of flashover test performed on an insulator and these are,
1. Power Frequency Dry Flashover test of Insulator
1. First the insulator to be tested is mounted in the manner in which it would be used
practically.
2. Then terminals of variable power frequency voltage source are connected to the both
electrodes of the insulator.
3. Now the power frequency voltage is applied and gradually increased up to the specified
value. This specified value is below the minimum flashover voltage.
4. This voltage is maintained for one minute and observe that there should not be any
flash-over or puncher occurred.
The insulator must be capable of sustaining the specified minimum voltage for one minute
without flash over.
2. Power Frequency Wet Flashover Test or Rain Test of Insulator
1. In this test also the insulator to be tested is mounted in the manner in which it would be
used practically.
2. Then terminals of variable power frequency voltage source are connected to the both
electrodes of the insulator.
3. After that the insulator is sprayed with water at an angle of 45o in such a manner that its
precipitation should not be more 5.08 mm per minute. The resistance of the water used for
spraying must be between 9 kΩ 10 11 kΩ per cm3 at normal atmospheric pressure and
temperature. In this way we create artificial raining condition.
4. Now the power frequency voltage is applied and gradually increased up to the specified
value.
5. This voltage is maintained for either one minute or 30 second as specified and observe
that there should not be any flash-over or puncher occurred.
The insulator must be capable of sustaining the specified minimum power frequency
voltage for specified period without flash over in the said wet condition.
3. Power Frequency Flashover Voltage test of Insulator
1. The insulator is kept in similar manner of previous test.
2. In this test the applied voltage is gradually increased in similar to that of previous tests.
3. But in that case the voltage when the surroundings air breaks down, is noted.
4. Impulse Frequency Flashover Voltage test of Insulator
The overhead outdoor insulator must be capable of sustaining high voltage surges caused
by lightning etc. So this must be tested against the high voltage surges.
1. The insulator is kept in similar manner of previous test.
2. Then several hundred thousands Hz very high impulse voltage generator is connected to
the insulator.
3. Such a voltage is applied to the insulator and the spark over voltage is noted.
The ratio of this noted voltage to the voltage reading collected from power frequency
flashover voltage test is known as impulse ratio of insulator.
Impulse Frequency Flashover Voltage
∴ Impulse Ratio =
Power Frequency Flashover Voltage
This ratio should be approximately 1.4 for pin type insulator and 1.3 for suspension type
insulators.
Performance Test of Insulator
Now we will discuss performance test of insulator one by one
1. Temperature Cycle Test of Insulator
1. The insulator is first heated in water at 70oC for one hour.
2. Then this insulator immediately cooled in water at 7oC for another one hour.
3. This cycle is repeated for three times.
4. After completion of these three temperature cycles, the insulator is dried and the glazing
of insulator is thoroughly observed.
After this test there should not be any damaged or deterioration in the glaze of the insulator
surface.
2. Puncture Voltage Test of Insulator
1. The insulator is first suspended in an insulating oil.
2. Then voltage of 1.3 times of flash over voltage, is applied to the insulator.
A good insulator should not puncture under this condition.
3. Porosity Test of Insulator
1. The insulator is first broken into pieces.
2. Then These broken pieces of insulator are immersed in a 0.5 % alcohol solution of
fuchsine dye under pressure of about 140.7 kg ⁄ cm2 for 24 hours.
3. After that the sample are removed and examine.
The presence of a slight porosity in the material is indicated by a deep penetration of the
dye into it.
4. Mechanical Strength Test of Insulator
1. The insulator is applied by 2½ times the maximum working strength for about one
minute.
The insulator must be capable of sustaining this much mechanical stress for one minute
without any damage in it.
Routine Test of Insulator
Each of the insulator must undergo the following routine test before they are recommended
for using at site.
1. Proof Load Test of Insulator
In proof load test of insulator, a load of 20% in excess of specified maximum working load
is applied for about one minute to each of the insulator.
2. Corrosion Test of Insulator
In corrosion test of insulator,
1. The insulator with its galvanized or steel fittings is suspended into a copper sulfate
solution for one minute.
2. Then the insulator is removed from the solution and wiped, cleaned.
3. Again it is suspended into the copper sulfate solution for one minute.
4.The process is repeated for four times.
Then it should be examined and there should not be any disposition
of metal on it.
Distributors fed at one end:
In this type of feeding, the distributor is connected to supply mains at one end and loads are
tapped at different points along the path of the distributor. In this type of distributor current in
the section away from the feeding point and voltage across the loads away from the feeding
point goes on decreasing.
The minimum voltage occurs on the farthest load point. It fault occurs in any section of
distributor, the whole distributor is required to be disconnected from the supply mans and thus
supply continuity is disturbed.
Distributors fed at both ends
In this type of feeding, the distributor is connected to supply mains at both ends. The voltage at
both feeding points may be different or equal. In this type of distributor, load voltage first goes
on decreasing, reaches the minimum value, then starts increasing and reaches the maximum
value, when we reach the other feeding point while going from one load point to another load
point. The point of minimum voltage is never fixed. It always shifts with the variation of load
on the different sections of the distributor.
Advantages. (i) In case of fault in any one feeder feeding the distributor, the continuity of
supply is maintained by feeding it from other end.
(ii) If any section of the distributor is isolated in case of fault, the continuity of supply is
maintained to the remaining sections.
(iii) Since x-section required for doubly fed distributors is much less as compared to singly
fed one, hence it is economical.
Ring Main or Ring Circuit or Ring Final Circuit
In electricity supply, a ring final circuit or ring circuit (often called a ring main or informally a ring) is
an electrical wiring technique developed and primarily used in the United Kingdom. This design enables
the use of smaller-diameter wire than would be used in a radial circuit of equivalent total current.
Appliances connected to sockets on a ring circuit are individually protected by a fused plug.
Ideally, the ring circuit acts like two radial circuits proceeding in opposite directions around the ring, the
dividing point between them dependent on the distribution of load in the ring. If the load is evenly split
across the two directions, the current in each direction is half of the total, allowing the use of wire with half
the current-carrying capacity. In practice, the load does not always split evenly, so thicker wire is used.
The ring starts at the consumer unit (also known as fuse box, distribution board, or breaker box), visits
each socket in turn, and then returns to the consumer unit. The ring is fed from a fuse or circuit breaker in
the consumer unit.
Ring circuits are commonly used in British wiring with fused 13 A plugs to BS 1363. They are generally
wired with 2.5 mm2 cable and protected by a 30 A fuse, an older 30 A circuit breaker, or
a European harmonised 32 A circuit breaker. Sometimes 4 mm2 cable is used if very long cable runs (to
help reduce volt-drop) or derating factors such as thermal insulation are involved. 1.5 mm2 mineralinsulated copper-clad cable (known as pyro) may also be used (as mineral insulated cable can withstand
heat more effectively than normal PVC) though more care must be taken with regard to voltage drop on
longer runs.
Un-balanced load of Three-phase AC Distributors
In electrical engineering, three-phase electric power systems have at least three conductors
carrying alternating current voltages that are offset in time by one-third of the period. A threephase system may be arranged in delta (∆) or star (Y) (also denoted as wye in some areas). A wye
system allows the use of two different voltages from all three phases, such as a 230/400V system
which provides 230V between the neutral (centre hub) and any one of the phases, and 400V
across any two phases
Three-Phase unbalance refers to the relative difference in voltage between each of the individual phases in
a three phase power supply. A phase unbalance occurs when the individual three phase voltages have
different amounts of load connected to each of them on one system. When voltage varies among the three
phases, the unbalanced voltage raises the current above the normal level in one or two of the phases. This
causes overheating of that particular phase winding in the motor.
Phase voltage unbalance is calculated as follows:
Example:
Assume the individual phase voltages for a motor are 228 Volts to Phase A, 220 Volts to Phase B, and 236
Volts to Phase C. The average voltage is 228 Volts.
The voltage unbalance is as follows:
Cable Resistance
There are often problems related to the bonding resistance between the earth reference bar (ERB)
and installed devices. This is common where a device is installed using normal house wiring
cable, although it will also occur whenever the installer fails to take into account the actual length
of run, which may be different to the apparent length of run shown on the circuit diagram.
Many medical devices draw only a small amount of current, so that the installer may be tempted
to use cable having a low cross sectional area, even though the length of cable run may be quite
significant. In a medical installation the cable resistance is particularly important.
The bonding resistance from the ERB to the earth terminal of the medical device should be less
than 100 mΩ. The table below shows that if a 1.0 mm2 earth conductor is used, the maximum
length of run would be only about 5.5 m, although the current carrying capacity of the cable is 15
A, sufficient to supply a single 13 A socket.
The cable commonly used to connect standard 13 A sockets is 2.5 mm2 flat-twin-and-earth, which
has a current carrying capacity of 27 A. However, the cross sectional area of the earth conductor
of normal 2.5 flat-twin-and-earth is usually only 1.0 mm2, or in higher grade cables, may be 1.5
mm2. This means that the maximum length of run may be only 5.5 m for a radial, or 11 m for a
ring, assuming that both ends of the cable are connected to the ERB.
In order to get an earth conductor of 2.5 mm2, you would need to use 6 mm2 flat-twin-and-earth
cable. A 10.0 mm2 flat-twin-and-earth cable will usually have a 4.0 mm2 earth conductor, and a
16 mm2 flat-twin-and-earth cable will have a 6 mm2 earth conductor. In general other types of
three core cable will have the same cross sectional area for each of the conductors.
When planning a medical installation both the resistance and current carrying capacity of all
cabling must be taken into account.
Inductance
Inductance (measured in henries, symbol H) is a measure of the generated emf for a unit change
in current. For example, an inductor with an inductance of 1 H produces an emf of 1 V when the
current through the inductor changes at the rate of 1 A·s−1.
An inductor is a passive electrical device used in electrical circuits for its property of inductance.
An inductor is usually made as a coil (or solenoid) of conducting material, typically copper wire,
wrapped around a core either of air or of ferromagnetic material.
Electrical current through the conductor creates a magnetic flux proportional to the current. A
change in this current creates a change in magnetic flux that, in turn, generates an emf that acts to
oppose this change in current.
The inductance of an inductor is determined by several factors:

the shape of the coil; a short, fat coil has a higher inductance than one that is thin and tall.

the material that the conductor is wrapped around.

how the conductor is wound; winding in opposite directions will cancel out the inductance
effect, and you will have only a resistor.
The inductance of a solenoid is defined by:
(2)
where
is the permeability of the core material (in this case air), A is the cross-sectional area of
the solenoid, N is the number of turns and l is the length of the solenoid.
Capacitance
A capacitor's energy exists in its surrounding electric fields. It is proportional to the
square of the field strength, which is proportional to the charges on the plates. If we assume the
plates carry charges that are the same in magnitude, +q and -q, then the energy stored in the
capacitor must be proportional to q2. For historical reasons, we write the constant of
proportionality as 1/2C,
EC12Cq2
The constant C is a geometrical property of the capacitor, called its capacitance.
Based on this definition, the units of capacitance must be coulombs squared per joule,
and this combination is more conveniently abbreviated as the farad, 1 F=1 C2/J. “Condenser” is a
less formal term for a capacitor. Note that the labels printed on capacitors often use MF to mean
μF, even though MF should really be the symbol for mega farads, not microfarads. Confusion
doesn't result from this nonstandard notation, since microfarad and microfarad values are the
most common, and it wasn't until the 1990's that even mill farad and farad values became
available in practical physical sizes. Figure a shows the symbol used in schematics to represent a
capacitor.
Methods of Cable Installation
Method A

A1 - Insulated single core conductors in conduit in a thermally insulated wall
 A2 - Multicore cable in conduit in a thermally insulated wall
This method also applies to single core or multicore cables installed directly in a thermally insulated
wall (use methods A1 and A2 respectively), conductors installed in mouldings, architraves and
window frames.
Method B


B1 - Insulated single core conductors in conduit on a wall
B2 - Multicore cable in conduit on a wall
This method applies when a conduit is installed inside a wall, against a wall or spaced less than
0.3 x D (overall diameter of the cable) from the wall. Method B also applies for cables installed
in trunking / cable duct against a wall or suspended from a wall and cables installed in building
cavities.
Method C

C - Single core or multi-core cable on a wooden wall
This method also applies to cables fixed directly to walls or ceilings, suspended from
ceilings, installed on unperforated cable trays (run horizontally or vertically) and installed
directly in a masonry wall (with thermal resistivity less than 2 K.m/W).
Method D

D1 - Multicore or single core cables installed in conduit buried in the ground

D2 - Multicore or single core cables buried directly in the ground
Method E

E - Multicore cable in free-air
This method applies to cables installed on cable ladder, perforated cable tray
or cleats provided that the cable is spaced more than 0.3 x D (overall
diameter of the cable) from the wall. Note that cables installed on
unperforated cable trays are classified under Method C.
Method F

F - Single core cables touching in free-air
This method applies to cables installed on cable ladder, perforated
cable tray or cleats provided that the cable is spaced more than 0.3 x D
(overall diameter of the cable) from the wall. Note that cables installed
on unperforated cable trays are classified under Method C.
Method G

G - Single-core cables laid flat and spaced in free-air
This method applies to cables installed on cable ladder,
perforated cable tray or cleats provided that the cable is spaced
more than 0.3 x D (overall diameter of the cable) from the wall
and with at least 1 x D spacings between cables. Note that cables
installed on unperforated cable trays are classified under Method
C. This method also applies to cables installed in air supported by
insulators.
Voltage Drop and Power Loss
Types of Cables used in industries
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
VFD Cable
Variable-frequency AC motor drive output cables are subject to harsh operating environments characterized by high
voltage spikes, high noise levels and adverse environmental conditions. Typical cabling solutions for this application
have been unshielded tray cables, single-conductor lead wire installed in conduit or continuously-welded armored
cable.
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
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
Localization of Cable Fault
Substation Design and Layout
The First Step in designing a Substation is to design an Earthing and
Bonding System.
Earthing and Bonding
The function of an earthing and bonding system is to provide an earthing
system connection to which transformer neutrals or earthing impedances may
be connected in order to pass the maximum fault current. The earthing system
also ensures that no thermal or mechanical damage occurs on the equipment
within the substation, thereby resulting in safety to operation and maintenance
personnel. The earthing system also guarantees eqipotential bonding such that
there are no dangerous potential gradients developed in the substation.
In designing the substation, three voltage have to be considered.
1. Touch Voltage: This is the difference in potential between the surface
potential and the potential at an earthed
equipment whilst a man is standing and touching the earthed
structure.
2. Step Voltage: This is the potential difference developed when a man bridges
a distance of 1m with his feet
while not touching any other earthed equipment.
3. Mesh Voltage: This is the maximum touch voltage that is developed in the
mesh of the earthing grid.
Substation Earthing Calculation Methodology
Calculations for earth impedances and touch and step potentials are based on
site measurements of ground resistivity and system fault levels. A grid layout
with particular conductors is then analysed to determine the effective substation
earthing resistance, from which the earthing voltage is calculated.
In practice, it is normal to take the highest fault level for substation earth grid
calculation purposes. Additionally, it is necessary to ensure a sufficient margin
such that expansion of the system is catered for.
To determine the earth resistivity, probe tests are carried out on the site. These
tests are best performed in dry weather such that conservative resistivity
readings are obtained.
Earthing Materials
1. Conductors: Bare copper conductor is usually used for the substation
earthing grid. The copper bars themselves
usually have a cross-sectional area of 95 square millimetres,
and they are laid at a shallow depth
of 0.25-0.5m, in 3-7m squares. In addition to the buried
potential earth grid, a separate above ground
earthing ring is usually provided, to which all metallic
substation plant is bonded.
2. Connections: Connections to the grid and other earthing joints should not be
soldered because the heat generated
during fault conditions could cause a soldered joint to fail.
Joints are usually bolted, and in this case, the
face of the joints should be tinned.
3. Earthing Rods: The earthing grid must be supplemented by earthing rods to
assist in the dissipation of earth fault
currents and further reduce the overall substation earthing
resistance. These rods are usually made of
solid copper, or copper clad steel.
4. Switchyard Fence
Earthing: The switchyard fence earthing practices are possible and
are used by different utilities. These are:
(i) Extend the substation earth grid 0.5m-1.5m beyond the
fence perimeter. The fence is then
bonded to the grid at regular intervals.
(ii) Place the fence beyond the perimeter of the switchyard
earthing grid and bond the fence to its
own earthing rod system. This earthing rod system is not
coupled to the main substation earthing
grid.
Layout of Substation
The layout of the substation is very important since there should be a Security
of Supply. In an ideal substation all circuits and equipment would be duplicated
such that following a fault, or during maintenance, a connection remains
available. Practically this is not feasible since the cost of implementing such a
design is very high. Methods have been adopted to achieve a compromise
between complete security of supply and capital investment. There are four
categories of substation that give varying securities of supply:
 Category 1: No outage is necessary within the substation for either
maintenance or fault conditions.
 Category 2: Short outage is necessary to transfer the load to an
alternative circuit for maintenance or fault conditions.
 Category 3: Loss of a circuit or section of the substation due to fault or
maintenance.
 Category 4: Loss of the entire substation due to fault or maintenance.
Different Layouts for Substations
Single Busbar
The general schematic for such a substation is shown in the figure below.
With this design, there is an ease of operation of the substation. This design
also places minimum reliance on signalling for satisfactory operation of
protection. Additionally there is the facility to support the economical operation
of future feeder bays.
Such a substation has the following characteristics.
 Each circuit is protected by its own circuit breaker and hence plant
outage does not necessarily result in loss of supply.
 A fault on the feeder or transformer circuit breaker causes loss of the
transformer and feeder circuit, one of which may be restored after
isolating the faulty circuit breaker.
 A fault on the bus section circuit breaker causes complete shutdown of
the substation. All circuits may be restored after isolating the faulty
circuit breaker.
 A busbar fault causes loss of one transformer and one feeder.
Maintenance of one busbar section or isolator will cause the temporary
outage of two circuits.
 Maintenance of a feeder or transformer circuit breaker involves loss of
the circuit.
 Introduction of bypass isolators between busbar and circuit isolator
allows circuit breaker maintenance facilities without loss of that circuit.
Mesh Substation
The general layout for a full mesh substation is shown in the schematic below.
The characteristics of such a substation are as follows.
 Operation of two circuit breakers is required to connect or disconnect a
circuit, and disconnection involves opening of a mesh.
 Circuit breakers may be maintained without loss of supply or protection,
and no additional bypass facilities are required.
 Busbar faults will only cause the loss of one circuit breaker. Breaker
faults will involve the loss of a maximum of two circuits.
 generally, not more than twice as many outgoing circuits as infeeds are
used in order to rationalise circuit equipment load capabilities and
ratings.
One and a half Circuit Breaker layout
The layout of a 1 1/2 circuit breaker substation is shown in the schematic
below.
The reason that such a layout is known as a 1 1/2 circuit breaker is due to the
fact that in the design, there are 9 circuit breakers that are used to protect the 6
feeders. Thus, 1 1/2 circuit breakers protect 1 feeder. Some characteristics of
this design are:
 There is the additional cost of the circuit breakers together with the
complex arrangement.


It is possible to operate any one pair of circuits, or groups of pairs of
circuits.
There is a very high security against the loss of supply.
Principle of Substation Layouts
Substation layout consists essentially in arranging a number of switchgear
components in an ordered pattern governed by their function and rules of
spatial separation.
Spatial Separation

Earth Clearance: this is the clearance between live parts and earthed
structures, walls, screens and ground.
 Phase Clearance: this is the clearance between live parts of different
phases.
 Isolating Distance: this is the clearance between the terminals of an
isolator and the connections thereto.
 Section Clearance: this is the clearance between live parts and the
terminals of a work section. The limits of this work section, or
maintenance zone, may be the ground or a platform from which the man
works.
Separation of maintenance zones
Two methods are available for separating equipment in a maintenance zone that
has been isolated and made dead.
1. The provision of a section clearance
2. Use of an intervening earthed barrier
The choice between the two methods depends on the voltage and whether
horizontal or vertical clearances are involved.
 A section clearance is composed of a the reach of a man, taken as 8 feet,
plus an earth clearance.
 For the voltage at which the earth clearance is 8 feet, the space required
will be the same whether a section clearance or an earthed barrier is
used.
Separation by earthed barrier = Earth Clearance + 50mm for barrier + Earth
Clearance
Separation by section clearance = 2.44m + Earth clearance
 For vertical clearances it is necessary to take into account the space
occupied by the equipment and the need for an access platform at higher
voltages.
 The height of the platform is taken as 1.37m below the highest point of
work.
Establishing Maintenance Zones
Some maintenance zones are easily defined and the need for them is self
evident as is the case of a circuit breaker. There should be a means of isolation
on each side of the circuit breaker, and to separate it from adjacent live parts,
when isolated, either by section clearances or earth barriers.
Electrical Separations
 Together with maintenance zoning, the separation, by isolating distance
and phase clearances, of the substation components and of the
conductors interconnecting them constitute the main basis of substation
layouts.
There are at least three such electrical separations per phase that are needed in a
circuit:
1. Between the terminals of the busbar isolator and their connections.
2. Between the terminals of the circuit breaker and their connections.
3. Between the terminals of the feeder isolator and their connections.
Classification of Substations
Based ON Nature Of Duties:
Step up or primary Electrical Power substation:
Primary substations are associated with the power generating plants where the voltage is stepped up
from low voltage (3.3, 6.6, 11, 33kV ) to 220kV or 400kV for transmitting the power so that huge
amount of power can be transmitted over a large distance to load centers.
Primary Grid Electrical Power Substation:
Such substations are located at suitable load centers along with the primary transmission lines. At
primary Grid Power Substations the primary transmission voltage (220kV or 400kV) is stepped down
to secondary transmission voltages (110kV) . This Secondary transmission lines are carried over to
Secondary Power Substations situated at the load centers where the voltage is further stepped down to
Sub transmission Voltage or Primary Distribution Voltages (11kV or 33kV).
Step Down or Distribution Electrical Power Substations:
Such Power Substations are located at the load centers. Here the Sub transmission Voltages of
Distribution Voltages (11kV or 33kV) are stepped down to Secondary Distribution Voltages (400kV
or 230kV). From these Substations power will be fed to the consumers to their terminals.
Basis Of Service Rendered:
Transformer Substation:
Transformers are installed on such Substations to transform the power from one voltage level to other
voltage level.
Switching Substation:
Switching substations are meant for switching operation of power lines without transforming the
voltages. At these Substations different connections are made between various transmission lines.
Different Switching Schemes are employed depends on the application to transmit the power in more
reliable manner in a network.
Converting Substation:
Such Substations are located where AC to DC conversion is required. In HVDC transmission
Converting Substations are employed on both sides of HVDC link for converting AC to DC and again
converting back from DC to AC. Converting Power Substations are also employed where frequency is
to be converted from higher to lower and lower to higher. This type of frequency conversion is
required in connecting to Grid Systems.
Based on Operation Voltage:
High Voltage Electrical Power Substation:
This type of Substation associated with operating voltages between 11kV and 66kV.
Extra High Voltage Electrical Power Substation:
This type of Substation is associated where the operating voltage is between 132kV and 400kV.
Ultra High Voltage Electrical Power Substation:
Substations where Operating Voltages are above 400kV is called Ultra High Voltage Substation
Based On Substation Design:
Outdoor Electrical Power Substations:
In Outdoor Power Substations , the various electrical equipments are installed in the switchyard below
the sky. Electrical equipment are mounted on support structures to obtain sufficient ground clearance.
Indoor Electrical Power Substation:
In Indoor Power Substations the apparatus is installed within the substation building. Such substations
are usually for the rating of 66kV. Indoor Substations are preferred in heavily polluted areas and
Power Substations situated near the seas (saline atmosphere causes Insulator Failures results in
Flashovers)
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Based on Design Configuration:
Air Insulated Electrical Power Substation:
In Air Insulated Power Substations busbars and connectors are visibe. In this Power Substations
Circuit Breakers and Isolators, Transformers, Current Transformers, Potential Transformers etc are
installed in the outdoor. Busbars are supported on the post Insulators or Strain Insulators. Substations
have galvanized Steel Structures for Supporting the equipment, insulators and incoming and outgoing
lines. Clearances are the primary criteria for these substations and occupy a large area for installation.
Gas Insulated Electrical Power Substation:
In Gas Insulated Substation Various Power Substation equipments like Circuit Breakers, Current
Transformers, Voltage Transformers, Busbars, Earth Switches,Surge Arresters, Isolators etc are in the
form of metal enclosed SF6 gas modules. The modules are assembled in accordance with the required
Configuration. The various Live parts are enclosed in the metal enclosures (modules) containing SF6
gas at high pressure. Thus the size of Power Substation reduces to 8% to 10% of the Air Insulated
Power Substation.
Hybrid Electrical Power Substation:
Hybrid Substations are the combination of both Conventional Substation and Gas Insulated
Substation. Some bays in a Power Substation are Gas Insulated Type and some are Air Insulated
Type. The design is based on convenience, Local Conditions available, area available and Cost.
Bus-Bar Arrangements
The aim of any particular arrangement of bus-bars is to achieve adequate operating flexibility,
sufficient reliability and minimum cost. The cost can be minimized by reducing the number of
circuit breakers to a minimum but by doing so the cost and complication of the protective gear
are increased. Typical bus-bar arrangements are described below:
1. Single Bus-bar Arrangement. In this arrangement a set of bus-bars is used for complete
power station and to this bus-bar are connected all generators, transformers and feeders through
circuit breakers and isolating switches. Such a bus-bar arrangement is cheaper in initial as well
as in maintenance cost and simple in operating and relaying but in case of a fault on the busbars in this arrangement whole of the supply is interrupted. Such an arrangement of bus-bars is
used in dc and small ac power stations.
2. Single Bus-bar System with Sectionalization. With increased number of generators and
outgoing feeders connected to the bus-bars, it becomes essential to provide arrangement for
sectionalizing the bus-bars so that a fault on any one section of the bus-bars may not cause a
complete shutdown. This is achieved by providing a circuit breaker and isolating switches
between the section.
3. Ring Bus-bar system. In this arrangement each feeder is supplied from two paths, so that in
case of failure of a section, supply is not interrupted.
4. Duplicate Bus-bars System. Duplicate bus-bar system with sectionalization is usually
adopted in order to maintain continuity of supply. Such a system consists of two-bus-bar
couplers and sectionalizing breaker converts the duplicate bus-bars into a ring system having
greater flexibility.
5. Double Main and Transfer Bus-bar Arrangement. This arrangement incorporates all
advantages of the double bus as well as transfer bus-scheme. This scheme needs a bus-coupler
for the on load transfer of circuits from one main bus to the other and a transfer coupler for
taking out circuit breaker of various circuits for maintenance.