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filtre ckt for harmonics


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hi friends


We have a filter circuit in order to reduce the harmonic current and improve the pf

Rating is: 600 MVA


Specification of air-cooled inductor:

Rated v= 11kv, rated L = 8.65 mH, rated I = 334A, rated f = 50HZ, harmonic A = 40A harmonic f = 150Hz,


Specification of capacitor:

Rated O/p = 500KVAR, rated V = 8.28KV, rated C = 23.2 micro F, rated I = 60A, rated f = 50Hz


The connection is such that with one coil (inductor) we have 6-cap bank (i.e. with 3 inductor we have 18 cap, 6 cap / inductor) in double star configuration.


I have find out the tuned frequency as


f filter = 1 / 2 *pi * root LC = ½ *3.14 * root (8.56 m H * (6*23.2 micro F)) = 145 Hz


1. We have fundamental frequency of 50 Hz, and hence the 5th harmonic frequency is 150 HZ. Why the harmonic frequency I have calculated differ from the 5ht harmonic i.e 150HZ, is there some correction is required?


2. Also the said filter ckt is connect to 30 MVA transformer, 132/11KV, %Z= 10


The inductance of transform is therefore

L Transformer = L total – L filter


L total = (1/2* pi *f)^2 * 1/ c

L total = (1/2* pi *50)^2 * 1/ 6*23.2 micro F = 72.7 mH


L transformer = 72.7 mH – 8.65mH = 64.05 mH (Is this correct?)


3 Is the 5th harmonic current is higher than the other harmonic current i.e. 7ht and other higher harmonics.



4. Why should we connect the capacitor in double star instead of delta connection? Is relation? (C delta = 3 C star)



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Hello tahira


This is quite a complex question and I believe that we would really need to see the complete circuit to comment, however 150Hz is the third harmonic, not the 5th. The 5th harmonic is 250Hz on a 50Hz system, or 300Hz on a 60Hz system.


It is not usually easy to calculate the filter frequencies, as you normally need to take into account the reactance and impedance of the circuit to which the filter is connected. Perhaps you should talk to a good consultant on this one.


Best regards,

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