power harmonic filters for technological excellence

8/18/2019 Power Harmonic Filters for Technological Excellence

1/18

1

POWER HARMONIC FILTERS FOR TECHNOLOGICAL EXCELLENCEBy

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 thefundamental 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 inindustrial 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 calledharmonic disorders.

HOW THE HARMONICS ARE GENERATED

Harmonics are generated due to increasing number of non-linear loads as explained

bellow:

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

VIa

Ir

Ih

At fundamental frequency ‘f’

At harmonic frequency ‘fh’

α Φ

Here, Ia = Active Component of Current

Ir = Apparent current measured by an ammeter

Ih = Actual current due to harmonic distortionsΦ = Displacement angle between the system Voltage and apparent

current defining the PF of the loadα = 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 currentharmonics.


2/18

2

When the system voltage and load are both non-linear, (A condition which is morecommon) 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 currentdrawn does not have the same waveform as

the supply voltage. The harmonic spectrumdepends on the type of load. i.e. switch-

mode power supplies, motors during start-

up, transformers during switch-on,

frequency-controled motors....

•Non-linear loads cause voltage and currentdistortion.

•Voltage harmonics depends on grid /source stability, current harmonics and

network impedance.

•Current harmonics are mainly loaddependent.

•Shunt connected electrical equipment aresensitive to voltage harmonic and crest.

•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.

V

Fundamental Frequency Waveform

Fifth Harmonic Waveform

Distorted Waveform


3/18

3

SOURCES OF HARMONICS

Transformers:

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:

Generators produce some 5th harmonic voltages due to magnetic flux distortions thatoccur 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-offstates.

Inverter circuits are notorious for producing harmonics, and are in widespread use todayin every spectrum of the industry. A variable speed drive is one application that makesuse 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 andvoltage 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 inconverting 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 doesnot 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.

The apparent power for a motor can be calculated using the equation,

KVA=√ (KW2+KVAR 2)


4/18

4

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 powerfactor 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

distortion angles.

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). Whenharmonics 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 RMSvalue of the voltage, since harmonic currents may cause voltage waveform distortion in

some systems.

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 effectivecurrent caused by all the harmonics present in the system can be expressed as:


5/18

5

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 atdifferent 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 wouldinflict 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)

1000

and IC = V

XC

Therefore, KVAR = √3 * V2

1000 * XC

(or) KVAR = √3 * V2 *2π * f * c

1000

Generalizing KVAR h is proportional to Vh .at fh

(or) KVAR h 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.

2

2 222 2

2 2 2 2 2


6/18

6

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 cannotindiscriminately add power factor correction capacitors to a system without

understanding how their presence will affect the system, especially in the presence of

harmonics.

RESONANCE:

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 thecapacitive reactance decreases as the frequency increases, as

shown in this fig.

PARALLEL RESONANCE:

At a given harmonic frequency in any system where a capacitor exists, there will be acrossover 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 harmonicsexists 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

resonant frequency.

Capacitance of

Capacitor Bank

Network

Inductance

Harmonic

source

Capacitor Bank

Harmonic generating load

PARALLEL RESONANT CIRCUIT AND ITS EQUIVALENT

HV Bus


7/18

7

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.

SERIES RESONANCE:

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 drawshigh harmonic current through the capacitor. Series resonance can create high voltage

distortion in the LV side of the transformer.

400 V

Capacitance of the

Capacitor BankHarmonic source

Transformer's

Inductance's

Capacitor BankHarmonic generating load

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 orderat 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 atthe 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 thecircuit 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 theinput power at resonant frequency will result in multiple increases in current.

HV Bus HV Bus


8/18

8

EFFECT OF HARMONICS ON POWER SYSTEM

DISTRIBUTION SIDE:

Tripping of circuit breakers and fuses

Due to resonance effects, the current levels may rise to multifold levels, which results intripping 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

sensitive processes.

Impact of Harmonics on Transformers

Transformers are designed to deliver power at network frequency (50/60Hz). The ironlosses 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 thecrowding 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 dueto the alternating magnetic field. Hysteresis losses are proportional to frequency, andeddy 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

and currents

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.CXc


9/18

9

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 otherin neutral, and resultant neutral current is zero. However, in a 4-wire system with single-

phase 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 likein 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, whichaffect 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.

GENERATION SIDE:

Generally power electronic devices cause line current to be non-sinusoidal. Thisharmonics 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

accounted:

2 2 2 2 2


10/18

10

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). Basicallyvoltage 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

frequencies. 5th

Harmonic current produces approximately 5 times more heat andsimilarly 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 thealternator 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.


11/18

11

CLASSIFICATION OF HARMONIC FILTERS

HARMONIC FILTERS

PASSIVE

FILTERS

ACTIVE FILTERS

DE-TUNEDFILTERS

TUNEDFILTERS

Suppression Filters Absorption Filters

Blocking Filters


12/18

12

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 classifiedas:

1. Detuned Filters2. Tuned Filters

The classification of de-tuned filters and tuned filters basically depends on the tuningfrequency 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 tuningfrequencies 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

loads.

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 distributionnetwork, it is necessary to select Blocking Filter systems of tuning frequency at 154.8 Hz.These filters are designed to block the effect of 3

rd 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 necessaryto 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 ofBlocking & Suppression Filters:

1. Avoids premature failure of capacitors due to the basic blocking nature of thefilters

2. Harmonic amplification due to impedance matching is avoided3. 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.


13/18

13

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 forthe filter circuit. Such filters are normally used for both harmonic suppression and power

factor improvement.

ACTIVE FILTER TECHNOLOGY

Active filters are IGBT based power electronic devicesinstalled in parallel to the harmonic generators. It

analyses the harmonic current produced by the non-

linear loads and supplies a 180 out-of-phasecompensating current, either over the entire spectrum

from the 2nd

to the 25th

harmonic or a specially selectedharmonic. 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 andharmonics. 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 contaminatedwith harmonics at the point of connection, provided that the system has been

appropriately dimensioned.

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 three-

phase feeder with or without a neutral conductor needs to be available. The currenttransformer 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 thecorrective power. Depending on the requirements, the filter can supply more or less

corrective current.

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 UPSunits 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 conditionschange, the filter automatically adapts to the new conditions within the scope of itsnominal rating.


14/18

14

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 ofthe 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

analysis.

Based on the tuning frequency of passive filter, the current harmonics of that particulartuning 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 levelless than

IEEE519

Harmonic level above IEEE 519 / IEC 1000-2-4.

/ IEC 1000-2-4.THD >10% &

20% &major odd

Harmonics

THD >20%& major 3

rd

Harmonics

Steady

Load

Power

Capacitors upto 85% of the

No-Load

MagentisingKVAR

Fixed De-TunedHarmonic

Suppression Filter

& P.F Correction

Fixed TunedHarmonic Filter& P.F Correction

Fixed

TriplenHarmonicFilter & P.F

Correction

VaryingLoads

APFC System

up to 100% ofthe Reactive

Power.

Variable De-Tuned

HarmonicSuppression Filter

& APFC

Variable Tuned

Harmonic Filter

& APFC

VariableTriplen

Harmonic

Filters &

APFC

Dynamical

ly VaryingLoads

----- Active HarmonicFilter Active HarmonicFilter

Active

HarmonicFilter


15/18

15

BENEFITS OF HARMONIC FILTERS

Harmonic filters offers superior harmonic filtration along with following incomparable

benefits:

Avoidance of frequent capacitor failures Elimination of Resonance

Safeguards Neutral Conductor Improves Voltage StabilityReduces System losses Shunts Harmonic Currents

Reduces Neutral Current Increases System Capacity

Reduces local Neutral to Ground Voltage Reduces Transformer Overloading

PERFORMANCE OF PASSIVE HARMONIC FILTERS

Improvement in Voltage Profile - With the installation of harmonic filters, the RMSvalue of voltage waveform is improved, thereby avoiding motor winding burn-outs,

repetitive fuse failures etc due to low voltage and improves voltage regulation.

-400

-300

-200

-100

0

100

200

300

400

0 0.005 0.01 0.015 0.02 0.025 0.03

Time in secs

C u r r e n t i n a m p s ,

V o l t a g e i n v o l t s

VoltageCurrent

-400

-300

-200

-100

0

100

200

300

400

0 0.005 0.01 0.015 0.02 0.025 0.03

Time in secs

C

u r r e n t i n a m p s ,

V o l t a g e i n v o l t s

VoltageCurrent

VOLTAGE & CURRENT WAVEFORM

BEFORE FILTERATION

VOLTAGE & CURRENT WAVEFORM

AFTER FILTERATION


16/18

16

Improvement in current profile- With the installation of harmonic filter the net currentis 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

etc.


17/18

17

PERFORMANCE OF ACTIVE HARMONICS FILTER


18/18

18

CONCLUSION

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 theharmonic 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 userand the harmonic filters supplier but will also benefit the utility and funding organization.

power harmonic filters for technological excellence

Documents