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23 May 2016 | c©0MPRCR-1
Reactors and shunt capacitor banks
Introduction
Shunt capacitor banks are installed for a variety ofreasons in industrial, distribution and transmissionsystems. A common thread to all installations is thequestion of what, if any series reactor should be in-stalled with the capacitor bank.
Series reactors are used with capacitor banks for twomain reasons:
• To dampen the effect of transients during capaci-tor switching, and to
• Control the natural frequency of the capacitorbank and system impedance to avoid resonanceor to sink harmonic current.
This note is based on a realistic example and dis-cusses the effect and consequences of different typesof reactor.
1 Capacitor switching transients
When a capacitor bank is energised, the bank and thenetwork are subject to transient voltage and current.The severity of the effect is determined by the size ofthe capacitor and the network impedance.
The worst case occurs when a capacitor bank is ener-gised close to a bank that is already connected. Theinrush current into the newly connected bank (and outof the connected bank) is determined by the size ofthe capacitor bank and the inductance between thetwo banks. The larger the banks, and the smallerthe inductance between banks, the higher will be theinrush current.
The frequency of the inrush current is determinedby the ratio of capacitor bank reactance and theimpedance between the banks. The smaller theimpedance, the higher will be the frequency.
Consider the case shown in the following simplifiedsingle line diagram. An 11 kV substation is suppliedvia a 132 kV/11 kV transformer. At 11 kV, a loadwith moderately poor power factor is overloading thesupply transformer. It is decided to compensate theload by means of 8 Mvar of reactive power, in twosteps of 4 Mvar.
External grid
T120 MVA Dyn11
11 kV bus Skss = 181 MVA
C1, C24 Mvar at 11 kV
L1P = 18 MW
Q = 12 MvarITHD = 6%
If the only impedance between the two banks is thatin the busbar and cabling between the banks, a verylarge, high frequency inrush current will flow betweenthe banks, as shown in the figure below.
Cur
rent
(pu)
−100
0
100
Time (ms)0 5 10 15 20
In this case, with only some micro-henrys between thebanks, a peak current of more than 120 times nominalcurrent, at a frequency of more than 8 kHz can beexpected.
Capacitor standards such as IEC 60871 state thatcapacitors should be able to withstand inrush currentsup to 100 times nominal. The standards suggest alower value if banks are switched frequently.
Large and high frequency inrush current can dam-age capacitors, circuit breakers and contactors. Allconnected equipment, and even remote substationsare subject to voltage transients and may result insporadic equipment malfunction or failure.
To avoid this problem, it is common practice to insertinrush limiting reactors in series with the capacitorbanks.
In the diagram below, 150 µH reactors have beeninserted in series with each bank.
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41/ 2 Benson Street Toowong QLD 4066
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External grid
T120 MVA Dyn11
11 kV bus Skss = 181 MVA
C1, C24 Mvar at 11 kV0.150 mH reactors
L1P = 18 MW
Q = 12 MvarITHD = 6%
The peak inrush current is now almost an order ofmagnitude smaller, at a much lower frequency, as canbe seen from the trace below.
From an inrush limiting perspective, this should be suf-ficient protection for the capacitor bank and associatedequipment.
2 Voltage distortion
Even if switching can be arranged in such a mannerthat no transient current occurs (such as promised bypoint of wave switching) due consideration must begiven to the natural frequency of the network, and theeffect of harmonic resonance.
The following traces indicate the impedance of thenetwork above, as seen from the 11 kV busbar.
One bankTwo banks
350 Hz250 Hz
Impe
danc
e (Ω
)
0.1
1
10
100
1000
Frequency (Hz)0 500 1000
It can be seen that the network presents highimpedance at 250 Hz and 350 Hz, depending onwhether one or both capacitor banks are energised.
Even if the current distortion is low, very high voltagedistortion can be expected as a result of harmoniccurrents flowing into this high impedance.
In the chart below, it is seen that the voltage distortion(VTHD) at the 11 kV busbar without any power factorcorrection is expected to be 4.3%.
When the first step is connected, the impedance traceabove predicts that resonance will occur at the seventhharmonic. This is clearly seen in the chart below, withtotal voltage distortion of 25%, mainly at the seventhharmonic.
Similarly, when the second step is connected reso-nance occurs at the fifth harmonic, resulting in voltagedistortion of more than 30%.
No PFCOne stepTwo steps
Volta
ge d
isto
rtion
(%)
0
10
20
30
40
Harmonic orderTHD 5 7 11 13 17 19
Such high levels of voltage distortion are beyond limitsof practical electricity distribution, and far exceed per-missible power quality levels. Protection equipmentwill disconnect equipment in such circumstances dueto the high currents experienced, but before such pro-tection operates capacitor life will be dramatically re-duced, cables, busbars, transformers and switchgearwill be thermally stressed, and connected equipmentsuch as control systems can malfunction or fail.
3 Avoiding resonance
This problem with harmonic resonance can be avoidedby selecting a different reactor in series with the capac-itor bank. The purpose of such reactors is specificallyto avoid tuning to any frequency where harmonic cur-rent may be present. For this reason these reactorsare termed detuning reactors and the capacitor bankas a whole is referred to as a detuned bank.
External grid
T120 MVA Dyn11
11 kV bus Skss = 181 MVA
C1, C24 Mvar at 11 kV7% reactors
L1P = 18 MW
Q = 12 MvarITHD = 6%
2
In the single line diagram the series reactors havebeen described as 7% reactors. This shorthand ter-minology infers that the reactor reactance is 7% ofthe capacitor reactance at the fundamental frequency.The resulting tuned frequency of the bank is 189 Hz —at this frequency, the reactor and capacitor have equalreactance.
Other detuning types commonly used are 6%, 5%or 14% in cases where third harmonic distortion isexpected.
A 7% reactor in this case equates to an inductanceof 7.25 mH. This is very significantly higher than the150 µH of the inrush limiting reactors. These reac-tors therefore also take up more physical space, areheavier and cost more.
The effect of the higher series inductance can be seenin the inrush transient below. It is clear that the peakinrush current is limited to less than four times thenominal current, and that the frequency of the tran-sient is much lower than in the case of inrush limitingreactors.
Cur
rent
(pu)
−4
−2
0
2
4
Time (s)0 0.5 1.0
The frequency dependent impedance of the two stepsare shown in the chart below. The natural frequency ofthe network is moved to below any of the frequencieswhere significant current harmonic distortion occurs.
One bankTwo banks
189 Hz
Impe
danc
e (Ω
)
0.1
1
10
100
Frequency (Hz)0 500 1000
It is also clear that the natural frequency is not in-fluenced by the number of capacitor steps that areconnected.
Not shown in this chart, but important to note is thefact that the detuning frequency is independent of the
system impedance — changes in network configura-tion will not influence the performance of the bankitself.
The effect of the detuned bank on harmonic distortionis indicated in the chart below. As before, the voltagedistortion with no capacitors connected is 4.3%.
No PFCOne stepTwo steps
Volta
ge d
isto
rtion
(%)
0
1
2
3
4
Harmonic orderTHD 5 7 11 13 17 19
With one step connected, the voltage distortion fallssomewhat, due to some absorption of harmonic cur-rent by the capacitor bank. The resulting voltage dis-tortion is 2.8%. With two steps connected, the voltagedistortion is slightly more than 2%.
The purpose of detuned banks is not to significantlyreduce harmonic distortion but rather to ensure thatthe capacitor bank does not resonate with the networkimpedance, irrespective of the number of connectedsteps, network configuration or load conditions.
In all cases thus far, the current distortion of the loadwas taken to be a low 6%. If the total load has alarge proportion of non-linear loads, for example inan industrial application with many variable speeddrives or rectifier loads, the current distortion will besignificantly higher, as shown in the chart below.
Low distortionHigh distortion
Cur
rent
dis
torti
on (%
)
0
5
10
15
20
Harmonic orderTHD 5 7 11 13 17 19
Given a total current distortion of 19%, the same net-work with 7% detuned capacitor banks would havevoltage distortion as indicated in the chart below.
3
No PFCOne stepTwo steps
Volta
ge d
isto
rtion
(%)
0
5
10
15
Harmonic orderTHD 5 7 11 13 17 19
Even with the reduction in voltage distortion resultingfrom the detuned bank, it is clear that the distortionlevel at the 11 kV busbar exceeds the permissiblecontribution of a single load at a point of commoncoupling. If compliance with voltage quality regulationsis required, alternative measures are required.
4 Reducing harmonic distortion
In the network below, the configuration of the capacitorbank has been modified with the specific purpose ofreducing the amount of harmonic distortion at the 11kV busbar.
External grid
T120 MVA Dyn11
11 kV bus Skss = 181 MVA
C1, C2, C35,3,2 Mvar at 11 kV4.2, 2.2, 0.87% reactors
L1P = 18 MW
Q = 12 MvarITHD = 6%
The bank is somewhat bigger and has an additionalstep. The series-connected reactors have been se-lected to tune the capacitor steps to the fifth, seventhand eleventh harmonic, as shown in the chart below.
One bankTwo banksThree banks
250 Hz
350 Hz550 HzIm
peda
nce
(Ω)
0.1
1
10
100
Frequency (Hz)0 500 1000
The network as seen from the 11 kV busbar now hasvery low impedance at these harmonic frequencies,exactly where most harmonic generation takes place.The capacitor banks will therefore by design absorbmost of the current at these frequencies.
The effect of the harmonic filter is clear from the har-monic spectra below. With all three steps connected,the voltage distortion on the 11 kV busbar is just over1%.
No PFCOne stepTwo stepsThree steps
Volta
ge d
isto
rtion
(%)
0
5
10
15
Harmonic orderTHD 5 7 11 13 17 19
5 Conclusion
This application note describes in simple terms thebenefits of installing reactors in series with capacitorbanks.
It is shown that for some applications, current limitingreactors may be all that is required for safe operationof the network and capacitor banks.
It is always necessary to understand the interactionbetween the capacitor banks and the network, and insome case detuned or even tuned harmonic filters arerequired.
Optimised Network Equipment can assist you in select-ing the correct reactor and design appropriate reactivepower compensation to suit your requirements. Findus at www.onegrid.com.au
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