Power Quality PRESENTATION Power quality topics

K . GA Y A T HR I
M A N A GE R , E M ( S S T P , S T P & T R T P )
An Introduction to Power Quality

DEFINITION
 Power quality is a generic term applied to a wide
variety of electromagnetic phenomena on the power
system.
 The duration of these phenomena ranges from a few
nanoseconds (e.g., lightning strokes) to a few
minutes (e.g., feeder voltage regulations) to steady-
state disturbances (harmonic distortions and voltage
fluctuations).
 Normally, voltage is the quantity causing the observed
disturbance and the resulting power will not necessarily
be directly proportional to the voltage.

General Classes of Power Quality Disturbances
 The term transient in the analysis of power system
variations denotes an event that is undesirable and
momentary in nature.
 In general, transients can be classified into two
categories, impulsive and oscillatory.
 These terms reflect the wave shape of a current or
voltage transient.

Impulsive Transient:
 An impulsive transient is a sudden, non power
frequency change in the steady state
condition of voltage, current, or both, that is
unidirectional in polarity (primarily either positive
or negative).
 They are normally characterized by their rise and
decay times in microseconds.
 Common cause of impulsive transient are lightning.

 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. It consists of a voltage or
current whose instantaneous value changes polarity
rapidly.
 It is described by its spectral content (predominate
frequency), duration, and magnitude.
 Frequency of the oscillation varies from 5 kHz to 500
kHz with a typical duration measured in Microseconds.

Results of transients
 Degradation or immediate dielectric failure.
 High magnitude and high/ fast rise time results in
insulation breakdown in switchgear, transformers, motors.
 Insulation damage in cables- insulation between wires
have capacitive properties. This provides path for
transient impulse.
 If transient has enough strength, it will damage insulation
(Xc= 1/2∏fC). As f , Xc . Lower path for transients.
 But since time period is very less, energy is less. Immediate
failure chance is less. But reduced life after repeated
failures.

Short-Duration Voltage Variations
 Short-duration voltage variations are caused by fault conditions,
the energization of large loads that 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), or
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 short duration variation until
protective devices operate to clear the fault.

Voltage Sags
 A sag or voltage dip is a decrease to between 10% and 90% in
RMS voltage at the power frequency for durations from 0.5
cycles to 1 min.
 Generally caused by staring large loads such as motors.
Voltage Swells
 A swell or momentary overvoltage is an increase to between 1.1
and 1.8 pu in RMS voltage at the power frequency for durations
from 0.5 cycle to 1 min.
 Swells can also be caused by switching off a large load or
energizing a large capacitor bank.

Interruption.
 An interruption occurs when the supply voltage or
load current decreases to less than 10 % for a
period of time not exceeding 1 min.
 Interruptions can be the result of power system
faults, equipment failures, and control malfunctions.
 The interruptions are measured by their duration
since the voltage magnitude is always less than 10%
of nominal.

Long-Duration Voltage Variations
 Long-duration voltage variations are the RMS deviations at
power frequencies for longer than 1 minute.
 Long-duration variations can be either over voltages (>
110 %) or under voltages (< 90%).
 Under voltage Causes: Faults, overloaded lines, etc.
 UV Results: Over heating in motors
 Over voltage Causes: Variation in capacitive banks
reactive compensation, incorrect tap of transformer, etc
 OV results: Saturation of motors, breakers to trip,
reduced life of lighting, etc.

Sustained Interruption
 When the supply voltage has been zero for a period
of time in excess of 1 min, the long duration
voltage variation is considered a sustained
interruption.
 Voltage interruptions longer than 1 min are often
permanent and require human intervention to repair
the system for restoration.

Voltage unbalance
 Voltage unbalance is defined as the maximum
deviation from the average of the 3-phase
voltages or currents, divided by the average of
the 3-phase voltages or currents, expressed in
percent.

Waveform Distortion
 Waveform distortion is defined as a steady-state deviation
from an ideal sine wave of power frequency principally
characterized by the spectral content of the deviation.
 There are five primary types of waveform distortion:
 DC offset
 Harmonics
 Inter-harmonics
 Notching
 Noise

DC Offset
 The presence of a dc voltage or current in an AC power
system is termed DC offset.
 This can occur as the result of asymmetry of electronic
power converters .
 DC in alternating current networks can have a detrimental
effect by biasing transformer cores so they saturate in
normal operation. This causes additional heating and loss
of transformer life.
 DC may also cause the electrolytic erosion of grounding
electrodes and other connectors.

Harmonics and Inter-harmonics
 Harmonics are sinusoidal voltages or currents having frequencies
that are integer multiples of the fundamental frequency
 Periodically distorted waveforms can be decomposed into a sum of the
fundamental frequency and the harmonics
 Voltages or currents having frequency components that are not
integer multiples of the fundamental frequency are called inter-
harmonics.
 The main sources of inter-harmonic waveform distortion are static
frequency converters,cycloconverters, induction furnaces, and arcing
devices.
 Power line carrier signals can also be considered as inter-harmonics

Notching
 Notching is a periodic voltage disturbance caused
by the normal operation of power electronics
devices when current is commutated from one phase
to another.
 During this period, there is a momentary short
circuit between two phases pulling the voltage as
close to zero as permitted by system impedances.

Noise.
 Noise is defined as unwanted electrical signals with broadband spectral content lower
than 200 kHz superimposed upon the power system voltage or current in phase
conductors, or found on neutral conductors or signal lines.
 Noise in power systems can be caused by power electronic devices, control circuits, arcing
equipment, loads with solid-state rectifiers, and switching power supplies.
 Noise problems are often exacerbated by improper grounding that fails to conduct
noise away from the power system.
 In principle, noise consists of any unwanted distortion of the power signal that cannot be
classified as harmonic distortion or transients.
 Noise disturbs electronic devices such as microcomputer and programmable controllers.
 The problem can be mitigated by using filters, isolation transformers, line conditioners and
screened cables.

Voltage Fluctuation
 Voltage fluctuations are systematic variations of the
voltage or a series of random voltage changes, the
magnitude of which does not normally exceed the voltage
ranges 0.9 to 1.1 pu.
 Loads that can exhibit continuous, rapid variations in the
load current magnitude can cause voltage variations that
are often referred to as flicker.
 Arc furnace, is one of the most common causes of voltage
fluctuations on utility transmission and distribution
systems.

 Power frequency variations are defined as the deviation of
the power system fundamental frequency from its specified
nominal value .
 The power system frequency is directly related to the rotational
speed of the generators supplying the system.
 There are slight variations in frequency as the dynamic balance
between load and generation changes. The size of the frequency
shift and its duration depends on the load characteristics and the
response of the generation control system to load changes.
 Frequency variations that go outside the accepted limits for normal
steady-state operation of the power system can be caused by faults
on the bulk power transmission system, a large block of load being
disconnected, or a large source of generation going offline.

Sources and Characteristics of Electrical Transients
 Two main sources of transient overvoltage on utility
systems are capacitor switching and lightning.
 Capacitor banks used to provide VAR compensation
is one of the most common source of oscillatory
transients when switched.
 Some capacitors are energized all the time (a fixed
bank) while others are switched according to load
levels.

Capacitor switching transient
 Capacitor switching transients can propagate into the local
power system and will generally pass through distribution
transformers into customer load facilities.
 If there are capacitors on the secondary system, the voltage
may actually be magnified on the load side of the
transformer if the natural frequencies of the systems are
properly aligned.
 Transient over-voltages on the end-user side may reach as
high as 3.0 to 4.0 pu on the low-voltage bus under these
conditions, with potentially damaging consequences for all
types of customer equipment.

Voltage magnification due to capacitive switching

Capacitive switching transients mitigation techniques
Various techniques used to mitigate transients
are
 MOV Surge arresters
 Pre insertion resistors for fraction of a cycle
 Line reactors for loads sensitive to transients.
 Isolation transformers.
 Synchronous closing to prevent closing of capacitor
switch at the peak voltage. This is to avoid step
change in the capacitor voltage.

Impulsive transient due to lightning
 Lightning is a potent source of impulsive transients and can have serious
impacts on power system and end-user equipment.
 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. 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.
 The power quality problems with lightning stroke currents entering the ground
system are
 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.
 They induce high voltages in phase conductors as they pass through cables on the way
to a better ground.

Lightning path in power system

Transformer Energizing
 Energizing a transformer produces inrush currents
that are rich in harmonic components for a period
lasting up to 1 s.
 If the system has a parallel resonance near one of the
harmonic frequencies, a dynamic overvoltage
condition results that can cause failure of arresters
and problems with sensitive equipment.
 This problem can occur when large transformers are
energized simultaneously with large power factor
correction capacitor banks in industrial facilities.

Power Quality PRESENTATION Power quality topics

Power System harmonics
Harmonic Distortion
 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.
 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.
 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.
 The sum of sinusoids is referred to as a Fourier series.

Power System harmonics
 Voltage distortion is the result
of distorted currents passing
through the linear, series
impedance of the power
delivery system.
 The harmonic currents passing
through the impedance of the
system cause a voltage drop for
each harmonic.
 This results in voltage
harmonics appearing at the
load bus. The amount of
voltage distortion depends on
the impedance and the current

Triplen Harmonics
 Triplen harmonics are the odd multiples of the third
harmonic (h = 3, 9, 15, 21, . .ie multiple of 3,but not
divisible by 2).
 They deserve special consideration because the
system response is often considerably different for
triplens than from rest of the harmonics.
 Triplens become an important issue for grounded-
wye systems with current flowing on the neutral.
 Two typical problems are over loading the neutral
and telephone interference.

Triplen Harmonics
 For the system with perfectly
balanced single-phase loads
illustrated in Fig., an assumption
is made that fundamental and
third harmonic components are
present.
 Summing the currents at node
N, the fundamental current
components in the neutral are
found to be zero, but the third
harmonic components are three
times the phase currents because
they naturally coincide in phase
and time.

Triplen Harmonics in Transformers
 In the wye-delta transformer , the
triplen harmonic currents are
shown entering the wye side. Since
they are in phase, they add in the
neutral. The delta winding
provides ampere-turn balance so
that they can flow, but they remain
trapped in the delta and do not
show up in the line currents on the
delta side.
 Using grounded-wye windings on
both sides of the transformer
allows balanced triplens to flow
from the low voltage system to the
high voltage system unimpeded.
They will be present in equal
proportion on both sides

Total Harmonic Distortion
 The total harmonic
distortion (THD) is a
measure of the effective
value of the harmonic
components of a distorted
waveform. That is, the
potential heating value of
the harmonics relative to
the fundamental.
 This index can be
calculated for either
voltage or current.

Detrimental effects of Harmonics
 Harmonic distortion of the voltage and current in an
industrial facility is caused by the operation of
nonlinear loads and devices on the power system.
 A nonlinear load is one that does not draw sinusoidal
current when a sinusoidal voltage is applied.
 Examples on nonlinear loads are arcing devices such
as arc furnaces, saturable devices such as
transformers, and power electronic equipment such
as adjustable-speed drives and rectifiers.

 High levels of distortion can lower power factors, overheat
equipment, and lead to penalties from the local utility for
exceeding recommended limits.
 Harmonic currents increase the volt-amperes required for a load
without increasing the watts, because true power factor is equal to
the watts divided by the volt-amperes, any increase in volt amperes
without a corresponding increase in watts will lower the power
factor.
 A lower power factor will affect industrial facilities in two ways.
Losses inside the facility will increase due to the higher level of
current required to perform the work.
 Utilities will also charge a penalty if the power factor falls below a
predetermined level. Both of these will increase utility bills.

 Overheating of transformers is another problem
associated with harmonic currents.
 A transformer can carry its rated current if the current
distortion is less than 5%.
 If the current distortion exceeds this value, then some amount
of derating is required.
 The overheating is caused primarily by the higher eddy-
current losses inside the transformer than were anticipated by
the designer.
 The overheating can be avoided by either derating the
transformer or by specifying a “k-rated” transformer that is
designed for the higher levels of eddy currents.

 Over heating of motors due to higher eddy current losses.
 Overheating of neutral in Y connected circuits occurs
because the tripplen harmonic and any odd do not cancel in
the neutral as do the other harmonic currents. The result is
trippen current in the neutral conductor if there are significant
nonlinear loads connected to the Y source.
 Usually the higher multiples of the third harmonic are of small
magnitude.
 The increase in the RMS value of current, can cause excessive
heating in the neutral wire.
 This potential for overheating can be addressed by over sizing
neutral conductors or reducing nonlinear currents with filters.

Techniques for reduction of harmonic effects
 IEEE Standard 1100-1992 recommends practices to reduce
harmonic content.
 Double-Size Neutrals, or Separate Neutrals per Phase.
 Harmonic Filters.
 Shielded Isolation Transformers.
 K-Rated Transformers.
 Screened cables.

POWER QUALITY ISSUES ON MOTORS
Voltage Unbalance
 A voltage unbalance exists when phase voltages at
the point of utilization are unequal.
 There are many possible causes of voltage unbalance.
 An unbalanced three-phase voltage causes three
phase motors to draw unbalanced current, which can
cause the rotor of a motor to overheat.
 Temperature rise caused by unbalanced current is
much greater than the rise caused by balanced
current.

 Distortion of source voltage due to current
harmonics:
 Source voltage with higher THD will over heat the
motors due to higher eddy current losses and hence
affects in life and capacity utilisation.

 Effect of VFDs on Motors
 Advances in semiconductor devices have made variable
frequency drives (VFDs) more affordable and thus more
prevalent in industrial processes.
 To drive small- and medium-sized induction motors, most
VFDs use pulse-width-modulation (PWM) inverters using
IGBTs with switching frequencies from 2 kHz to 20 kHz.
 The waveform of the inverter output voltage is composed
of step-like functions, which are in effect voltage pulses
with extremely quick changes in voltage magnitude (as
fast as from 0 to 600 volts in 0.1 μs).

Long-Lead Effect
 In many industrial applications, a VFD and the motor it drives are
separated by long cable to connect the two together.
 Fast-changing PWM voltage pulses can interact with the distributed
inductance and capacitance of the motor leads, which can result in an
amplified peak voltage as high as 1600 volts at the motor terminals.
This phenomenon, known as the long-lead effect, can stress and
consequently degrade the insulation around the stator windings of the
motor.
 The magnitude of peak voltage at the motor terminals depends upon
the characteristics of the motor leads and the surge impedance of the
motor.
 The smaller the motor and the longer the leads, the greater the peak
voltage.

Premature Motor Bearing Failure
 Another problem associated with motors controlled
by VFDs is bearing failure.
 The root cause of premature bearing failure in
inverter-motor applications is electrical discharge
machining (EDM) due to capacitively coupled shaft-
to-ground voltage build-up. EDM, or fluting as it is
more commonly called, is the passage of electrical
current through the bearing.

Solutions to Motor Issues
 The most effective way to solve the problems of motors
overheating due to voltage unbalance is to eliminate the
unbalance.
 The unbalance can be caused by unbalanced single-phase loads,
faulty connections etc.
 If the voltage unbalance cannot be eliminated, the motor must be
de-rated to ensure long life.
 NEMA recommends de-rating an induction motor when the
voltage unbalance exceeds one percent and recommends not
operating a motor at all when the voltage unbalance exceeds five
percent (Voltage unbalance is 100 times the maximum deviation
of the line voltage from the average voltage of all three phases
divided by the same average)

 The application of PWM VFDs in a process can damage conventional motors
by damaging the winding insulation and by damaging the motor bearings.
 NEMA has developed a standard for performance requirements for motors
to be used with VFDs. NEMA MG-1, Section IV, Part 31, Definite- Purpose
Inverter-Fed Motors, specifies the requirement for squirrel-cage
induction motors rated at 5000 HP or less and at 7200 volts or less, intended
for use with PWM inverters.
 In order to address the specific concerns of transient over-voltages, the
standard requires that motors that are built to this specification will have a
stator insulation system that can withstand voltage pulses up to 1600 volts
with a rise time greater than 0.1 microseconds for motors with a base rating
of less than 600 volts.

Motors that conform to this specification usually
employ one or more of the following additional
features to augment the insulation system:
 Wire with increased dielectric strength (ISR wires)
 Improved insulation on end turns, in the slots, and
between phases.
 Heavy duty lacing taping of end-turns.
 Extra cycles of varnish dip.

 EDM can be reduced by reducing switching frequency of the
inverter .
 Note that reduced switching frequency will enhance the THD
and motor may get heated due to eddy current.
 From a motor standpoint, the most effective way to solve this
problem is to install a shaft grounding system.
 This system effectively provides a low-impedance path from
the shaft to the ground and minimizes the magnitude of the
shaft voltage.
 Grounding the motor shaft with a system of brushes creates a
low-impedance path to ground for otherwise damaging
discharge currents.

K-rated transformer
 K-Rated transformer
 A standard transformer is not designed for high harmonic
currents produced by non-linear loads and will overheat and
fail prematurely when connected to these loads.
 K-rated transformers are able to handle the heat generated by
harmonic currents; not effected by harmonics; very efficient
when used under their K-factor value manufactured with
heavier gauge copper and a double sized neutral conductor; and
have higher cross section core to operate at low flux density
than a standard transformer. All these properties are needed to,
reduce the heating and distortion effects of nonlinear loads

K-factor
 Total amount of harmonic current present is called "Total Harmonic
Distortion (THD)". Since this value has a wide range, there needs to be
an appropriate way to size the K-rated transformer to the load.
 K-rated transformers have an associated K-factor rating.
 K-factor ratings range between 1 and 50. The higher the K-factor, the
more heat from harmonic currents the transformer is able to handle.
 A standard transformer that is designed for linear loads is said to have
a K-factor of 1, whereas a transformer with a K factor of 50 is designed
for the harshest harmonic current environment possible.
 Transformers rated with K-factors of 40 and 50 are extremely rare, very
expensive and generally are not used. Making the correct selection of K-
factor is extremely important because it affects cost and safety.
 Sometimes a de-rated K-1 transformer is used to obtain a greater K-
rating. For example to achieve a K- 13 rating, a K-1 rated transformer
would have to be oversized 200%.

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