ONE OPTIMISED NETWORK EQUIPMENT 17 September 2014 | c 0MPRIH-2 Harmonic mitigation in modern networks Introduction Significant change has occurred in electricity networks in the last few years: electronic converters are used widely in industrial networks due to the increased flex- ibility, efficiency and level of control, and are now ap- plied at all levels of the electricity network, from do- mestic applications such as air conditioning and grid- connected solar photo-voltaic systems, to commercial buildings, large scale renewable energy sources and distribution and transmission utilities. The harmful effects of harmonics in power systems are clear and range from excessive heating, reduced equipment life expectancy, to loss of supply and de- structive failures. Appropriate care and mitigation be- gins with an understanding of the sources of harmon- ics. The most common sources of harmonics are described below. An example of how harmonics prop- agate through an electrical network is presented, then attention turns to what can be done to mitigate ex- cessive harmonics in networks. Practical guidance is given on the measurement of harmonic distortion, and real examples of measurements taken in networks are presented. Finally operating principles and types of harmonic filters are described together with guide- lines on the selection of appropriate filters for various applications. 1 Sources of harmonics Most, but not all network harmonics are caused by electronic loads that are now commonly used in light- ing and heating, large industrial applications and trans- mission networks. We describe some interesting as- pects of a number of such loads here, from the thyris- tor converter to arc furnaces. 1.1 Thyristor converters The current drawn from the supply network by a thyris- tor converter is not sinusoidal. Thyristor converters are treated as harmonic current sources because of this non-sinusoidal characteristic. The order of the harmonic fed into the supply depends on the construc- tion of the convertor. A characteristic feature of the converter is its pulse number. The order of harmonics produced by a converter can be approximated by the equation: n = kp ± 1 (1) where n = order of harmonic p = pulse number k = 1, 2, 3, . . . Six-pulse converters are in common use and feed har- monics in the order 5, 7, 11, 13 etc. into the supply system as predicted by the above equation. An exam- ple of a simple six-pulse converter topology feeding a DC load is presented in figure 1. M L 1 L 3 L 2 Figure 1: Six-pulse thyristor bridge If the converter has a twelve-pulse configuration, har- monics of the fifth and seventh order do not appear in general. Twelve-pulse (and higher pulse numbers) are more expensive to manufacture than six-pulse devices and are generally used only for high power applications. In general we can say that the higher the pulse num- ber, the lower the magnitude of harmonic distortion fed into the supply. Lack of symmetry (as a result of supply system phase unbalance or component mismatches), and specific faults in the converter can cause unchar- acteristic harmonic orders such as the second, third and fourth to appear in the supply current. In the ideal case the magnitude of the harmonic cur- rents produced by the converter is dependent only on the magnitude of the fundamental current and the order of the harmonic concerned according to the following equation: In = I1 n (2) where I1 = fundamental frequency current n = order of harmonic In = n th harmonic current Equation 2 assumes that the converter is fed from a strictly symmetrical, stiff three phase network and that the direct current output contains no ripple. In practice the magnitude of the harmonic currents pro- duced is, in addition to order of the harmonic and the magnitude of the fundamental frequency current, also affected by the short-circuit power at the connection point of the converter and the ripple on the direct cur- rent. Decrease of the short circuit power (increased network impedance) lengthens the commutation time, in other words the commutation angle increases and the supply side current distortion is reduced. The circuit inductance is not able to smooth the direct current completely, therefore ripple remains in the DC current. This has the effect of increasing some of the harmonics and decreasing others. Table 1 provides some insight to the difference between the idealised harmonic current magnitudes predicted by equation 2 and typical levels encountered in practice. The differ- ences are mainly caused by the effects of overlap and DC ripple. Optimised Network Equipment Pty Ltd ABN 56 151 739 374 www.onegrid.com.au 1A/70 Prospect Terrace Kelvin Grove QLD 4059 PO Box 1951 Toowong QLD 4066 [email protected]
Significant change has occurred in electricity networksin the last few years: electronic converters are usedwidely in industrial networks due to the increased flex-ibility, efficiency and level of control, and are now ap-plied at all levels of the electricity network, from do-mestic applications such as air conditioning and grid-connected solar photo-voltaic systems, to commercialbuildings, large scale renewable energy sources anddistribution and transmission utilities.
The harmful effects of harmonics in power systemsare clear and range from excessive heating, reducedequipment life expectancy, to loss of supply and de-structive failures. Appropriate care and mitigation be-gins with an understanding of the sources of harmon-ics. The most common sources of harmonics aredescribed below. An example of how harmonics prop-agate through an electrical network is presented, thenattention turns to what can be done to mitigate ex-cessive harmonics in networks. Practical guidance isgiven on the measurement of harmonic distortion, andreal examples of measurements taken in networksare presented. Finally operating principles and typesof harmonic filters are described together with guide-lines on the selection of appropriate filters for variousapplications.
1 Sources of harmonics
Most, but not all network harmonics are caused byelectronic loads that are now commonly used in light-ing and heating, large industrial applications and trans-mission networks. We describe some interesting as-pects of a number of such loads here, from the thyris-tor converter to arc furnaces.
1.1 Thyristor converters
The current drawn from the supply network by a thyris-tor converter is not sinusoidal. Thyristor convertersare treated as harmonic current sources because ofthis non-sinusoidal characteristic. The order of theharmonic fed into the supply depends on the construc-tion of the convertor. A characteristic feature of theconverter is its pulse number. The order of harmonicsproduced by a converter can be approximated by theequation:
n = kp ± 1 (1)
wheren = order of harmonicp = pulse numberk = 1, 2, 3, . . .
Six-pulse converters are in common use and feed har-monics in the order 5, 7, 11, 13 etc. into the supplysystem as predicted by the above equation. An exam-ple of a simple six-pulse converter topology feeding aDC load is presented in figure 1.
M
L1
L3
L2
Figure 1: Six-pulse thyristor bridge
If the converter has a twelve-pulse configuration, har-monics of the fifth and seventh order do not appearin general. Twelve-pulse (and higher pulse numbers)are more expensive to manufacture than six-pulsedevices and are generally used only for high powerapplications.
In general we can say that the higher the pulse num-ber, the lower the magnitude of harmonic distortion fedinto the supply. Lack of symmetry (as a result of supplysystem phase unbalance or component mismatches),and specific faults in the converter can cause unchar-acteristic harmonic orders such as the second, thirdand fourth to appear in the supply current.
In the ideal case the magnitude of the harmonic cur-rents produced by the converter is dependent onlyon the magnitude of the fundamental current and theorder of the harmonic concerned according to thefollowing equation:
In =I1n
(2)
where
I1 = fundamental frequency currentn = order of harmonicIn = nth harmonic current
Equation 2 assumes that the converter is fed froma strictly symmetrical, stiff three phase network andthat the direct current output contains no ripple. Inpractice the magnitude of the harmonic currents pro-duced is, in addition to order of the harmonic and themagnitude of the fundamental frequency current, alsoaffected by the short-circuit power at the connectionpoint of the converter and the ripple on the direct cur-rent. Decrease of the short circuit power (increasednetwork impedance) lengthens the commutation time,in other words the commutation angle increases andthe supply side current distortion is reduced.
The circuit inductance is not able to smooth the directcurrent completely, therefore ripple remains in the DCcurrent. This has the effect of increasing some of theharmonics and decreasing others. Table 1 providessome insight to the difference between the idealisedharmonic current magnitudes predicted by equation 2and typical levels encountered in practice. The differ-ences are mainly caused by the effects of overlap andDC ripple.
The relationship between fundamental frequency andharmonics fed into the supply network by a frequencyconverter does not differ materially from what hasbeen stated regarding the thyristor converter. The or-der of the harmonics is also in accordance with equa-tion 1. There are considerable differences, however,in the harmonic currents fed into the supply systemby different types of frequency converters, due to thefact that the fundamental frequency currents taken atthe same torque and frequency are of different magni-tudes for different frequency converters.
Two different front end topologies are shown in fig-ure 2.
L1
L3
L2
P1
P3
P2DC/AC
L1
L3
L2
P1
P3
P2DC/AC
Figure 2: Controlled and fixed converter front ends
The magnitude of the fundamental current at a specifictorque and frequency is a function of the frequencyconverter input circuit, namely whether this is a thyris-tor bridge or a diode bridge. Since the thyristor bridgealso takes reactive power from the supply network,the fundamental current drawn by such a converter isgreater than the fundamental current drawn by a diode
bridge and consequently the harmonic currents sup-plied by the thyristor currents are greater than thosefrom a diode bridge.
1.3 Distributed renewable energy sources
Whereas domestic and commercial air conditioningloads are significant users of the frequency convertersabove, the large scale application of distributed renew-able energy sources requires controlled bi-directionalpower flow. Modern inverters allow for four-quadrantoperation, with independent direction of flow of ac-tive and reactive power. Furthermore, sophisticatedswitching control and inverter topologies have enableddevelopment of devices with low harmonic emissionand flexible fundamental frequency power control.
The examples in figure 3 demonstrate that indepen-dent of the source of energy, a utility interface thattransfers power to the network is necessary. Suchinterfaces are almost always electronic and inject har-monic current into the network. Strict standards applyto the amount of harmonic distortion that can be in-jected by such devices.
L1
L3
L2
AC
DC
DC
DC
L1
L3
L2
AC
DC
DC
AC
Figure 3: Common distributed generation systems
A large variety of topologies exist for inverters con-necting distributed generation systems to the network.Most employ some form of pulse-width modulationand line filters to reconstruct a nearly perfectly sinu-soidal voltage waveform to the network, an exampleof which is illustrated in figure 4.
Inverter outputLine side voltage
Out
put v
olta
ge (p
u)
−1
0
1
Time (s)0 0.01 0.02
Figure 4: PWM generation and filtering
Reducing the amount of voltage distortion presentedto the network requires increased switching frequen-cies in the inverters with associated increased losses.Most systems strike a balance between acceptablelosses and compliance with emission limits. A typical
2
voltage harmonic spectrum at the network connectionpoint is shown in figure 5. Regardless of topology,harmonic contributions are generally characterised byhigh frequency components, and side-bands aroundmultiples of the switching frequency.
V n/V
1 (p
u)
0.01
0.1
1
Harmonic order, n11 21 42
Figure 5: PWM spectrum and limits
Predicting the harmonic contribution of this class ofload is made complicated by the fact that changesto the harmonic spectrum can be brought about bychanges in the inverter control system, an approachthat is often used to circumvent a known problem withharmonic resonance, only to re-appear as a differentcomplication at a different frequency. The combina-tion of large cable networks and the relatively highfrequency components generated by these devices,the unpredictable and widely varying spectra result inthese devices being a special challenge for harmonicfilter design, which is only slightly mitigated by the factthat the devices have relatively low absolute harmonicemissions.
1.4 Thyristor switches
In recent years there has been an increase in theindustrial application of thyristors controlling resistiveloads. The most general control methods are the so-called integral cycle or burst firing control, and phasecontrol.
The burst firing control thyristors always conduct forone or more complete cycles, after which the ignitionpulse is removed, again for one or more cycles.
Figure 6 illustrates some examples for three differentdegrees of control (the meaning of the parameter “a”is defined in the top set of traces, for example for aratio of a=0.5, the thyristor conducts for one cycleout of every two.) The bottom figure presents theharmonic current spectrum for each degree of con-trol. It is clear that this type of control results in non-integer (harmonic current at frequencies that are notinteger multiples of the fundamental frequency) andsub-harmonics (harmonic current at frequencies lowerthan the fundamental current.)
In phase control, thyristors are fired during each halfcycle and power is controlled by varying the firingangle α depicted in figure 7. The fundamental andlower order current harmonics for various firing anglesis shown in the bottom image.
a=0.50
a=0.33
a=0.25
a=0.5a=0.33a=0.25
I n/I 1
0
0.2
0.4
Harmonic order, n0 1 2 3 4
Figure 6: Burst control waveforms and harmonics
I1
I
0 φ1 α
I1I3I5I7
I n/I
0.01
0.1
1
Firing angle α(degrees)30 60 90 120 150 180
Figure 7: Phase control waveforms and harmonics
In controlling resistive loads by phase control it is ob-served that the current fed also includes a laggingreactive component. A simple resistive load controllerand the amount of reactive power drawn by the con-troller is shown in figure 8.
1.5 Cyclo-converters
Cyclo-converters are static frequency converters thatconvert multi-phase fundamental frequency voltage tosingle or multi-phase voltage at a lower frequency. Acharacteristic of cyclo-converters is that they operatein most cases without circulating current.
A three-phase cyclo-converter generally consists ofthree inverse parallel connected three-phase thyris-tor converters that together provide the output for
3
PQ
P,Q
(pu)
0
0.5
1.0
Firing angle (degrees)0 30 60 90 120 150 180
R
Figure 8: Phase control and reactive power
the lower frequency three phase system. A simpleschematic is shown in figure 9.
M
f1
f2 < f1
Figure 9: Cyclo-converter schematic
The order of the harmonics in the input current ofthe cyclo-converter depends on the pulse number(equation 1). In addition to these so-called typicalharmonics a three-phase cyclo-converter producesharmonics the order of which depends on the cyclo-converter output frequency as follows:
n = (kp ± 1) ± 6mf2f1
(3)
where
n = order of harmonicp = pulse numberk = 1, 2, 3, . . .m = 0, 1, 2, . . .f1 = supply network frequencyf2 = cyclo-converter output frequency
If the output frequency of the cyclo-converter is 3 Hz,for example, there appear components on each sideof the fifth harmonic. Additional harmonics will alsoappear as a function of the output and line frequency.
The cyclo-converter harmonic amplitudes depend onthe load, load power factor, degree of firing angleand control mode. In some cases the non-harmoniccomponents may be of greater magnitude than theharmonic components. “Harmonic” is taken to meaninteger multiples of the fundamental frequency. Distor-tion at frequencies such as 268 Hz is non-harmonicper definition.
1.6 Arc furnaces
Since the current drawn by arc furnaces is, partic-ularly in the initial melting phases, appreciably non-sinusoidal, these furnaces are also sources of har-monics. Measurements on different arc furnaces haveshown that furnace current includes almost all harmon-ics. Average and instantaneous harmonic currents arepresented in table 2.
Table 2: Average and peak furnace harmonics
Harmonic % of fundamental currentorder, n Average Maximum
The harmonic producing part of the total load hasincreased continuously over recent years. Power elec-tronics is being used more extensively in industry forthe control of various processes. A consequence ofthis has been an increase in the harmonic voltagesand currents in the supply networks of industrial es-tablishments and electricity undertakings. Harmonicscause additional losses in network components andmay disrupt communication and control equipment,causing loss of time, product and equipment.
Capacitance – whether in the form of power factorcorrection equipment or stray capacitance from ca-ble networks – in networks subject to harmonics maycause harmonic currents and voltages to be amplified.In order to avoid the harmful effects of harmonics itis necessary to be informed of the possibility of theirappearance whilst still in the planning phase and ifnecessary to carry out a network harmonic study. Ina network already in use a harmonic analysis willbecome necessary if, for example, converter powerincreases considerably. An increase in compensationpower also generally makes a harmonic analysis nec-essary. The better the electrical values of the differentnetwork components and their frequency dependentimpedance are known the more exact will be the pic-ture of harmonic distribution given by the harmonicstudy.
For the analysis of harmonics there are now availablevarious software programs that also take into accountthe frequency dependence of different components,and that can calculate the distribution of harmonics,find possible resonance situations and are of assis-tance in the electrical design of harmonic filters..
Sources of harmonics are generally represented asconstant current sources injecting harmonic current
4
M
10 MVAXd
” = 0.12
�1.5 MVAIk/In = 5
31.5 MVAUk
= 10%
2 MVAUk
= 6%2 MVAUk
= 6%
5 : 100%7 : 100%11:100%13:100%
P = 1.2 MWQ = 1.2 Mvar
15% 21%
64%
G
110 kV 1500 MVA
6 kV ~350 MVA
0.4 kV ~30 MVA
Figure 10: Distribution of harmonics
into the network. Harmonic currents are distributedthroughout the different components in the network sothat, at any given frequency, the part of the networkwith the lowest impedance carries relatively the great-est part of the harmonic currents. The impedancesused are the short circuit impedances of the variouscomponents. Figure 10 shows the distribution in anindustrial network. The spectrum of a non-linear loadis normalised for reference, and the total harmonic cur-rent distortion through various network componentsas a result of this non-linear load is shown.
M
10 MVAXd
” = 0.12
�1.5 MVAIk/In = 5
31.5 MVAUk
= 10%
2 MVAUk
= 6%2 MVAUk
= 6%
5 : 100%7 : 100%11:100%13:100%
5 : 142%7 : 361%11:113%13:100%
P = 1.2 MWQ = 1.2 Mvar
5 : 242%7 : 261%11:13%13:0%
600 kvar
G
110 kV 1500 MVA
6 kV ~350 MVA
Figure 11: Amplification of harmonics
In figure 11 the new distribution of harmonics is cal-culated when compensation capacitors are applied
to the same busbar as the non-linear load. It is clearthat in this case substantial amplification of harmoniccurrent is taking place: for example, for every 1 A ofseventh harmonic generated by the non-linear load,3.6A will flow into the capacitor bank and 2.6 A willflow into the supply transformer feeding that busbar.This is a clear example of harmonic resonance.
In examining what factors affect the network reso-nance frequency and the distribution of harmonics itis noticed that increase of short circuit power (networkinductance decrease) raised the network resonancefrequency and a still greater part of the generatedharmonics flow into the network. Squirrel cage mo-tors loading the network also increases the resonancefrequency, but at the same time damp the resonance.
R=0, M=0R>0, M=0R=0, M>0
Har
mon
ic a
mpl
ifica
tion
0
10
20
Frequency (Hz)200 300 400 500
R M
Figure 12: Effect of different loads on resonance
A resistive load does not affect the resonance fre-quency markedly, but damps the resonance consider-ably. Raising the degree of reactive power compensa-tion by capacitors reduces the resonance frequencyof the network. Figure 12 shows the effect of dif-ferent loads in damping and shifting the resonancefrequency.
3 Measurement of harmonics
Measurement of harmonics in industrial networks andthose belonging to electricity utilities is usually carriedout by making use of current and voltage transformersthat are already part of the network. In some cases, aseparate clamp-on current transformer may be used.
For reliable measurement results it is necessary to as-certain whether the voltage and current transformersused in the measurements are capable of reproduc-ing the higher frequencies reliably. In general it maybe said that current transformers reproduce reliablyharmonics with frequencies of some kilo-herz whilethe reproduction range of voltage transformers mayonly be some hundreds of herz.
The following should be considered in all measure-ments:
1. Take note of the characteristics of the instrumenttransformers, specifically accuracy class, burden,and construction.
5
2. Know what network configuration exists. Anynumber of possible scenarios (fault level, busconfigurations, transformer connections and tapposition, etc.) may occur and filter designs willbe based on the network conditions prevailingwhen measurements were done.
3. Be aware of the nature of the loads in the network.With modern IGBT inverters the interesting, highfrequency and non-integer harmonics may occuroutside the range of the measurement device.
4. Plan the analysis process before planning themeasurement regime. One normally only getsone go at performing site measurements dueto the cost of equipment, travel and access ar-rangements, so make sure everything you needto measure to construct the model is covered,and that the locations where the measurementsare made are consistent with the requirementsof the model.
5. Harmonic measurements create a surprisinglylarge amount of data. Make sure you have aplan for the storage, manipulation, verificationand traceability management of the data. Forexample, measurements over a two week periodusing 3 second data on three phase of voltageand current on two feeders results in 240 milliondiscrete measurements if all harmonics to thefiftieth are recorded.
Modern measurement equipment can easily providethe voltage and current harmonics phase angles, fromwhich the direction of the harmonic with respect tothe network may be determined. In theory it can besaid that a part of the network is receiving harmonicswhen the angle between voltage and current at thatfrequency is in the range −90◦ . . .+ 90◦. Correspond-ingly harmonics are being produced if the phase angleis in the range +90◦ . . . + 270◦.
Care should be taken in interpreting phase angle mea-surements at harmonic frequencies: current and volt-age transformers, depending on technology, burden,and accuracy can have large errors in magnitude andphase and without magnitude and phase calibrationharmonic direction measurements can be meaning-less.
3.1 Verification of coincidence factors
A critical aspect of model construction is the use ofcoincidence factors or diversity of distributed harmonicsources. In a wind farm of 100 turbines, the modelcannot assume that all turbines are operating at thesame level and with the same harmonic generation— some means of taking the load/generation diversitymust be incorporated in the model. This is clearly avery important lever that can be used to manipulatethe outcome of harmonic studies and the assumptionsmade in the model must be stated explicitly. The bestway to justify the use of a specific diversity factor is torelate its use to existing installations where a modelhas been set up and calibrated by field measurements.
3.2 Recording existing harmonic levels
It is very important that existing background harmonicdistortion be recorded, whether in greenfield or brown-field applications. Most filter design will (or at leastshould) require post-installation measurements to ver-ify the performance of the equipment. Such measure-ments are almost meaningless unless the pre-existingharmonic distortion was measured and taken into ac-count when the filters were designed.
These measurements should cover a time span of atleast a few weeks in order to obtain statistical figures,keeping in mind the requirements of the measurementstandards.
3.3 Measurements during commissioning
Apart from the normal tests of voltage rise, reactivepower output and verification of the tuned frequency,it is highly recommended to install a transient recorderduring energisation of a harmonic filter to capturetransient conditions inside the bank and imposed onthe network and switching equipment.
Verification of the harmonic performance of the filteris naturally an important test, keeping in mind thatthe measurement conditions will probably never corre-spond to the worst case conditions for which the filterdesign has been made.
3.4 Measurement results
Measurements were carried out of the supply trans-former the secondary current in the network shown infigure 13.
If1 If2
IV
Figure 13: Plant with multiple converter loads
The sources of harmonics were two identical six-pulsethyristor bridges. The firing angles of the two bridgeswere independent of each other. Table 3 presents themeasured harmonic currents in the supply transformer,IV .
As can be seen from the results of the measurementsthe fifth harmonic is only 3.4% of the fundamental fre-quency component, while the expectation from equa-tion 2 is that this component should be approximately20%. The reason for the low fifth harmonic content isrevealed from the voltage waveform on the supply busas shown in figure 14.
6
Table 3: Harmonics with independent converters
Harmonic order IV//A IV//%
1 817 1005 28 3.47 29 3.5
11 57 713 32 3.917 11 1.319 18 2.223 18 2.2
It is clear that the control angles (firing angles) of theconverter bridges are different — the commutationnotches are not aligned. The fifth harmonic currentsare therefore not in phase and as can be seen from thevector summation of the harmonics there is substantialcancellation of the fifth harmonic under these specificconditions.
Regulating the thyristor bridge control angles so thatthey were almost identical as illustrated in figure 15resulted in measurement of harmonic currents as re-ported in table 4. This is much closer to what is ex-pected from equation 2.
Table 4: Harmonics with converters aligned
Harmonic order IV /A IV /%
1 800 1005 158 19.97 71 8.9
11 51 6.413 35 4.417 23 2.919 16 2.023 9 1.1
The harmonic sources of the network in figure 16 arefrequency convertors equipped with fixed intermedi-ate circuit voltage (input circuit consisting of uncon-trolled diode bridges) with a total power rating of about1 MVA.
For frequency convertors with diode bridge input cir-cuits the harmonics sum arithmetically since there isno differences between the control angles.
Table 5 presents measurements of harmonics on thelow voltage side of the supply transformer. Values are
If1
If2
If1+If2
Fifth harmoniccurrent vectors
Figure 15: Harmonic summation with converters al-most aligned
If
IV
f1
f2
150 kvar 150 kvar
Ic Ic
Figure 16: Plant with frequency converters
of the same order of magnitude as predicted by equa-tion 2. The table includes the effect of switching twosteps of power factor correction in to compensate forreactive power. These non-tuned capacitors clearlycause parallel resonance as can be seen from theincrease in current distortion through the supply trans-former and increase in harmonic current distortion.
Non-linear loads affect the power system, specificallyin terms of voltage distortion due to harmonic currentgenerated by the load and passed into the network.Power systems also affect non-linear loads. Voltagedistortion on the busbar feeding the converter will re-sult in a different harmonic spectrum drawn by the loadthan if there was no distortion of the busbar voltage.In figure 17 a simple network is shown that containsan uncontrolled rectifier converter and a fifth harmonicfilter.
The results of measurements of the converter currentare shown in table 6. It is clear that the presence ofthe shunt connected filter has a very significant impact
7
If
IV
Fifth harmonic filter, 640 kvar 660 V
Figure 17: Effect of filter on converter current
Table 6: Effect of filter on converter current
Harmonic IV /% Changeorder No filter With filter %
on the converter current, increasing some harmoniccurrent and reducing others.
The measurement results presented above under-score the importance of performing and understandingdetailed measurements of harmonic distortion in anyfacility, before any design work is carried out.
4 Reactive power compensation andharmonics
4.1 Operation principle
The function of a harmonic filter is to remove harmon-ics appearing in a network and to produce capaci-tive reactive power at the fundamental frequency. Byfrequency tuning, filters present a low impedance be-tween phase and star point or between phases, so thatthe frequency tuned harmonic flows into the filter anddoes not spread into the feeding network. Harmonicfilters are connected at an appropriate voltage levelin each network. Harmonic filters consist, dependingon the requirements of each application, of one ormore branches, each of which is tuned to harmonicfrequencies appearing in the network in question.
4.2 Single-tuned filter
A filter tuned to one frequency consists of a capacitorbank and reactor connected in series, as shown infigure 18.
The capacitance of the capacitor bank is generallydetermined by the compensating power required for
2 Mvar6 Mvar
|Zf|
()
0
10
20
30
Frequency (Hz)0 250 500 750 1000
L
C
Figure 18: Single-tuned harmonic filter
the fundamental frequency. The inductance of the filterreactor is chosen so that together with the capacitor itforms a a resonant circuit at the desired frequency.
4.3 Wideband filter
In a wideband filter a resistor is connected in parallelwith the reactor, for example as shown in figure 19.The result of this configuration is that harmonics abovethe tuned frequency are also filtered.
|Zf|
()
10
20
30
Frequency (Hz)0 250 500 750 1000
C2
L
C1
R
Figure 19: Wideband filter
The resistor reduces the filtering effect at the tuned fre-quency. The purpose of the capacitor C2 is to reducethe fundamental frequency current flowing through theresistance and consequently to reduce the losses inthe resistor.
4.4 Information required for filter design
Filters are always designed according to the require-ments at the specific location where they will be usedso that the relevant technical and economic factorscan be taken into account in the best possible way.
For filter design, the following information is requiredfrom the client:
1. Desired reactive power at the fundamental fre-quency (maximum and minimum)
2. Service voltage and possible range of variation
3. Rated frequency
4. Insulation requirements if these are different fromnormal values at that service voltage
8
5. Required fault withstand capability
6. Frequency dependent network impedance for allpossible operating conditions for which the filteris expected to provide the desired mitigation, or ifnot available then the actual short circuit currentat the filter connection point and possible rangeof variation
7. Information of loads generating harmonics
8. Permissible harmonic content
9. Details of the installation environment (in-door/outdoor, pollution, temperature range, windand seismic requirements, etc)
4.5 The need for harmonic filters
All networks have limits for the extent of harmonicdistortion that can be present in the network. In almostall networks there are statutory limitations, for examplelegislation relating to the conduct of energy users thatrefer to technical standards to determine exactly whatthose limits should be.
Two broad categories of standards are used: thosethat aim to limit voltage distortion in the network by ap-plying limits to the amount of harmonic current distor-tion caused by individual customers or loads (the IEEEStd 519 approach) and those that allocate permissiblecontribution to voltage distortion at specific nodes inthe network based on network and load characteris-tics (the IEC 61000 approach). Neither approach isperfect or uniformly fair, and both rely on either makingsimplifying assumptions about the nature of the sup-ply network (for example the IEEE approach does notexpect any reactive power compensating capacitorsto be in the supply network) or require detailed knowl-edge of the network ( for example the IEC approachassumes that the frequency dependent nature of thesupply network is known).
As networks become more exposed to harmonic pro-ducing loads and simultaneously are expected to lastlonger and transfer more power than before, there iswidespread implementation of reactive power com-pensation in networks. These can be implemented assimple capacitor banks, possibly fitted with inrush cur-rent damping reactors, or as detuned or filter capacitorbanks.
Any addition of capacitor banks to an existing, purelyinductive and resistive network will result in a signifi-cant change in the network impedance. If a parallelcapacitor alone is used in a network containing har-monics, then the capacitance together with the induc-tance of the feeding network forms a resonant circuit,with:
fr = f1
√SK
QC(4)
where
fr = resonant frequency in Hzf1 = fundamental frequency in Hz, for
example 50 HzSK = short circuit power at the capacitor
bank connection, in MVAQC = capacitor bank compensating power, in
Mvar
If the natural frequency calculated from equation 4is near to some harmonic appearing in the network,then that harmonic will be amplified considerably. Thebiggest amplification factor may be in the order of 20.
4.6 Choice of filter
In designing a filter the aim is the simplest and leastcost construction that will satisfy the requirements forreactive power compensation and filtering. In practisethat may mean that the fifth and seventh harmonicsmay be filtered by single tuned filters, and the upperharmonics by a single wide-band filter.
Filter compensation power affects filter characteristics:the greater the compensation power the better the har-monic suppression. The different branches of a har-monic filter may be connected each to its own circuitbreaker and use, for example, a reactive power regu-lator to control them according to the reactive powerrequirement, or can use a common circuit breaker.When each branch has its own breaker, connectionof the different branches to the network should takeplace in the order of the harmonics, beginning withthe lowest, and disconnection should take place in thereverse order. Switching order is important in order toavoid harmonic resonance.
In choosing and designing a filter a central factor isthe distribution of compensating power between thedifferent branches of the filter. This should be carriedout so that:
1. The same capacitor units may be used in differ-ent branches
2. Capacitor banks are used at rated voltage (noexcess voltages)
3. Parallel resonant frequencies occurring at fre-quencies between those of the absorption cir-cuits do not coincide with harmonics that mayappear in the network, for example even harmon-ics
4. Branch powers are suitable for (existing)switchgear
5. Filter inductors are reasonable to assemble
6. Filtering results are adequate.
4.7 Effects of filters
Filters are able to reduce from 60% – 90% of harmon-ics. Filtering results depend on the relation betweenthe impedance of the supply network and the filter.
9
The solution is always a compromise since the filter re-moves those harmonic for which it has been designedbut increases harmonics at intermediate frequencies.
An example of a filter application is shown in figure 20,where the harmonics produced by a nonlinear loadare mitigated by a harmonic filter with three branches:single tuned filters for the fifth and seventh harmonics,and a wideband filter tuned to the eleventh harmonic.
In what follows, Zn is the frequency dependentimpedance of the network (dominated by the sup-ply transformer, but with the additional impedance ofthe network in series with the supply transformer),and Zf is the equivalent impedance of the three filterbranches connected in parallel.
5th filter, 4 Mvar
110 kV, SK = 1500 MVA
11 kV, SK = 250 MVA
Sn = 31.5 MVAZK=10%
7th filter, 3 Mvar
Wideband filter, 3.7 Mvar
6 pulse S=12.7 MVA
=
I1 = 666 AI5 = 200 AI7 = 80 AI11 = 47 AI13 = 40A
Zn
In
Z5
I5
Z7
I7
Zwb
Iwb
Zn
In
Zf
If
IH IfIH If
Figure 20: Example of a harmonic filter in industry
The frequency dependent impedance of the networkand the filter alone are presented in figure 21.
ZfZn
|Z| (Ω
)
0
5
10
15
Frequency (Hz)0 250 500 750 1000
Figure 21: Filter and network impedance
The extent to which harmonics are absorbed by thefilter or injected into the network is determined by thecurrent divider expressions:
In = IHZf
Zn + Zf(5)
If = IHZn
Zn + Zf(6)
whereIH = harmonic current produced by the loadIn = current flowing into the networkIf = current flowing into the filter
Zn = network impedanceZf = filter impedance
When reviewing the impedance of the network with thefilters connected, illustrated in figure 22, three clearresonance points can be observed. The lowest fre-quency resonance point is produced by the interactionof the total capacitance of the filter and the inductanceof the network, and this frequency is always lowerthan the lowest tuned frequency of the filter. Two otherresonance resonance points represent resonancesbetween the separate branches of the filter.
Zn || ZfZn
|Z| (Ω
)
0
5
10
15
Frequency (Hz)0 250 500 750 1000
Figure 22: Network impedance with filters connected
Where the common impedance of the filter and net-work is less than the network impedance, harmonicsare filtered according to the expressions in equation 5,and where the common impedance is greater than thenetwork impedance, harmonics are amplified.
It is important to note that in most cases, Zn will notbe a simple inductance — the presence of one ormore capacitor banks in the external network andchanges in supply network topology may result inwide variations in impedances at fundamental andhigher frequencies. One of the major challenges infilter design is to gain an accurate understanding of thenature of the network impedance. As this is largely outof the control of the filter designer, and may changeat any time and over time, any filter design must takesuch variations into account and be robust enough tobe effective under a variety of network conditions.
The art of harmonic filter design is to select the shapeof the impedance Zf to modify the network impedanceZn in such a manner that the necessary reduction inharmonic current is achieved without causing ampli-fication of other harmonics, taking into account thepossible variations in network impedance, and to pro-duce this solution at the lowest possible cost.
Acknowledgements
Most of the above work was done by Martti Tuomainen,then at Nokian Capacitors, in an article titled “SpecialQuestions of Industrial Networks Harmonics” with ref-erence EN-TH03-11/2004. Thank you to Martti forpermission to repackage the document.
