JRE SCHOOL OF Engineering
Pre-University Test (Solution) - April 2015
Subject Name Power Quality
Roll No. of Student
Date
27th April 2015
For EE(VIIIth Semester) branches only
Subject Code
Max Marks
Max Duration
Time
EEE-061
100 Marks
3 Hr.
10:00 am to 1: 00 pm
Note: (1) Attempt All questions. Write precisely and neatly.
(2) All questions carry equal marks.
1. Attempt any Four parts of following:
(5 marks x 4 = 20)
(a). What are the general classes of power quality problems?
Solution:
Voltage quality and Current quality
Power quality is the combination of voltage quality and current quality. Voltage quality is
concerned with deviations of the actual voltage from the ideal voltage. The ideal voltage
is a single frequency sine wave of constant frequency and constant amplitude. The
complementary definition would be current quality. Current quality is concerned with
deviations of the actual current from the ideal current. The ideal current is also of constant
amplitude and frequency, but additionally the current frequency and phase are the same as
the frequency and phase of the voltage. Any deviation of voltage or current from the ideal
is a power quality disturbance.
A disturbance can be a voltage disturbance or a current disturbance, but it is often not
possible to distinguish between the two. Any change in current gives a change in voltage
and the other way around. Where we use a distinction between voltage and current
disturbances, we use the cause as a criterion to distinguish between them. Voltage
disturbances originate in the power network and potentially affect the customers, whereas
current disturbances originate with a customer and potentially affect the network. Again
this classification is due to fail. Starting a large induction motor leads to an over current.
Seen from the network this is clearly a current disturbance. However, the resulting
voltage dip is a voltage disturbance for a neighboring customer. For the network operator
this is a current disturbance, whereas it is a voltage disturbance for the neighboring
customer. The fact that one underlying event (the motor start in this case) leads to
different disturbances for different customers or at different locations is very common for
power quality issues.
Steady-state
Steady state disturbances can be classified into two categories namely, long duration
variations and short duration variations.
Long duration variations:
A variation is considered to be long duration when the limits are exceeded for more than
one minute. Long duration variations can be classified into overvoltage, under voltage
and sustained interruptions.
Overvoltage
An overvoltage is an increase in the rms ac voltage greater than 110 percent at the power
frequency for duration longer than 1 min. Over voltages are usually the results of load
switching (e.g. switching off a large load or energizing a capacitor bank). The over
voltages result because either the system is too weak for the desired voltage regulation or
voltage controls are inadequate. Incorrect tap settings on transformers can also result in
system over voltages.
Undervoltage
An under voltage is a decrease in the rms ac voltage to less than 90 percent at the power
frequency for duration longer than 1 min. Under voltages are the result of switching
events that are the opposite of the events that cause over voltages. A load switching on or
a capacitor bank switching off can cause an under voltage until voltage regulation
equipment on the system can bring the voltage back to within tolerances. Overloaded
circuits can result in under voltages also. The term brownout is often used to describe
sustained periods of under voltage initiated as a specific utility dispatch strategy to reduce
power demand. Because there is no formal definition for brownout and it is not as clear as
the term under voltage when trying to characterize a disturbance, the term brownout
should be avoided.
Short duration variations
This category encompasses the IEC category of voltage dips and short interruptions. Each
type of variation can be designated as instantaneous, momentary, or temporary,
depending on its duration. Short-duration voltage variations are caused by fault
conditions, the energization of large loads which require high starting currents, or
intermittent loose connections in power wiring. Depending on the fault location and the
system conditions, the fault can cause either temporary voltage drops (sags), voltage rises
(swells), or a complete loss of voltage (interruptions). The fault condition can be close to
or remote from the point of interest. In either case, the impact on the voltage during the
actual fault condition is of the short-duration variation until protective devices operate to
clear the fault.
Voltage sag
Voltage sag (also called a “dip”) is defined as a decrease in voltage magnitude below
90% of nominal, but not a complete interruption. The typical duration is from 3 to 10
cycles, 50 to 167 milliseconds. Typical end-use equipment sensitive to voltage sags are:
computers, programmable logic controllers, controller power supplies, motor starter
contactors, control relays and adjustable speed drives.
Voltage sag is a short duration decrease in voltage values. Voltage sags longer than two
minutes are classified as under voltages. Common causes of voltage sags and under
voltages are short circuits (faults) on the electric power system, motor starting, customer
load additions, and large load additions in the utility service area. Sags can cause
computers and other sensitive equipment to malfunction or simply shut off. Under voltage
conditions can damage certain types of electrical equipment.
Voltage swells
A voltage swell is the converse to the sag. A swell is a short duration increase in rms line
voltage values of 110 to 180 percent of the nominal line-voltage for duration of 0.5 cycles
to 1 minute. Voltage swells lasting longer than two minutes are classified as over
voltages. Voltage swells and over voltages are commonly caused by large load changes
and power line switching. If swells reach too high a peak, they can damage electrical
equipment. The utility's voltage regulating equipment may not react quickly enough to
prevent all swells or sags. Sources of voltage swells are line faults and incorrect tap
settings in tap changers in substations.
Interruption
Interruptions occur when voltage levels drop to zero. Interruptions are classified as
momentary, temporary, or long-term. Momentary interruptions occur when service is
interrupted, but then is automatically restored in less than two seconds.
Temporary interruptions occur when service is interrupted for more than two seconds, but
is automatically restored in less than 2 minutes. Long-term interruptions last longer than
two minutes and may require field work to restore service. In some cases, momentary
outages may go unnoticed or cause no apparent problems. However, even momentary
outages can last long enough to shut down computers and disrupt the operation of
sensitive electrical equipment.
Wave shape changes or distortions
Wave shape changes are caused by disruptions, which results in a deviation from an ideal
sine wave at power frequency. The change in the wave shape can be due to notching,
harmonics, dc offset etc.
Distortion (harmonics)
Distortion occurs when harmonic frequencies are added to the 60 Hertz (60Hz) voltage or
current waveform, making the usually smooth wave appear jagged or distorted. Distortion
can be caused by solid state devices such as rectifiers, adjustable speed controls,
fluorescent lights, and even computers themselves. At high levels, distortion can cause
computers to malfunctions and cause motors, transformers, and wires to heat up
excessively. Distortion is probably the most complicated and least understood of all
power disturbances.
Major causes
Power electronic equipment, arcing, transformer saturation
Major Consequences
Equipment overheating, high voltage/current, protective device operations
Inter-harmonics
Inter-harmonics are defined as frequency components of voltages or currents that are not
an integer multiple of the normal system frequency (e.g., 60 or 50 Hz). The main sources
of inter-harmonics are static frequency converters, cyclo-converters, induction motors,
and arcing devices. Power line carrier signals can be considered as inter-harmonics. The
effects of inter-harmonics are not well known but have been shown to affect power line
carrier signaling and induce visual flicker in display devices such as cathode ray tubes
(CRTs).
Noise
Noise, or more specifically electrical noise, is a rapid succession of transients tracking up
and down along the voltage waveform. The magnitude of these rapid transients is usually
much less than that of an isolated transient. Noise often originates in electrical motors and
motor control devices, electric arc-furnaces, electric welders, relays, and remote
atmospheric discharges such as lightning. Although less destructive than a large rapid
transient, electrical noise can cause computers to malfunction and can interfere with the
operation of communications equipment or other sensitive electronic equipment.
(b). Differentiate the contribution of harmonics generated by power electronic devices,
fast control and electrical noise.
Solution: Categories of dirty power quality sources include power system events,
nonlinear loads, and poor wiring and grounding. Examples of dirty power quality sources
include lightning, adjustable speed drives, and loose connections. Power quality experts
prefer not to use the term dirty power but like to instead use the term power quality
problems. The source of a power quality problem often causes a disturbance or power
quality variation. The disturbance can then affect the operation of end-user equipment.
This may seem confusing. To make sense of the confusing causes and effects of power
quality problems, the power quality engineer breaks down a power quality problem into
three parts: sources (initiating events), causes, and effects of power quality.
(c). Define transients in power quality. What is flicker?
Solution:
Transients are sudden but significant deviations from normal voltage or current levels.
Transients typically last from 200 millionths of a second to half a second. Transients are
typically caused by lightning, electrostatic discharges, load switching or faulty wiring.
Transients can erase or alter computer data, resulting in difficult-to-detect computational
errors. In extreme cases, transients can destroy electronic circuitry and damage electrical
equipment.
Oscillatory & Impulsive Transient
Oscillatory transients are a sudden, non-power frequency change in the steady-state
condition of voltage, current, or both, that includes both positive and negative polarity
values. It can also be distinctive by its fast-rise time and oscillations that decay over time.
An oscillatory transient is commonly a result of load and of capacitor banks switching.
Impulsive transient is a sudden, non – power frequency change in the steady-state
condition of voltage, current, or both, unidirectional in polarity. Common cause of
impulsive transients is lightning.
Impulsive Transient
Oscillatory Transient
Flicker
Flicker can be defined as small amplitude changes in voltage levels occurring at
frequencies less than 25 Hertz (25 Hz). Flicker is caused by large, rapidly fluctuating
loads such as arc furnaces and electric welders. Flicker is rarely harmful to electronic
equipment, but is more of a nuisance because it causes annoying, noticeable changes in
lighting levels.
Frequency deviations
Deviation of the power system fundamental frequency from its specified nominal value
(e.g. 50 Hz or 60 Hz).
(d). Give a schematic diagram of frequency deviation monitoring and explain.
Solution:
It is clear that power quality phenomena cover a wide range of frequencies. They include
everything from very fast transient over voltages (microsecond time frame) to longduration outages (hours or days time frame). Power quality problems also include steadystate phenomena, such as harmonic distortion, and intermittent phenomena, such as
voltage flicker. There are other instruments that can be used to help solve power quality
problems by measuring ambient conditions:
 Infrared meters can be very valuable in detecting loose connections and
overheating conductors. An annual procedure of checking the system in this
manner can help prevent power quality problems due to arcing, bad connections,
and overloaded conductors.
Noise problems related to electromagnetic radiation may require measurement of field
strengths in the vicinity of affected equipment. Magnetic gauss meters are used to
measure magnetic field strengths for inductive coupling concerns. Electric field meters
can measure the strength of electric fields for electrostatic coupling concerns.
(e). Explain the term phase angle imbalances and reasons responsible for this.
Solution:
There are a number of important factors that should be considered when selecting the
instrument. Some of the more important factors include
 Number of channels (voltage and/or current)
 Temperature specifications of the instrument
 Ruggedness of the instrument
 Input voltage range (e.g., 0 to 600 V)
 Power requirements
 Ability to measure three-phase voltages
 Input isolation (isolation between input channels and from each input to ground)
 Ability to measure currents
 Housing of the instrument (portable, rack-mount, etc.)
 Ease of use (user interface, graphics capability, etc.)
 Documentation
 Communication capability (modem, network interface)
(f). What are predominant harmonics experienced with IGBT operation?
Solution:
Power quality data gathered over time can be analyzed to provide information relating to
specific equipment performance. For example, a repetitive arcing fault from an
underground cable may signify impending cable failure, or repetitive capacitor-switching
re-strikes may signify impending failure on the capacitor-switching device. Equipment
maintenance can be quickly ordered to avoid catastrophic failure, thus preventing major
power quality disturbances which ultimately will impact overall power quality
performance.
The monitoring program must be designed based on the appropriate objectives, and it
must make the information available in a convenient form and in a timely manner (i.e.,
immediately). The most comprehensive monitoring approach will be a permanently
installed monitoring system with automatic collection of information about steady-state
power quality conditions and energy use as well as disturbances.
2. Attempt any Four parts of following
(5 marks x 4 = 20)
(a). Explain the following causes of sags.
(i) Voltage sag due to motor
(ii) Voltage sag due to single line to ground fault.
(iii) Voltage sag due to transformer energizing.
Solution:
Motor-Starting Sags
Motors have the undesirable effect of drawing several times their full load current while
starting. This large current will, by flowing through system impedances, cause voltage
sag which may dim lights, cause contactors to drop out, and disrupt sensitive equipment.
The situation is made worse by an extremely poor starting displacement factor usually in
the range of 15 to 30 percent. The time required for the motor to accelerate to rated speed
increases with the magnitude of the sag, and an excessive sag may prevent the motor from
starting successfully. Motor starting sags can persist for many seconds.
Motor-Starting Methods
Energizing the motor in a single step (full-voltage starting) provides low cost and allows
the most rapid acceleration. It is the preferred method unless the resulting voltage sag or
mechanical stress is excessive.
Autotransformer starters have two autotransformers connected in open delta. Taps
provide a motor voltage of 80, 65, or 50 percent of system voltage during start-up. Line
current and starting torque vary with the square of the voltage applied to the motor, so the
50 percent tap will deliver only 25 percent of the full-voltage starting current and torque.
The lowest tap which will supply the required starting torque is selected.
Resistance and reactance starters initially insert impedance in series with the motor. After
a time delay, this impedance is shorted out. Starting resistors may be shorted out over
several steps; starting reactors are shorted out in a single step. Line current and starting
torque vary directly with the voltage applied to the motor, so for a given starting voltage,
these starters draw more current from the line than with autotransformer starters, but
provide higher starting torque.
Reactors are typically provided with 50, 45, and 37.5 percent taps. Part-winding starters
are attractive for use with dual-rated motors (220/440 V or 230/460 V). The stator of a
dual-rated motor consists of two windings connected in parallel at the lower voltage
rating, or in series at the higher voltage rating. When operated with a part winding starter
at the lower voltage rating, only one winding is energized initially, limiting starting
current and starting torque to 50 percent of the values seen when both windings are
energized simultaneously.
Delta-wye starters connect the stator in wye for starting and then, after a time delay,
reconnect the windings in delta. The wye connection reduces the starting voltage to 57
percent of the system line-line voltage; starting current and starting torque is reduced to
33 percent of their values for full-voltage start.
(b). Give the various types of harmonic filtering devices and suggest the most applicable
one.
Solution:
Solving any diagnostic problem requires the right tools and the ability to use them. Like
doctors trying to solve a health problem, power quality engineers and technicians need
meters and other measurement tools to solve electrical facility health problems. The first
step in solving a power quality problem is to determine the cause of the problem. Making
either visual inspections or electrical measurements of the electrical distribution system
can do this.
―Power Quality Surveys show how to make visual inspections of a facility’s electrical
distribution system.
―Power Quality Measurement Tools explain how to perform and analyze power quality
measurements using power quality measurement tools. There are a myriad of power
quality measurement tools available today. They include instruments that measure and
display the basic electrical parameters of voltage, current, frequency, and impedance of
an electrical distribution system. These tools include ammeters, voltmeters, multimeters,
oscilloscopes, flicker meters, electrostatic voltmeters, infrared detectors, radio-frequency
interference and electromagnetic interference meters, harmonic and spectrum analyzers,
power quality monitors, and various types of wiring and grounding testers. These
instruments measure display, and store electrical parameters for the purpose of helping
solve power quality problems. In addition to these electrical measurement tools, there are
devices, such as video cameras and audiotape recorders, for recording the effects of
power quality problems.
With all these choices, power quality experts as well as novices must know how to choose
and use the right instrument.
(c). What is Voltage sag and swell? What are the sources of voltage sag and
interruptions?
Solution:
Voltage sag
Voltage sag (also called a “dip”) is defined as a decrease in voltage magnitude below
90% of nominal, but not a complete interruption. The typical duration is from 3 to 10
cycles, 50 to 167 milliseconds. Typical end-use equipment sensitive to voltage sags are:
computers, programmable logic controllers, controller power supplies, motor starter
contactors, control relays and adjustable speed drives.
Voltage sag is a short duration decrease in voltage values. Voltage sags longer than two
minutes are classified as under voltages. Common causes of voltage sags and under
voltages are short circuits (faults) on the electric power system, motor starting, customer
load additions, and large load additions in the utility service area. Sags can cause
computers and other sensitive equipment to malfunction or simply shut off. Under voltage
conditions can damage certain types of electrical equipment.
Voltage swells
A voltage swell is the converse to the sag. A swell is a short duration increase in rms line
voltage values of 110 to 180 percent of the nominal line-voltage for duration of 0.5 cycles
to 1 minute. Voltage swells lasting longer than two minutes are classified as over
voltages. Voltage swells and over voltages are commonly caused by large load changes
and power line switching. If swells reach too high a peak, they can damage electrical
equipment. The utility's voltage regulating equipment may not react quickly enough to
prevent all swells or sags. Sources of voltage swells are line faults and incorrect tap
settings in tap changers in substations.
Major causes
Power electronic equipment, arcing, transformer saturation
Major Consequences
Equipment overheating, high voltage/current, protective device operations
Inter-harmonics
Inter-harmonics are defined as frequency components of voltages or currents that are not
an integer multiple of the normal system frequency (e.g., 60 or 50 Hz). The main sources
of inter-harmonics are static frequency converters, cyclo-converters, induction motors,
and arcing devices. Power line carrier signals can be considered as inter-harmonics. The
effects of inter-harmonics are not well known but have been shown to affect power line
carrier signaling and induce visual flicker in display devices such as cathode ray tubes
(CRTs).
(d). Define the terms: CBEMA Curve, Coupling, Electromagnetic Compatibility, FerroResonance, Ground Electrode, Harmonic Resonance, Instantaneous Reclosing, and
Notch
Solution:
CBEMA Curve
A set of curves representing the withstand capabilities of computers in terms of the
magnitude and duration of the voltage disturbance developed by the Computer Business
Equipment Manufacturers Association (CBEMA). CBEMA has been replaced by the
Information Technology Industry Council (ITI), and a new curve has been developed that
is commonly referred to as the ITI curve.
Coupling
A circuit element, or elements, or a network that may be considered common to the input
mesh and the output mesh and through which energy may be transferred from one to
another.
Electromagnetic Compatibility
The ability of a device, equipment, or system to function satisfactorily in its
electromagnetic environment without introducing intolerable electromagnetic
disturbances to anything in that environment.
Ferro Resonance
An irregular, often chaotic type of resonance that involves the nonlinear characteristic of
iron-core (ferrous) inductors. It is nearly always undesirable when it occurs in the power
delivery system, but it is exploited in technologies such as constant-voltage transformers
to improve the power quality.
Ground Electrode
A conductor or group of conductors in intimate contact with the earth for the purpose of
providing a connection with the ground.
Harmonic Resonance
A condition in which the power system is resonating near one of the major harmonics
being produced by nonlinear elements in the system, thus exacerbating the harmonic
distortion.
Instantaneous Reclosing
A term commonly applied to reclosing of a utility breaker as quickly as possible after an
interrupting fault current. Typical times are 18 to 30 cycles.
Notch
A switching (or other) disturbance of the normal power voltage waveform, lasting less
than a half-cycle, which is initially of opposite polarity than the waveform and is thus
subtracted from the normal waveform in terms of the peak value of the disturbance
voltage. This includes complete loss of voltage for up to a half-cycle.
(e). What are the different voltage sag mitigation techniques? Explain in details.
Solution:
Following are mitigation techniques
 Motor - generator set
 Ferro resonance transformer
 Electric tap changer
 Uninterrupted power supply
 Static transfer switch
 Series – connected voltage source converter
 Shunt connected back – up source
 Superconducting magnetic energy storage (SMES) devices
Motor Generator Set
Motor-generator (M-G) sets
come in a wide variety of
sizes and configurations.
This is a mature technology
that is still useful for
isolating critical loads from
sags and interruptions on the
power system. The concept
is very simple.
Motor Generator Sets are a
Block diagram of typical M-G set with flywheel
combination of an electrical
generator and an engine
mounted together to form a single piece of equipment. Motor generator set is also referred
to as a gen-set, or more commonly, a generator. Motor generator sets are used throughout
industry to provide electrical power on demand.
A motor powered by the line drives a generator that powers the load. Flywheels on the
same shaft provide greater inertia to increase ride-through time. When the line suffers a
disturbance, the inertia of the machines and the flywheels maintains the power supply for
several seconds.
This arrangement may also be used to separate sensitive loads from other classes of
disturbances such as harmonic distortion and switching transients. While simple in
concept, M-G sets have disadvantages for some types of loads:
There are losses associated with the machines, although they are not necessarily larger
than those in other technologies described here.
Noise and maintenance may be issues with some installations. The frequency and voltage
drop during interruptions as the machine slows. This may not work well with some loads.
Another type of M-G set uses a special synchronous generator called a written-pole motor
that can produce a constant 60-Hz frequency as the machine slows. It is able to supply a
constant output by continually changing the polarity of the rotor’s field poles.
Thus, each revolution can have a different number of poles than the last one. Constant
output is maintained as long as the rotor is spinning at speeds between 3150 and 3600
revolutions per minute (rpm). Flywheel inertia allows the generator rotor to keep rotating
at speeds above 3150 rpm once power shuts off. The rotor weight typically generates
enough inertia to keep it spinning fast enough to produce 60 Hz for 15 s under full load.
Another means of compensating for the frequency and voltage drop while energy is being
extracted is to rectify the output of the generator and feed it back into an inverter. This
allows more energy to be extracted, but also introduces losses and cost.
Ferro-resonance Transformer
Ferro-resonant transformers, also called constantvoltage transformers (CVTs), can handle most
voltage sag conditions.
CVTs are especially attractive for constant,
lowpower loads. Variable loads, especially with
high inrush currents, present more of a problem for
CVTs because of the tuned circuit on the output.
Ferro-resonant transformers are basically 1:1
transformers which are excited high on their
saturation curves, thereby providing an output
Ferro-resonance Transformer
voltage which is not significantly affected by input
voltage variations. A typical ferro-resonant transformer schematic circuit diagram is
shown above.
Electric Tap Changer
The actual tap-changer which is used in the Indian Railways locos, it will better to
understand in general what tap-changers are. The output voltage of a transformer varies
according to the turn’s ratio of the primary and the secondary windings of the
transformer. It can appreciated that at any point of the primary or the secondary winding
the voltage is different from any other point on the same winding because these points are
at different ratios with respect to the other winding.
Hence each and every tap brought out from the winding gives a different voltage. Broadly
tap-changers can be divided into two categories-namely off-load and on-load. Off-load
tap-changers cannot be operated while current is flowing in the circuit. Off-load tap
changers are used mainly for non-critical applications where a momentary interruption in
the current can be tolerated. Hence, such tap-changers have no use in traction duty. In
traction only On-Load Tap-Changers (OLTC) are used. They are capable of changing the
taps rapidly without interrupting the flow of current.
Uninterrupted Power Supply
An Uninterruptible Power Supply (UPS), also known as an Uninterruptible Power
Source, Uninterruptible Power System, Continuous Power Supply (CPS) or a
battery backup is a device which maintains a continuous supply of electric power to
connected equipment by supplying power from a separate source when utility power is
not available. There are two distinct types of UPS: off-line and line-interactive (also
called on-line).
An off-line UPS remains idle until a power failure occurs, and then switches from utility
power to its own power source, almost instantaneously. An on-line UPS continuously
powers the protected load from its reserves (usually lead-acid batteries or stored kinetic
energy), while simultaneously replenishing the reserves from the AC power. The on-line
type of UPS, in addition to providing protection against complete failure of the utility
supply, provides protection against all common power problems, and for this reason it is
also known as a power conditioner and a line conditioner.
On-line UPS
Figure shows a typical configuration
of an on-line UPS. In this design, the
load is always fed through the UPS.
The incoming ac power is rectified
into dc power, which charges a bank
of batteries. This dc power is then
inverted back into ac power, to feed
the load. If the incoming ac power
fails, the inverter is fed from the
On-line UPS
batteries and continues to supply the
load. In addition to providing ride-through for power outages, an on-line UPS provides
very high isolation of the critical load from all power line disturbances. However, the online operation increases the losses and may be unnecessary for protection of many loads.
Standby UPS
A standby power supply is sometimes termed off-line UPS since the normal line power is
used to power the equipment until a disturbance is detected and a switch transfers the load
to the battery backed inverter. The transfer time from the normal source to the battery
backed inverter is important. The CBEMA curve shows that 8 ms is the lower limit on
interruption through for
power-conscious
manufacturers.
Therefore, a transfer
time of 4 ms would
ensure continuity of
operation for the critical
load. A standby power
Standby UPS
supply does not typically
provide any transient protection or voltage regulation as does an on-line UPS. This is the
most common configuration for commodity UPS units available at retail stores for
protection of small computer loads. UPS specifications include kilovolt ampere capacity,
dynamic and static voltage regulation, harmonic distortion of the input current and output
voltage, surge protection, and noise attenuation. The specifications should indicate, or the
supplier should furnish, the test conditions under which the specifications are valid.
Hybrid UPS
Similar in design to the
standby UPS, the hybrid UPS
utilizes a voltage regulator on
the UPS output to provide
regulation to the load and
momentary ride-through when
the transfer from normal to
UPS supplies is made.
Static Transfer Switch and fast transfer switches
The Static Transfer Switch provides break-before-make switching between two
independent AC power sources for uninterrupted power to sensitive electronic equipment.
When used with redundant AC power sources, the switch permits maintenance without
shutting down critical equipment. The switch utilizes solid-state switching devices close
to the critical load, thus producing high levels of power availability and power system
tolerance. The switch is suited for data processing, distributed computing,
telecommunications equipment, and high-tech manufacturing applications. Features
include:
 0.25 cycle maximum transfers between AC power sources
 Manual and automatic transfers
 Selectable preferred input sources
 AccuVar TVSS for both AC inputs
Superconducting magnetic energy storage (SMES) devices
An SMES device can be used to alleviate voltage sags and brief interruptions. The energy
storage in an SMES-based system is provided by the electric energy stored in the current
flowing in a superconducting magnet. Since the coil is lossless, the energy can be released
almost instantaneously. Through voltage regulator and inverter banks, this energy can be
injected into the protected electrical system in less than 1 cycle to compensate for the
missing voltage during a voltage sag event. The SMES-based system has several
advantages over battery-based UPS systems:
i. SMES-based systems have a much smaller footprint than batteries for the same
energy
storage and power delivery capability.
ii. The stored energy can be delivered to the protected system more quickly.
iii. The SMES system has virtually unlimited discharge and charge duty cycles. The
discharge and recharge cycles can be performed thousands of times without any
degradation to the superconducting magnet.
(f). Differentiate between power conditioner and UPS. Draw the circuit diagram of power
conditioner.
Solution:
Power quality monitoring is the process of gathering, analyzing, and interpreting raw
measurement data into useful information. The process of gathering data is usually
carried out by continuous measurement of voltage and current over an extended period.
The process of analysis and interpretation has been traditionally performed manually, but
recent advances in signal processing and artificial intelligence fields have made it
possible to design and implement intelligent systems to automatically analyze and
interpret raw data into useful information with minimum human intervention.
Power quality monitoring programs are often driven by the demand for improving the
system wide power quality performance. Many industrial and commercial customers have
equipment that is sensitive to power disturbances, and, therefore, it is more important to
understand the quality of power being provided. Examples of these facilities include
computer networking and telecommunication facilities, semiconductor and electronics
manufacturing facilities, biotechnology and pharmaceutical laboratories, and financial
data-processing centers. Hence, in the last decade many utility companies have
implemented extensive power quality monitoring programs.
3. Attempt any Two parts of following
(10 marks x 2 = 20)
(a). What are the international power quality standards? How to achieve minimum THD
distortion?
Solution:
At the fundamental frequency, power systems are primarily inductive, and the equivalent
impedance is sometimes called simply the short-circuit reactance. Capacitive effects are
frequently neglected on utility distribution systems and industrial power systems. One of
the most frequently used quantities in the analysis of harmonics on power systems is the
short-circuit impedance to the point on a network at which a capacitor is located. If not
directly available, it can be computed from
short-circuit study results that give
either the short-circuit megavoltampere (MVA) or the short-circuit current as follows
Z sc  Rsc  jX sc
kV 2
kV *1000


MVAsc
3 * I sc
Where,
ZSC = short-circuit impedance
RSC = short-circuit resistance
XSC = short-circuit reactance
kV = phase-to-phase voltage, kV
MVASC = three-phase short-circuit MVA
ISC = short-circuit current, A)
ZSC is a phasor quantity, consisting of both resistance and reactance. However, if the
short-circuit data contain no phase information, one is usually constrained to assuming
that the impedance is purely reactive. This is a reasonably good assumption for industrial
power systems for buses close to the mains and for most utility systems.
(b). What are transient over voltages? Explain the different types of transient over
voltages.
Solution:
There are two main sources of transient over voltages on utility systems, capacitor
switching and Lightning. These are also sources of transient over voltages as well as
many of other switching phenomena within end-user facilities. Some power electronic
devices generate significant transients when they switch. Transient over voltages can be
generated at high frequency (load switching and lightning), medium frequency (capacitor
energizing), or low frequency.
Capacitor switching
Capacitor switching is one of the most common switching events on utility systems.
Capacitors are used to provide reactive power (in units of vars) to correct the power
factor, which reduces losses and supports the voltage on the system. They are a very
economical and generally trouble-free means of accomplishing these goals. Alternative
methods such as the use of rotating machines and electronic var compensators are much
more costly or have high maintenance costs. Thus, the use of capacitors on power systems
is quite common and will continue to be. One drawback to the use of capacitors is that
they yield oscillatory transients when switched. Some capacitors are energized all the
time (a fixed bank), while others are switched according to load levels. Various control
means, including time, temperature, voltage, current, and reactive power, are used to
determine when the capacitors are switched. It is common for controls to combine two or
more of these functions, such as temperature with voltage override.
Oscillatory transient
An oscillatory transient is a sudden, non–power frequency change in the steady-state
condition of voltage, current, or both, that includes both positive and negative polarity
values. An oscillatory transient consists of a voltage or current whose instantaneous value
changes polarity rapidly. It is described by its spectral content (predominate frequency),
duration, and magnitude. The spectral content subclasses defined in Table 3.1 are high,
medium, and low frequency. The frequency ranges for these classifications are chosen to
coincide with common types of power system oscillatory transient phenomena.
Oscillatory transients with a primary frequency component greater than 500 kHz and a
typical duration measured in microseconds (or several cycles of the principal frequency)
are considered high-frequency transients. These transients are often the result of a local
system response to an impulsive transient.
Categories
Impulsive Transient
1. Nanosecond
2. Microsecond
3. Milli second
Oscillatory Transient
1. Low frequency
2. Medium
frequency
3. High frequency
Typical
Content
Spectral Typical duration
Typical Voltage
5 ns rise
1 ns rise
0.1 ms rise
<50 ns
50 ns-1 ms
>1ms
<5kHz
5-500kHz
0.3-50 ms
20s
0-4pu
0-8pu
0.5-5 MHz
5s
0-4pu
A transient with a primary frequency component between 5 and 500 kHz with duration
measured in the tens of microseconds (or several cycles of the principal frequency) is
termed a medium-frequency transient.
A transient with a primary frequency component less than 5 kHz, and a duration from 0.3
to 50 ms, is considered a low-frequency transient. This category of phenomena is
frequently encountered on utility sub-transmission and distribution systems and is caused
by many types of events. The most frequent is capacitor bank energization, which
typically results in an oscillatory voltage transient with a primary frequency between 300
and 900 Hz. Back-to-back capacitor energization results in oscillatory transient currents
in the tens of kilohertz. Cable switching results in oscillatory voltage transients in the
same frequency range. Medium-frequency transients can also be the result of a system
response to an impulsive transient.
(c). Define lightning? Discuss in detail about the over voltages due to lightning and the
problems associated with it.
Solution:
Lightning is a powerful source of impulsive transients. We will concentrate on how
lightning causes transient over voltages to appear on power systems. Figure shows some
of the places where lightning can strike that result in lightning currents being conducted
from the power system into loads. The most obvious conduction path occurs during a
direct strike to a phase wire, either on the primary or the secondary side of the
transformer. This can generate very high over voltages, but some analysts question
whether this is the most common way that lightning surges enter load facilities and cause
damage. Very similar transient over voltages can be generated by lightning currents
flowing along ground conductor paths. Note that there can be numerous paths for
lightning currents to enter the grounding system. Common ones, indicated by the dotted
lines in figure, include the primary ground, the secondary ground, and the structure of the
load facilities. Note also that strikes to the primary phase are conducted to the ground
circuits through the arresters on the service transformer. Thus, many more lightning
impulses may be observed at loads than one might think.
It is to be noted that grounds are never perfect conductors, especially for impulses. While
most of the surge current may eventually be dissipated into the ground connection closest
to the strike, there will be substantial surge currents flowing in other connected ground
conductors in the first few microseconds of the strike.
A direct strike to a phase conductor generally causes line flashover near the strike point.
Not only does this generate an impulsive transient, but it causes a fault with the
accompanying voltage sags and interruptions. The lightning surge can be conducted a
considerable distance along utility lines and cause multiple flashovers at pole and tower
structures as it passes. The interception of the impulse from the phase wire is fairly
straightforward if properly installed surge arresters are used. If the line flashes over at the
location of the strike, the tail of the impulse is generally truncated. Depending on the
effectiveness of the grounds along the surge current path, some of the current may find its
way into load apparatus. Arresters near the strike may not survive because of the severe
duty (most lightning strokes are actually many strokes in rapid-fire sequence).
Lightning does not have to actually strike a conductor to inject impulses into the power
system. Lightning may simply strike near the line and induce an impulse by the collapse
of the electric field. Lightning may also simply strike the ground near a facility causing
the local ground reference to rise considerably. This may force currents along grounded
conductors into a remote ground, possibly passing near sensitive load apparatus.
Many investigators in this field postulate that lightning surges enter loads from the utility
system through the interwinding capacitance of the service transformer. The concept is
that the lightning impulse is so fast that the inductance of the transformer windings blocks
the first part of the wave from passing through by the turn ratio. However, the interwinding capacitance may offer a ready path for the high-frequency surge. This can permit
the existence of a voltage on the secondary terminals that is much higher than what the
turn ratio of the windings would suggest. The degree to which capacitive coupling occurs
is greatly dependent on the design of the transformer. Not all transformers have a
straightforward high-to-low capacitance because of the way the windings are constructed.
The winding-to-ground capacitance may be greater than the winding-to-winding
capacitance, and more of the impulse may actually be coupled to ground than to the
secondary winding. In any case, the resulting transient is a very short single impulse, or
train of impulses, because the inter-winding capacitance charges quickly.
Arresters on the secondary winding should have no difficulty dissipating the energy in
such a surge, but the rates of rise can be high. Thus, lead length becomes very important
to the success of an arrester in keeping this impulse out of load equipment.
This is a particular problem if the load system offers a better ground and much of the
surge current flows through conductors in the load facility on its way to ground. The chief
power quality problems with lightning stroke currents entering the ground system are
i. They raise the potential of the local ground above other grounds in the vicinity
by several
kilovolts. Sensitive electronic equipment that is connected between two
ground
references, such as a computer connected to the telephone
system through a modem, can
fail when subjected to the lightning surge voltages.
ii. They induce high voltages in phase conductors as they pass through cables on
the way to
a better ground.
The problems are related to the so-called low-side surge problem. Lightning causes more
flashovers of utility lines than previously thought. Evidence is also mounting that
lightning stroke current wave fronts are faster than previously thought and that multiple
strikes appear to be the norm rather than the exception. Durations of some strokes may
also be longer than reported by earlier researchers. These findings may help explain
failures of lightning arresters that were thought to have adequate capacity to handle large
lightning strokes.
4. Attempt any Two parts of following
(10 marks x 2 = 20)
(a). What are the equipments needed to minimize the voltage disturbance? Identify the
location of these.
Solution:
Many power quality problems stem from lightning. Not only can the high-voltage
impulses damage load equipment, but the temporary fault that follows a lightning strike to
the line causes voltage sags and interruptions.
Arresters and TVSS devices protect equipment from transient over voltages by limiting
the maximum voltage, and the terms are sometimes used interchangeably. However,
TVSSs are generally associated with devices used at the load equipment. A TVSS will
sometimes have more surge-limiting elements than an arrester, which most commonly
consists solely of MOV blocks. An arrester may have more energy-handling capability.
The elements that make up these devices can be classified by two different modes of
operation, crowbar and clamping.
Crowbar devices are normally open devices that conduct current during overvoltage
transients. Once the device conducts, the line voltage will drop to nearly zero due to the
short circuit imposed across the air or a special gas. The gap arcs over when a sufficiently
high overvoltage transient appears. Once the gap arcs over, usually power frequency
current, or follow current,‖ will continue to flow in the gap until the next current zero.
Thus, these devices have the disadvantage that the power frequency voltage drops to zero
or to a very low value for at least one-half cycle. This will cause some loads to drop
offline unnecessarily. Clamping devices for ac circuits are commonly non- linear resistors
(varistors) that conduct very low amounts of current until an overvoltage occurs. Then
they start to conduct heavily, and their impedance drops rapidly with increasing voltage.
These devices effectively conduct increasing amounts of current (and energy) to limit the
voltage rise of a surge. They have an advantage over gap-type devices in that the voltage
is not reduced below the conduction level when they begin to conduct the surge current.
(b). What are the tools for transient analysis for electrical power system?
Solution:
The most widely used computer programs for transient’s analysis of power systems are
the Electromagnetic Transients Program, commonly known as EMTP, and its derivatives
such as the Alternate Transients Program (ATP). EMTP was originally developed by
Hermann W. Dommel at the Bonneville Power Administration (BPA) in the late 1960s15
and has been continuously upgraded since. One of the reasons this program is popular is
its low cost due to some versions being in the public domain. Some of the simulations
presented in this book have been performed with a commercial analysis tool known as
PSCAD/EMTDC, a program developed by the Manitoba HVDC Research Center. This
program features a very sophisticated graphical user interface that enables the user to be
very productive in this difficult analysis. Some power system analysts use computer
programs developed more for the analysis of electronic circuits, such as the well-known
SPICE program16 and its derivatives.
Although the programs just discussed continue to be used extensively, there are now
many other capable programs available. We will not attempt to list each one because
there are so many and, also, at the present rate of software development, any such list
would soon be outdated. The reader is referred to the Internet since all vendors of this
type of software maintain websites.
Nearly all the tools for power systems solve the problem in the time domain, re-creating
the waveform point by point. A few programs solve in the frequency domain and use the
Fourier transform to convert to the time domain. Unfortunately, this essentially restricts
the addressable problems to linear circuits. Time-domain solution is required to model
nonlinear elements such as surge arresters and transformer magnetizing characteristics.
The penalty for this extra capability is longer solution times, which with modern
computers becomes less of a problem each day.
It takes considerably more modeling expertise to perform electromagnetic transient’s
studies than to perform more common power system analyses such as of the power flow
or of a short circuit. Therefore, this task is usually relegated to a few specialists within the
utility organization or to consultants.
While transients programs for electronic circuit analysis may formulate the problem in
any number of ways, power systems analysts almost uniformly favor some type of nodal
admittance formulation. For one thing, the system admittance matrix is sparse allowing
the use of very fast and efficient sparsity techniques for solving large problems. Also, the
nodal admittance formulation reflects how most power engineers view the power system,
with series and shunt elements connected to buses where the voltage is measured with
respect to a single reference.
To obtain conductances for elements described by differential equations, transients
programs discretize the equations with an appropriate numerical integration formula. The
simple trapezoidal rule method appears to be the most commonly used, but there are also
a variety of Runge-Kutta and other formulations used. Nonlinearities are handled by
iterative solution methods. Some programs include the nonlinearities in the general
formulation, while others, such as those that follow the EMTP methodology, separate the
linear and nonlinear portions of the circuit to achieve faster solutions. This impairs the
ability of the program to solve some classes of nonlinear problems but is not usually a
significant constraint for most power system problems.
(c). Explain in detail about classification of linear loads and non linear loads used in
harmonic studies. Explain the concept of harmonic phenomena under the presence of
harmonic producing loads.
Solutions:
Electricity generation is normally produced at constant frequencies of 50 Hz or 60 Hz and
the generators’ e.m.f. can be considered practically sinusoidal. However, when a source
of sinusoidal voltage is applied to a nonlinear device or load, the resulting current is not
perfectly sinusoidal. In the presence of system impedance this current causes a nonsinusoidal voltage drop and, therefore, produces voltage distortion at the load terminals,
i.e. the latter contains harmonics. Harmonic distortion is caused by nonlinear devices in
the power system. A nonlinear device is one in which the current is not proportional to
the applied voltage. Figure illustrates this concept by the case of a sinusoidal voltage
applied to a simple nonlinear resistor in which
Voltage function, v(t) = V sin (ωt)
Current function, i(t) = I sin (ωt ± Ø)
The voltage and current vary according
to the curve shown. While the applied
voltage is perfectly sinusoidal, the
resulting current is distorted. Increasing
the voltage by a few percent may cause
the current to double and take on a
different wave shape. This is the source
of most harmonic distortion in a power
system.
Current distortion caused by non-linear resistance
Next figure illustrates that any periodic,
distorted waveform can be expressed as a sum of sinusoids. When a waveform is identical
from one cycle to the next, it can be represented as a sum of pure sine waves in which the
frequency of each sinusoid is an integer multiple of the fundamental frequency of the
distorted wave. This multiple is called a harmonic of the fundamental, hence the name of
this subject matter. The sum of sinusoids
is referred to as a Fourier series, named
after the great mathematician who
discovered the concept.
Because of the above property, the
Fourier series concept is universally
applied in analyzing harmonic problems.
The system can now be analyzed
separately at each harmonic. In addition,
finding the system response of a
sinusoid of each harmonic individually
is much more straightforward compared
Fourier series representation of distorted wave
to that with the entire distorted waveforms. The outputs at each frequency are then
combined to form a new Fourier series, from which the output waveform may be
computed.
When both the positive and negative half cycles of a waveform have identical shapes, the
Fourier series contains only odd harmonics. This offers a further simplification for most
power system studies because most common harmonic-producing devices look the same
to both polarities. In fact, the presence of even harmonics is often a clue that there is
something wrong either with the load equipment or with the transducer used to make the
measurement.
There are notable exceptions to this such as half-wave rectifiers and arc furnaces when
the arc is random. Usually, the higher-order harmonics (above the range of the 25th to
50th, depending on the system) are negligible for power system analysis. While they may
cause interference with low-power electronic devices, they are usually not damaging to
the power system. It is also difficult to collect sufficiently accurate data to model power
systems at these frequencies. A common exception to this occurs when there are system
resonances in the range of frequencies. These resonances can be excited by notching or
switching transients in electronic power converters. This causes voltage waveforms with
multiple zero crossings which disrupt timing circuits. These resonances generally occur
on systems with underground cable but no power factor correction capacitors.
If the power system is depicted as series and shunt elements, as is the conventional
practice, the vast majority of the nonlinearities in the system are found in shunt elements
(i.e., loads). The series impedance of the power delivery system (i.e., the short-circuit
impedance between the source and the load) is remarkably linear.
In transformers, also, the source of harmonics is the shunt branch (magnetizing
impedance) of the common T model. The leakage impedance is linear. Thus, the main
sources of harmonic distortion will ultimately be end-user loads. This is not to say that all
end users who experience harmonic distortion will themselves have significant sources of
harmonics, but that the harmonic distortion generally originates with some end-user’s
load or combination of loads.
5. Attempt any Two parts of following
(10 marks x 2 = 20)
(a). Define for the following terms related with IEEE standards.(i)System Characteristics
Response (ii) System Impedance
Solutions:
System Response Characteristics
In power systems, the response of the system is equally as important as the sources of
harmonics. In fact, power systems are quite tolerant of the currents injected by harmonic
producing loads unless there is some adverse interaction with the impedance of the
system. Identifying the sources is only half the job of harmonic analysis. The response of
the power system at each harmonic frequency determines the true impact of the nonlinear
load on harmonic voltage distortion. There are three primary variables affecting the
system response characteristics, i.e., the system impedance, the presence of a capacitor
bank, and the amount of resistive loads in the system.
System Impedance
At the fundamental frequency, power systems are primarily inductive, and the equivalent
impedance is sometimes called simply the short-circuit reactance. Capacitive effects are
frequently neglected on utility distribution systems and industrial power systems. One of
the most frequently used quantities in the analysis of harmonics on power systems is the
short-circuit impedance to the point on a network at which a capacitor is located. If not
directly available, it can be computed from
short-circuit study results that give
either the short-circuit megavoltampere (MVA) or the short-circuit current as follows
Z sc  Rsc  jX sc 
kV 2
kV *1000

MVAsc
3 * I sc
Where,
ZSC = short-circuit impedance
RSC = short-circuit resistance
XSC = short-circuit reactance
kV = phase-to-phase voltage, kV
MVASC = three-phase short-circuit MVA
ISC = short-circuit current, A)
ZSC is a phasor quantity, consisting of both resistance and reactance. However, if the
short-circuit data contain no phase information, one is usually constrained to assuming
that the impedance is purely reactive. This is a reasonably good assumption for industrial
power systems for buses close to the mains and for most utility systems.
(b). Explain Proactive monitoring. Discuss in detail about the selection of power quality
monitoring sites.
Solutions:
Power quality monitoring is the process of gathering, analyzing, and interpreting raw
measurement data into useful information. The process of gathering data is usually
carried out by continuous measurement of voltage and current over an extended period.
The process of analysis and interpretation has been traditionally performed manually, but
recent advances in signal processing and artificial intelligence fields have made it
possible to design and implement intelligent systems to automatically analyze and
interpret raw data into useful information with minimum human intervention.
Power quality monitoring programs are often driven by the demand for improving the
system wide power quality performance. Many industrial and commercial customers have
equipment that is sensitive to power disturbances, and, therefore, it is more important to
understand the quality of power being provided. Examples of these facilities include
computer networking and telecommunication facilities, semiconductor and electronics
manufacturing facilities, biotechnology and pharmaceutical laboratories, and financial
data-processing centers. Hence, in the last decade many utility companies have
implemented extensive power quality monitoring programs.
Monitoring Considerations
Before embarking on any power quality monitoring effort, one should clearly define the
monitoring objectives. The monitoring objectives often determine the choice of
monitoring equipment, triggering thresholds, methods for data acquisition and storage,
and analysis and interpretation requirements. Several common objectives of power
quality monitoring are summarized here.
Monitoring to characterize system performance
This is the most general requirement. A power producer may find this objective important
if it has the need to understand its system performance and then match that system
performance with the needs of customers. System characterization is a proactive approach
to power quality monitoring. By understanding the normal power quality performance of
a system, a provider can quickly identify problems and can offer information to its
customers to help them match their sensitive equipment’s characteristics with realistic
power quality characteristics.
Monitoring to characterize specific problems
Many power quality service departments or plant managers solve problems by performing
short-term monitoring at specific customer sites or at difficult loads. This is a reactive
mode of power quality monitoring, but it frequently identifies the cause of equipment
incompatibility, which is the first step to a solution.
Monitoring as part of an enhanced power quality service
Many power producers are currently considering additional services to offer customers.
One of these services would be to offer differentiated levels of power quality to match the
needs of specific customers. A provider and customer can together achieve this goal by
modifying the power system or by installing equipment within the customer’s premises.
In either case, monitoring becomes essential to establish the benchmarks for the
differentiated service and to verify that the utility achieves contracted levels of power
quality.
Monitoring as part of predictive or just-in-time maintenance
Power quality data gathered over time can be analyzed to provide information relating to
specific equipment performance. For example, a repetitive arcing fault from an
underground cable may signify impending cable failure, or repetitive capacitor-switching
re-strikes may signify impending failure on the capacitor-switching device. Equipment
maintenance can be quickly ordered to avoid catastrophic failure, thus preventing major
power quality disturbances which ultimately will impact overall power quality
performance.
The monitoring program must be designed based on the appropriate objectives, and it
must make the information available in a convenient form and in a timely manner (i.e.,
immediately). The most comprehensive monitoring approach will be a permanently
installed monitoring system with automatic collection of information about steady-state
power quality conditions and energy use as well as disturbances.
(c). Bring out the significance of Power quality monitoring. What are important power
quality monitoring objectives?
Solutions:
i.
ii.
iii.
iv.
v.
vi.
Energy and demand profiling with identification of opportunities for energy
savings and demand reduction
Harmonics evaluations to identify transformer loading concerns, sources of
harmonics, problems indicating disoperation of equipment (such as
converters), and resonance concerns associated with power factor correction
Voltage sag impacts evaluation to identify sensitive equipment and possible
opportunities for process ride-through improvement
Power factor correction evaluation to identify proper operation of capacitor
banks, switching concerns, resonance concerns, and optimizing performance
to minimize electric bills.
Motor starting evaluation to identify switching problems, inrush current
concerns, and protection device operation.
Short-circuit protection evaluation to evaluate proper operation of protective
devices based on short-circuit current characteristics, time-current curves, etc.