POWER HARMONIC FILTERS FOR TECHNOLOGICAL EXCELLENCE
Shaikh Shamser Ali, BE, PMP, P Engr, MBA, BEE Certified Energy Auditor & Manager
Harmonics are AC voltages and currents with frequencies that are integer multiples of the
fundamental frequency. In the earlier years, harmonics were not prevalent in most of the
industries due to the balanced linear loads using 3 phase induction motors along with
incandescent lighting, heating etc. but the rapid advancement of the power electronics in
industrial application makes industrial loads non-linear type. These non-linear loads draw
non-sinusoidal current from the sinusoidal voltage waveform. The distortions thus
produced in the voltage and current waveform from the sinusoidal waveform are called
HOW THE HARMONICS ARE GENERATED
Harmonics are generated due to increasing number of non-linear loads as explained
When the system voltage is linear but the load is non linear, the current will be distorted
and become non-sinusoidal. The actual current will become higher than an ammeter or
any other measuring instrument, at the fundamental frequency, could measure that.
Following figure illustrates the difference between the apparent current measured by an
instrument and the actual current
At fundamental frequency ‘f’
At harmonic frequency ‘fh’
Ia = Active Component of Current
Ir = Apparent current measured by an ammeter
Ih = Actual current due to harmonic distortions
Φ = Displacement angle between the system Voltage and apparent
current defining the PF of the load
α = Actual Phase displacement due to harmonic distortions
When the supply system itself contains harmonics and the voltage is already distorted, the
linear loads will also respond to such voltage harmonics and draw harmonic currents
against each harmonic present in the system and generate the same order of current
When the system voltage and load are both non-linear, (A condition which is more
common) the voltage harmonics will magnify and additional harmonics will be
generated, corresponding to the non-linearity of the load and hence will further distort an
already distorted voltage waveform
Harmonics will thus mean:
1. Higher voltage and current than apparent.
2. Adding to line loading and losses.
3. Reducing the actual load PF.
A load is “non-linear“ when the current
drawn does not have the same waveform as
the supply voltage. The harmonic spectrum
depends on the type of load. i.e. switchmode power supplies, motors during startup, transformers during switch-on,
•Non-linear loads cause voltage and current
•Voltage harmonics depends on grid /
source stability, current harmonics and
•Shunt connected electrical equipment are
sensitive to voltage harmonic and crest.
Fundamental Frequency Waveform
Fifth Harmonic Waveform
•Series connected devices like cable /
transformer are effected by current
harmonics thus increases distribution losses
and demagnetic interference.
•Major harmonic related problems are due
to current and voltage harmonics.
SOURCES OF HARMONICS
One common source of harmonics is iron core devices like transformers. The magnetic
characteristics of iron are almost linear over a certain range of flux density, but quickly
saturate as the flux density increases. This non-linear magnetic characteristic is described
by a hysteresis curve. Because of the non-linear hysteresis curve, the excitation current
waveform is not sinusoidal. A fourier analysis of the excitation current waveform reveals
a significant third harmonic component.
Generators produce some 5th harmonic voltages due to magnetic flux distortions that
occur near the stator slots and non-sinusoidal flux distribution across the air gap.
Other producers of harmonics include Rectifiers, Inverters, Variable Speed Drives,
Welders, Arc furnaces, Voltage Controllers, Frequency Converters etc.
Semiconductor switching devices produce significant harmonic voltages as they
abruptly chop voltage waveforms during their transition between conducting and cut-off
Inverter circuits are notorious for producing harmonics, and are in widespread use today
in every spectrum of the industry. A variable speed drive is one application that makes
use of inverter circuits, often using pulse width modulation (PWM) synthesis to produce
the AC output voltage. Various synthesis methods produce different harmonic spectrum.
Regardless of the method used to produce an AC output voltage from a DC input voltage,
harmonics will be present on both sides of the inverter and must be mitigated.
IMPACT OF HARMONICS ON POWER FACTOR
Power Factor (PF) is a measure of the efficiency of utilization of a power distribution
system. The closer the PF to unity, the more will be the efficiency to do the useful work.
With linear loads, the PF depends on the phase relationship between the current and
voltage sine waves. When these two waves are in phase, the PF is unity and no system
capacity is wasted.
Linear loads, such as resistance heaters and incandescent lights are 100% efficient in
converting real power to heat and therefore have a PF of unity. Induction motors require
real power and reactive power, which is measured in KVAR. The reactive current that
flows in the system creates a magnetic field that enables the motor to operate, but does
not contribute to the work done by the motor. Reactive current also causes the current
wave to lag behind the voltage wave. This process is called displacement.
Since the apparent power for a motor is larger than the active power, the PF is less than
unity. The PF for a system powering only linear loads is called the displacement power
factor. Unless the loads are pure resistance, the PF will be less than unity.
Today however, many electrical systems also have harmonic currents on their lines.
Harmonics are caused by non-linear or pulsed loads and their current causes the apparent
power to exceed the active power by a substantial amount.
The apparent power for a non-linear load can be calculated using the equation,
KVA =√ (P2+Q2+DVA2)
The presence of harmonics increases the apparent power that must be delivered to do a
certain amount of work, therefore lowering the PF. In these situations, the form of power
factor present is called distortion power factor. In a System consisting of both linear and
non-linear loads the true power factor (TPF) is a sum of cosine of both displacement and
If harmonic currents are introduced into a system, the true PF will always be lower than
the displacement PF. For example, the displacement PF for a computer is close to unity
(usually about 0.95) whereas the true PF, which includes harmonics, is around 0.7. For
both linear and non-linear loads, the result of extra current that does NO real work
(Whether it is reactive current or harmonic current) is a reduced capability for the system
to support useful loads.
For linear loads, measurements can be carried out to determine displacement power
factor with a number of instruments. These instruments can measure Kilowatts (KW) and
Kilo-Volt-Amperes (KVA) and some can directly read Power Factor (PF). When
harmonics are present, meters with true RMS capability must be used to accurately
account for the total current, which includes the current at the Fundamental 50/60 Hz and
the harmonic currents to determine the true PF. Also, it is advisable to read the true RMS
value of the voltage, since harmonic currents may cause voltage waveform distortion in
IMPACT OF HARMONICS ON CAPACITORS
Harmonic component affects the performance of a capacitor unit significantly due to the
diminishing reactance at higher frequencies, which adds to its loading substantially and
can be analysed as follows:
Xc = 1/(2πfc)
i.e Xc α 1/f
This means that the capacitor will offer a low reactance to the higher harmonics and will
tend to magnify the harmonic effect due to higher harmonic currents. In fact, harmonic
currents have a greater heating effect compared to fundamental current. The effective
current caused by all the harmonics present in the system can be expressed as:
Ich = √ (Ic + 9 Ich3 + 25 Ich5 + 49 I ch7 + …………..n I chn)
Where, Ic = Rated current of the Capacitor
Ich3, Ich5, Ich7 .. etc. = amplitude of the harmonic current components at
different Harmonic orders
To compensate for the harmonic effects, capacitor unit is designed for a minimum of 70 100 % continuous overload capacity. Summarizing the above, the harmonic quantities
when present in a system on which are connected a few capacitor banks affect the
capacitors as follows:
1. Over current resulting in higher losses.
2. Over current resulting to an over voltage across the capacitor units, which would
inflict greater dielectric stress on capacitor elements.
3. Since the harmonic disorders occur at higher frequencies than the fundamental,
they cause higher dielectric losses
Harmonic output of a capacitor unit:
KVAR = √3 * V * IC (V in Volts and Ic in Amperes)
and IC = V
Therefore, KVAR = √3 * V2
1000 * XC
KVAR = √3 * V2 *2π * f * c
Generalizing KVARh is proportional to Vh .at fh
(or) KVARh is proportional to
V1 + 3. Vh3 + 5 Vh5 + 7 Vh7 + …n.Vhn
The rating of the capacitor unit will thus vary in a square proportion of the effective
harmonic voltage and in direct proportion to the harmonic frequency. This rise in the
KVAR, however will not contribute to the improvement of system PF, but only to the
overloading of the capacitors themselves.
IMPACT OF HARMONICS ON VARYING LOADS
Wherever variation in loading pattern is observed, Automatic Power Factor Correction
systems are installed for maintenance of healthy power factor. However, one cannot
indiscriminately add power factor correction capacitors to a system without
understanding how their presence will affect the system, especially in the presence of
The operation of non-linear loads in a power distribution system
creates harmonic currents that flow throughout the power system.
The inductive reactance of the power system increases and the
capacitive reactance decreases as the frequency increases, as
shown in this fig.
At a given harmonic frequency in any system where a capacitor exists, there will be a
crossover point where the network impedance and capacitive reactances are equal. This
crossover point, called the parallel resonant point, is where the power system has
coincidental similarity of system impedances. Every system with a capacitor has a
parallel resonant point. Parallel resonance causes problems only if a source of harmonics
exists at the frequency where the impedances match. This is typically called harmonic
resonance. Harmonic resonance results in very high harmonic currents and voltages at the
Harmonic generating load
PARALLEL RESONANT CIRCUIT AND ITS EQUIVALENT
At resonant frequency the resultant impedance of a parallel resonant circuit increases to a
very high value. This leads to excitation of parallel resonance circuit between the power
factor correction capacitor and the network inductance resulting in a high voltage across
the inductors and very high circulating current inside the loop.
The increased use of non-linear loads distorts the current waveform thereby affecting the
voltage profile. In case of voltage distortion the series resonant circuit formed by the
capacitance of the capacitor and the short circuit inductance of the transformer draws
high harmonic current through the capacitor. Series resonance can create high voltage
distortion in the LV side of the transformer.
Harmonic generating load
Capacitance of the
SERIES RESONANT CIRCUIT AND ITS EQUIVALENT
For example, consider a 1500 KVA transformer and a capacitor bank rating of 250
KVAR. Substitute, these values in the equation given below will yield the harmonic order
at which this combination will form a parallel resonant circuit
A 1500 KVA transformer with 5% impedance yields about 30 MVAsc (1.5 MVA ÷
0.05). So the 250KVAR capacitor bank will be resonant with that source impedance at
the 11th harmonic. If any amplitude of 11th harmonic current flows on the power system
at that bus, the effect could be catastrophic.
Due to the combination of inductance and capacitance in series the net impedance of the
circuit reduces to a bare minimum level at the resonant frequency and this impedance is
basically resistive in nature at resonant frequency, since at resonant frequency, the
capacitive reactance and the inductive reactance are equal. This low impedance to the
input power at resonant frequency will result in multiple increases in current.
EFFECT OF HARMONICS ON POWER SYSTEM
Tripping of circuit breakers and fuses
Due to resonance effects, the current levels may rise to multifold levels, which results in
tripping of the breakers and melting fuses. This situation results into serious problems in
industries, which rely on the quality of power for the continuous operation of their
Impact of Harmonics on Transformers
Transformers are designed to deliver power at network frequency (50/60Hz). The iron
losses are composed of the eddy current loss (which increase with the square of the
frequency) and hysterics losses (which increase linearly with the frequency). Eddy
current concentrations are higher at the ends of the transformer windings due to the
crowding effect of the leakage magnetic fields at the coil extremities. Very often, the
damage to the coils in a transformer is not known until a failure occurs. With increasing
frequencies the losses also increase, causing an additional heating of the transformer.
Impact of Harmonics on Motors
Hysteresis and eddy current losses are part of iron losses that are produced in the core due
to the alternating magnetic field. Hysteresis losses are proportional to frequency, and
eddy current losses vary as the square of the frequency. Therefore, higher frequency
voltage components produce additional losses in the core of AC motors, which in turn,
increase the operating temperature of the core and the windings surrounding the core.
Application of non-sinusoidal voltages to motors results in harmonic current circulation
in the windings of motors.
Stray motor losses, which include winding eddy current losses, high frequency rotor and
stator surface losses, and tooth pulsation losses, also increase due to harmonic voltages
The interaction between the positive and negative sequence magnetic fields and currents
produces torsional oscillations of the motor shaft. These oscillations result in shaft
vibrations. If the frequency of oscillations coincides with the natural mechanical
frequency of the shaft, the vibrations are amplified and severe damage to the motor
shaft may occur.
Overloading of the capacitors
The rated current through capacitor is calculated as follows:
Ic = V = V.2.C
However due to harmonics, the capacitors are overloaded according to the following
equation producing excessive heat.
Ich = √ ( Ic + 9 Ich3 + 25 Ich5 + 49 I ch7 + …………..n I chn)
As the capacitive reactance decreases with the frequencies, even smaller amplitudes of
the harmonic voltages result into higher currents, which are detrimental to the capacitors.
Losses in distribution equipment
Harmonics in addition to the fundamental current cause additional losses in the cables,
fuses and also the bus bars.
Excessive currents in the neutral conductor
Under balanced load conditions without harmonics, the phase currents cancel each other
in neutral, and resultant neutral current is zero. However, in a 4-wire system with singlephase non-linear loads, odd numbered multiples of the third harmonics (3rd, 9th, 15th) do
not cancel, rather add together in the neutral conductor.
In systems with substantial amount of the non-linear single-phase loads, the neutral
currents may rise to a dangerously high level. There is a possibility of excessive heating
of the neutral conductor since there are no circuit breakers in the neutral conductors like
in the phase conductors.
Malfunctioning of the Electronic Controls and Computers
Electronic controls and computers relay on power quality for their reliable operation.
Harmonics result into distorted waveforms, neutral currents and over voltages, which
affect the performance of these gadgets.
Measurement errors in the metering systems
The accuracy of metering systems is affected by the presence of harmonics. Watt-hour
meters accurately register the direction of power flow at harmonic frequencies, but they
have amplitude errors, which increase with frequency.
Generally power electronic devices cause line current to be non-sinusoidal.
harmonics increases the losses in the stator conductors and it affects the field and damper
currents, thus distorting the voltage profile. The voltage profile distortion depends on the
current harmonics and the alternator characteristics. The following points needs to be
Increased summated net current flows through the alternator winding.
Reverse harmonic current flow in the windings: Harmonic current flows back into the
alternator, thereby distorting the voltage waveform (voltage harmonics). Basically
voltage harmonics affects all the loads either linear or non-linear and the voltage
harmonics also induces current harmonics even for linear loads.
Increased winding resistance for harmonic frequencies: Harmonic current produces
large heating than the fundamental current due to the increase in resistance at different
5th Harmonic current produces approximately 5 times more heat and
similarly higher order of harmonics produces higher heating. This leads to higher
temperature in the winding.
Rotor Jerking: Due to the reverse flow of harmonics, there is blocking movement in the
alternator speed i.e., jerking of the rotor takes place due to harmonic current flow.
Rotor Retardation: Due to inherent distortion in the voltage waveform, induced EMF
rotates at different frequencies. Depending on the phase angle of the 5th Harmonics, this
may have retardation or acceleration effect on the rotor.
REMIDIES TO OVERCOME POWER HARMONICS
Elimination of voltage and current harmonics by using harmonic filters is an easy option
to any harmonics problem.
CLASSIFICATION OF HARMONIC FILTERS
TYPES OF PASSIVE FILTER SYSTEMS
Passive harmonic filters are reactor-based systems basically used for the suppression of
harmonics and maintenance of healthy power factor. These filters are broadly classified
1. Detuned Filters
2. Tuned Filters
The classification of de-tuned filters and tuned filters basically depends on the tuning
frequency of the filter reactor & capacitor circuit and the selection of harmonic filter type
depends on the level & order of harmonics present in the distribution network.
De-Tuned Harmonic Filters:
The de-tuned filters are effective in circuits where variation of Q and different order of
harmonics level are anticipated. Such filters are shunt connected with matching tuning
frequencies below the predominant harmonic frequency, thus having most reliable life,
but also achieving the required harmonic reduction. The selection of the tuning
frequency depends on the system impedance behavior under varying loads or constant
The system impedance needs a detailed harmonic behavioral study to arrive at the correct
tuning frequency. However, depending on the predominant level of harmonics present,
following tuning frequencies are generally selected:
Blocking Filter –Wherever the level of 3rd harmonics is predominant in the distribution
network, it is necessary to select Blocking Filter systems of tuning frequency at 154.8 Hz.
These filters are designed to block the effect of 3rd harmonics affecting the life of shunt
connected capacitors and reduces the risk of harmonic resonance and amplification.
Suppression Filter – Wherever the level of 5th harmonics is predominant, it is necessary
to select Suppression Filter systems of tuning frequency at 279 Hz. These filters are also
designed to block the effect of 5th harmonics affecting the life of the shunt connected
capacitors & suppress the line current harmonics. In general following are the benefits of
Blocking & Suppression Filters:
1. Avoids premature failure of capacitors due to the basic blocking nature of the
2. Harmonic amplification due to impedance matching is avoided
3. Possibility of harmonic resonance is avoided
Tuned Harmonic Filters:
The tuned filters are basically designed to match to the predominant harmonic frequency,
but slightly tuned away from the harmonic frequency, since the supply frequency
variations needs to be accounted. Even the tuned filters act like de-tuned filters when the
frequency is at the normal level, but they operate as tuned filters only at the minimum
defined supply frequency.
These types of filters are tuned above the detuned filter frequencies, thus having the
impact of blocking filter and harmonic absorption capability. In other words, these filters
not only improve the power factor, but also absorb the harmonics. Here, the capacitors
are fully protected due to the blocking nature of the filters, thus ensuring a long life for
the filter circuit. Such filters are normally used for both harmonic suppression and power
ACTIVE FILTER TECHNOLOGY
Active filters are IGBT based power electronic devices
installed in parallel to the harmonic generators. It
analyses the harmonic current produced by the nonlinear loads and supplies a 180 out-of-phase
compensating current, either over the entire spectrum
from the 2nd to the 25th harmonic or a specially selected
harmonic. This technique is called as active injection
mode (AIM). This is not done by absorbing currents, but
by injecting additional currents whenever required.
A current transformer first measures the current being drawn momentarily by the load.
The control unit in the harmonic filters then analyses this current for amplitude and
harmonics. It consequently feeds a current into the supply system whose amplitude and
individual harmonic numbers is exactly equal to the current drawn by the load but which
is, however, 180 out of phase with it. The harmonic currents cancel each other out and
the supply network only has to supply the fundamental frequency and is not contaminated
with harmonics at the point of connection, provided that the system has been
The combination of harmonic filter and harmonic load appears to the network as an
overall linear load drawing a sinusoidal current. Installation is quite simple. A threephase feeder with or without a neutral conductor needs to be available. The current
transformer is then installed in the line to the non-linear load. One great advantage of the
active filter compared to conventional techniques is its flexibility in adapting the
corrective power. Depending on the requirements, the filter can supply more or less
Even on overload, the filter does not switch off, but assumes a current-limiting mode i.e.,
the filter supplies its maximum current and in doing so compensates for a large
proportion of the harmonics. Interaction with other system components, such as UPS
units is therefore reduced to a minimum that is not critical. There is no problem to extend
the system or install a combination of several filters. If operating or network conditions
change, the filter automatically adapts to the new conditions within the scope of its
SELECTION OF HARMONIC FILTERS:
Harmonics filter systems should be best based on harmonic study. Harmonic filter system
design depends on the filter reactive power output, tuning frequency and impedance of
the network at the point of connection. The followings should be considered while
selecting the appropriate harmonic filter:
Considering the amplitude of harmonic current measured, sizing of the filter circuit is
arrived based on the distributed network harmonics consisting of different orders.
Appropriate tuning frequency is selected based on the measurement and network
Based on the tuning frequency of passive filter, the current harmonics of that particular
tuning frequency will be absorbed by the filter. Such filters can be tuned for a particular
order (Frequency) of harmonics.
HARMONIC FILTER SELECTION CHART – (Examples)
Harmonic level above IEEE 519 / IEC 1000-2-4.
/ IEC 1000-2-4.
THD >10% &
to 85% of the
BENEFITS OF HARMONIC FILTERS
Harmonic filters offers superior harmonic filtration along with following incomparable
Avoidance of frequent capacitor failures
Safeguards Neutral Conductor
Reduces System losses
Reduces Neutral Current
Reduces local Neutral to Ground Voltage
Elimination of Resonance
Improves Voltage Stability
Shunts Harmonic Currents
Increases System Capacity
Reduces Transformer Overloading
PERFORMANCE OF PASSIVE HARMONIC FILTERS
Current in amps , Voltage in volts
Current in amps , Voltage in volts
Time in secs
Time in secs
VOLTAGE & CURRENT WAVEFORM
VOLTAGE & CURRENT WAVEFORM
Improvement in Voltage Profile - With the installation of harmonic filters, the RMS
value of voltage waveform is improved, thereby avoiding motor winding burn-outs,
repetitive fuse failures etc due to low voltage and improves voltage regulation.
Improvement in current profile- With the installation of harmonic filter the net current
is brought down due to the elimination of harmonic current thereby reducing losses in the
electrical distribution system and leads to enhance life of transformers, cables, switchgear
PERFORMANCE OF ACTIVE HARMONICS FILTER
Power harmonics is an electrical phenomenon. The negative effects due to power
harmonics is not necessarily be only electrical but can also be mechanical as explained on
“Impact of harmonics” earlier in this article. Awareness and acceptance of having better
power quality is increasing and the industries are gradually realizing the positive impact
of having electrical power without harmonics.
Primary aim of the end user to install a harmonics filter could be the elimination of
harmonics but an economic analysis will prove that the expenditure to install the
harmonic filter can generate an ROI of more than 30%, should it be looked at as an
investment. Implementation of harmonic filters as Demand Side Management (DSM)
project operating on “Performance Contracting” basis will not only benefit the end user
and the harmonic filters supplier but will also benefit the utility and funding organization.