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Abstract
An analysis of four shunt reactive power compen-sation configurations is presented. The performanceof the banks is compared in terms of the level ofabsorption of load control signals, exposure todamage from background harmonic distortion levels,and sensitivity to switching transients. In each casecomponent tolerances, fault and load levels arevaried to determine performance under a wide rangeof network conditions.
It is concluded that inrush current limited bankswith audio frequency blocking filters effectivelyavoid absorption of load control signals, but thesesystems are exposed to damage from harmonicdistortion and switching transients in the network.Detuned capacitor banks are not exposed toharmonic resonance or network switching events,while optimum performance is obtained fromdetuned banks with blocking filters.
A brief description of the approach to designcomponent ratings in detuned capacitor banks ispresented.
Introduction
Distribution network capacity is stretched to thelimit in many electricity markets. Shunt reactivepower compensation is installed at all levels of thenetworks to improve the voltage profile, free upgeneration and transmission capacity and reducesystem losses.
The configuration and functionality of compensationsystems are determined to a large extent by localpreferences and requirements for flexibility, noiselevels and availability. As a result design andimplementation of systems vary widely inter-nationally, or even within a single country.
Selecting the most suitable configuration for a givenapplication can be challenging in the face of thisvariety. The physical conditions behind selection ofa particular implementation in one network may notexist in another network, and the reasoning behindthe selection will most likely be unknown to buyers.
The purpose of this paper is to investigate theperformance of different power factor correctiontopologies in the context of a practical application.
It is vitally important that all systems connected tothe network are capable of operating successfullyregardless of network fault level, size of load andcomponent drift within manufacturers' tolerancelimits. Typical variations have been used in thispaper to construct a number of operating scenariosto evaluate the performance of each configuration.
COMPARISON OF SHUNTCOMPENSATION SYSTEMSIN MEDIUM VOLTAGENETWORKS
The performance of each configuration will bereviewed in terms of attenuation of audio frequencycontrol signals, performance in avoiding amplificationof harmonic distortion, and susceptibility to networkswitching transients.
Once a configuration is selected, it is necessary todetermine the correct rating for the criticalcomponents.
A brief description of a suitable design approach isincluded to assist in the detailed specification ofshunt reactive power plant.
Configuration
The network used as the basis of analysis is shownbelow.
Figure 1: Network section used in analysis
A Thevenin equivalent of the complete utility systemfeeds a zone substation via a short overheadtransmission line. The zone substation contains a132 kV busbar, two transformers, and a number ofloads. These loads are lumped together in thisanalysis to form a single load of 12.8 MVA at apower factor of 0.78, connected to the 11 kV busbarof the zone substation. The transformers are intendedfor operation in N-1 redundancy, but it is possiblefor both transformers to feed the load via the normallyclosed bus coupler. Parallel operation of thetransformers results in a significantly higher faultlevel at 11 kV (155 MVA) than with a singletransformer in service (81 MVA.)
Audio frequency load control signals are injectedupstream from the zone substation. The injectionequipment is connected to a remote zone substation
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TEshown shaded in the figure above. The injectionfrequency has been taken as 1045 Hz.
It has been decided for operational purposes toconnect a shunt capacitor bank at the zonesubstation. This may consist of multiple steps, butfor purposes of this analysis, the power factorcorrection system has been lumped into a singlestep of 5 Mvar at 11 kV.
Four configurations were considered for theapplication. The single line diagram below indicatesthe two different circuits for the four configurations.
The four configurations can be described as inrushcurrent limited, inrush current limited with audiofrequency blocking, detuned, and detuned withaudio frequency blocking.
The table below indicates the nominal values ofinductance and capacitance for an 11 kV, 5 Mvarstep. The detuned banks include a 7 % reactor, andthe audio injection frequency is 1045 Hz.
Lm Lt Cm Ct(mH) (mH) (µF) (µF)
Inrush current limited 0.15 - 131 -
Inrush currentlimited withblocking circuit 0.15 1 131 23
Detuned 5.8 122.3
Detuned withblocking circuit 5.22 0.58 122.3 39.6
Detuned capacitor banks also consist of a capacitorbank and a series connected reactor. In this case,the reactor is chosen such that the combination ofreactor and capacitor has low impedance at afrequency where no harmonic distortion is expected.This frequency is generally less than the fifth, andoccasionally less than the third harmonic. To achievethis tuning frequency, the reactor has to besubstantially larger than in the case of the inrushcurrent limited bank.
A blocking circuit can be added in the same manneras before to detuned banks, to present highimpedance at the audio injection frequency.
Network variables
Three network parameters were varied for thepurposes of this analysis - the fault level at the11 kV busbar, the load and the actual value ofcomponents in the capacitor bank.
Parallel operation of the transformers results in asignificantly higher fault level at 11 kV (155 MVA)than with a single transformer in service (81 MVA).
The load was changed between high (or full load)and low load, which was taken as 10 % of full load.Load level variation results in variation of the dampingin the network.
Variation in component values resulted in the tuningfrequency of any blocking circuit to be at minimum,nominal and maximum values. The minimum tuningfrequency occurs at maximum component values.
IEC 60289 and AS1089 state the tolerance range forreactors in tuned circuits as -3 – +3 %. IEC 60871-1requires capacitor banks between 3 and 30 Mvar tofall within 0 – +10%. This tolerance spread was usedfor inrush current limited banks as well as detunedbanks.
The values of the main components of the bank -the capacitor banks and inrush or detuning reactor- were varied according to the same tolerance limits.The variations in component values, load and faultlevel result in twelve scenarios of unique networkconditions.
Audio frequency load control
Ripple control, or audio frequency load control, iscommonly used to control single phase domesticloads, typically hot water systems. The carrierfrequency for the control signal varies widely.
Power networks are designed for optimalperformance at power frequency, and thereforeattenuation of control signals is a concern in allapplications. Since capacitor impedance is inverselyproportional to frequency, capacitor banks tend toabsorb much of these signals. Special capacitorbank design measures are generally required to avoidthis, such as the blocking filters discussed above.
Component mismatch - due to deviations within thetolerance limits from nominal value and componentdrift - are serious concerns in the application of
Inrush current limited capacitor banks consist of amain capacitor circuit, with a small series reactor.The purpose of the reactor is to limit the currenttransient that occurs when the capacitor isenergised, or when a capacitor bank close by isenergised. The purpose is to restrict the peakcurrent to within what switchgear and capacitorunits can withstand. The reactors are generallysmall, in the order of 150 H.
A blocking circuit can be added to the inrush currentlimited bank, by inserting an additional series reactor,and placing a capacitor in parallel with this tuningreactor. The values of the tuning circuit are selectedsuch that the bank has a high impedance at theinjection frequency.
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TEthese filters. The following charts describe theimpact of the above-mentioned tolerance spread,changes in fault level and variations in load level onthe tuning frequency and impedance at the carrierfrequency.
In the analysis carried out below a carrier signal at1045 Hz is injected at a busbar remote from the11 kV busbar where the power factor correction isapplied. The current flowing into the sourceimpedance sets up a voltage at the carrier frequencyat all busbars in the network.
It is clear from the charts below that the tolerancevariations permissible for capacitors and reactorsresult in wide variations in tuned frequency.
Most utilities specify tuning reactors with multipletap positions so that the tuning frequency can bevaried somewhat on blocking filters consisting ofcapacitors and reactors with the standard tolerancespread.
Figure 2: Impedance vs frequency for inrush currentlimited bank with blocking filter
It is clear from Figure 2 that good rejection of thecarrier frequency (shown as the solid vertical line)will be achieved if all components are at nominalvalues. However, the impedance at the carrierfrequency can vary by more than an order ofmagnitude.
Figure 3 shows that the situation is also potentiallyserious in the case of detuned banks with blockingfilters.
Multiple tap positions on reactors are costly, requireadditional maintenance, and unless the filters arecarefully tuned, do not permanently solve the problemof increased attenuation when component valuesdrift.
It is recommended that tuning components arematched during design and manufacture. If theabove analysis is repeated with typical componentvariations instead of the tolerance range allowed forby the standards, the spread of tuning frequenciesand impedance at the blocking frequency is greatlyreduced, as shown in Figure 4.
Figure 3: Impedance vs frequency for detuned bankwith blocking filter
Figure 4: Impedance vs frequency for detuned bankwith blocking filter, matched components
The table below indicates the level of carrierfrequency signal that remains on the 11 kV busbarafter the power factor correction is applied. A valueof 80 %, for example, indicates that the signalstrength when the particular configuration of powerfactor correction is applied is 80 % of what it wouldbe without any power factor correction.
The minimum, average and maximum valuesobtained under all network conditions are presentedfor each bank configuration.
Values in italics indicate the outcome with typicalmanufacturing tolerances, i.e. when the capacitorand reactor components are manufactured to matchthe application.
Table 1: Audio frequency signal remaining after connection of capacitor bank
Min Avg Max
Inrush current limited 1% 2% 3%
Inrush current limited with blocking 90% 101% 117%
94% 96% 99%
Detuned 73% 77% 81%
73% 77% 81%
Detuned with blocking circuit. 48% 81% 100%
94% 99% 103%
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Detuned banks with blocking filters perform verywell provided that the components of the filter arematched during manufacturing, or adjusted to matchduring construction.
Detuned banks without any blocking filters tend toabsorb more than the acceptable amount of controlsignal, but are the least influenced by networkconditions, especially variations in componentvalues.
Exposure to harmonic distortion
The traces below indicate the impedance of thenetwork in Figure 1, as seen from the 11 kV busbar.In each case, the effects of changes in load, faultlevel, and equipment tolerance are shown.
In the case of the inrush current limited banks,irrespective of the presence of blocking filters, thenetwork presents high impedance close to 250 Hz.
When the network impedance is high at or close toa frequency at which non-linear loads, such asvariable speed drives or arc furnaces inject currentinto the network, a high voltage appears across thenetwork impedance.
Figure 5: Impedance of an inrush currentlimited bank
When this occurs, a comparatively small non-linearload can cause high levels of voltage distortion. Thephenomenon is referred to as amplification of voltagedistortion due to parallel resonance.
Figure 6: Impedance of an inrush current limited bank with blocking filter
The effect of a detuning reactor in the capacitorbank is shown clearly in the trace below. Thecharacteristic is independent of the presence of ablocking circuit.
Figure 7: Impedance of a detuned bank
The network has low impedance at the tunedfrequency. This frequency is selected to be belowthe fifth harmonic, and is entirely a function of thecomponents of the capacitor bank. The capacitorbank is inductive at frequencies above the tunedfrequency, and therefore resonance is impossible.
The table below presents a summary of the outcomesunder all network and load conditions, for each ofthe configurations.
A nonlinear load with a total current harmonicdistortion of 8 % was used in this analysis, to indicatean industrial load with a low level of harmonicdistortion.
The voltage harmonic distortion at the 11 kV busbaris just below the IEC 61000-3-6 compatibility limitof 8 % without PFC connected. This occurs whenthe fault level is low and the full load is connected.
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No PFC connected 0.4% 3.1% 7.8%
Inrush current limited 1.1% 8.2% 16.8%
Inrush current limited with blocking 0.4% 4.8% 14.0%
Detuned 0.2% 1.3% 2.9%
Detuned with blocking circuit 0.2% 1.4% 3.2%
Table 2: Harmonic distortion at 11 kV busbar
The voltage harmonic spectra for the maximumdistortion level in each configuration are presentedin Figure 8.
It is very clear from this graph that detuned banksprovide protection against harmonic amplification,with harmonic levels somewhat reduced comparedwith distortion without any power factor correctionconnected.
Inrush current limited banks do not provide anyprotection against harmonic resonance. Thepredicted distortion levels, even with low level loadcurrent distortion, far exceed compatibility levelsaccording to IEC 61000.
Figure 8: Maximum voltage distortion
Apart from non-compliance to statutoryrequirements, such high levels of harmonic distortioncould result in premature failure of capacitors,reactors, switchgear and electronic systems.
The chart below indicates the harmonic spectrumof the current into the capacitor bank. The inrushcurrent limited banks have very high levels of fifthharmonic distortion, exceed the 84 % distortionlimit imposed by the requirement for reactors to berated to 1.43 x Inom, with an overload factor of10 % for fundamental frequency over current, and30 % in total for harmonic current.
Figure 9: Capacitor bank current distortion
Switching transients
Voltage transients may arise from any number ofevents in the network, and propagate through thenetwork. One such event is the energisation of acapacitor bank. In this analysis, a large capacitorbank is energised at the 132 kV busbar N132, shownshaded in Figure 1.
Two alternatives have been reviewed in this analysis:where the inrush current into the bank is restrictedby means of inrush current limiting reactors, andwhere the bank is supplied with a detuning reactor.
The voltage transient at the 132 kV bus is shownbelow.
Figure 10: 132 kV bus transients due to transmissioncapacitor bank energisation
Figure 10 illustrates that the voltage transientpropagating through the network is significantlymore severe - higher frequency and peak values -when inrush current limiting reactors are used.
The chart below indicates the voltage transient atthe 11 kV busbar when the inrush current limitedbank is connected. The transient event at this busbarwithout any capacitor banks at 11 kV is shown asa reference.
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TE Figure 11: Voltage transient at 11 kV bus with inrushcurrent limited bank connected
The peak transient value is significantly amplifiedwhen the inrush current limited 11 kV capacitorbank is already connected - to a peak value of closeto 3 p.u.
Figure 12: Effect of detuned transmission bank oninrush current limited capacitor bank.
The outcome with the same configuration at the11 kV bus, but with a detuning reactor used in the132 kV bank is shown in Figure 12. The transientwith the 132 kV bank fitted with inrush currentlimiting reactors only is shown as a reference.
The peak voltage transient at the 11 kV busbar issignificantly reduced by the use of detuning reactorsat 132 kV.
Figure 13 demonstrates that transient amplificationalso takes place in capacitor banks with blockingfilters, in this case due to a switching operation ofa 132 kV bank with inrush current limiting reactors.
Figure 13: Voltage transient at 11 kV bus with inrushcurrent limited and blocking bank connected
This transient amplification can be largely avoidedby making use of detuned banks at 11 kV,independent of whether a blocking filter is required.
Figure 14: Voltage transient at 11 kV bus with detunedbank connected
The traces in Figure 14 compare the voltage transientat the 11 kV busbar when a 132 kV bank is energised,and a detuned bank is connected to the 11 kV withthe transient when there is no 11 kV bank connected.
Figure 15: Voltage transient at 11 kV bus with detuned, blocking bank connected
The situation is not altered to any significant extentby the presence of a blocking filter in the detunedbank, as shown in Figure 15. The detuned bank hasa small damping effect on the voltage transient, andno amplification takes place.
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The total bank reactance can be calculated fromthe nominal reactive power requirement of the bankat nominal voltage, XT = VN
2/QN .This value and therelationships indicated in the figure above result invalues of capacitance 122.3 µF and inductance of5.8 mH.
The combination of inductor and capacitor iscapacitive at frequencies less than the tuningfrequency of 189 Hz, and inductive at frequenciesgreater than 189 Hz. Resonance between thecapacitor bank and the network impedance istherefore impossible above this tuning frequency.
AS 2897 and IEC 60871-1 require that the voltagerating of the capacitor be determined as thearithmetic sum of fundamental and harmonicvoltages.
The fundamental frequency voltage across the bankis determined by considering the current INflowing through the bank at 50 Hz, and then usingthe capacitor reactance XC,50 at 50 Hz to determinethe voltage across the capacitor, VC,1 = I1 x XC,1, whereI1 = VN,1 x 1.1/XT,1 .
These results lead directly to the required voltagerating of the capacitors and the current rating of thereactor:
n VN,n (V) XT,n ( ) In (A) VC,n (V)
1 12100 24 287 7512
5 440 4 65 339
7 330 9 21 78
The rated output of the capacitor at rated voltageis 7.25 Mvar.
Conclusion
The table below presents a summary of the outcomesobtained above.
Table 3: Overall outcome for configurations
Capacitor banks with inrush limiting reactors arenot suitable for applications where audio frequencycontrol signals are present, where any level ofharmonic distortion may be present, or whereswitching transients may occur.
The addition of a blocking circuit ensures that audiofrequency signals are not attenuated, but harmonicresonance remains likely and amplification oftransients in the network may still occur.
Detuned banks do absorb audio frequency signalsto a certain extent, but are not susceptible toharmonic resonance and switching transients.Improved performance of detuned banks can beobtained where required by the addition of a blockingcircuit, especially when matched components areused.
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Inrush current limited
Inrush current limitedwith blocking
Detuned
Detuned withblocking circuit.
Recommended ratings
A detuned filter arrangement is selected to simplifythe bank design and to avoid the problems ofresonance and exposure to switching transients.The design approach assumes that networkconditions with the bank connected are known, orthat values published as worst case figures in networkdistribution codes are to be used.
The nominal output of the capacitor bank is 5 Mvarat nominal voltage VN of 11 kV. The reactor rating is7 % of the capacitor. The busbar voltage can beexpected to operate at 10 % above nominal voltagefor substantial periods, including the voltage rise asa result of the capacitor bank.
It is assumed that the voltage total harmonicdistortion VTDH = 5%. It is assumed that this arisesfrom 5th and 7th harmonic voltages only whereVN,5 = 4% and VN,7 = 3%, in percentage of thefundamental frequency nominal voltage.
The current at each harmonic is determined in asimilar fashion: In = VN,n /XT,n and VC,n = In x XC,n.
