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Switchgear for capacitor

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#1 hkharmonic


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Posted 22 May 2006 - 03:30 PM

Hi, I have a question and want seeking for your advise.

For sizing the correct switchgear's rating for the "general & detuned" type capacitors, do we need to reserve extra rating for the detuned one ?

For example, we have two 3 phase capacitors (general & detuned) with the same capacity of 50KVar. How's the rating of switchgears we have to select ? Is there any difference in sizing the switchgear for detuned and general type capacitors ?

Switchgear for general capacitor = ( 50k / (sqrt(3)*380) ) * 1.35 .............. Correct ??

Switchgear for detuned capacitor = ???

Thank you in advance.


#2 marke


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Posted 22 May 2006 - 06:55 PM

Hello HKHarmonic.

If you use the same capacitor i.e. 50KVAR with a detuning reactor, then the effective KVAR will be reduced. Commonly, 7% detuning reactors are used. This reduces the effective KVAR by 7% so your 50KVAR becomes 46.5KVAR when used with a detuning reactor.

The other effect of the detuning reactor is to reduce the inrush curent into the capacitor at switch on. This will put less stress on the contactors used. I wold still use the same switchgear however and use the manufacturers suggested ratings for switching capacitors.

Best regards,

#3 hkharmonic


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Posted 23 May 2006 - 03:27 PM

Hello Marke.

Thanks for your advise. Actually, the rating of 50KVar is the total reactive power for the capacitor after adding the detuned reactor. The manufacturer catalogue suggest the detuned capacitor bank should be used with a larger rating of switchgear. So, I am curiosity for why we have to pay for it ?

I think it may due to the system's harmonic and capacitor's detuning frequency.(i.e. 189 Hz) Is it right ?

I can realize for the reduction of inrush current as the impedance of capacitor increases as well. But, in this point of view, the switchgear for the detuned capacitor should be used with a smaller rating than the general one. Agree?



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