# Derating Calculation of Capacitor Bank

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### #1 Guest__*

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Posted 22 September 2005 - 03:29 PM

Hi, do you know how to calculate the derating of a capcitor bank? If a 440V 100kVar capacitor bank installed in a 380V electrical system ? How much of the capacitor bank will be derated?

Is it right if I calculate like that 100k x 380 / 440 ?

=86.363kVar ??

Stephen

### #2 marke

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Posted 22 September 2005 - 06:43 PM

Hello Stephen

The capacitor provides an impedance that is frequency dependant. If you change the frequency, then the impedance changes and this will affect the current. If the frequency does not change, then the impedance is constant and the current will reduce in proportion to the voltage reduction.
As KVAR is a product of both voltage and current, the KVAR wil reduce by the voltage reduction squared.

Best regards,

### #3 Guest__*

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Posted 23 September 2005 - 04:07 PM

Dear Marke,

Thanks. For the above example,

= 100kVar - (440-380) (440-380))
= 100kVar - 3.6kVar
= 96.4kVar

Right?

Stephen

### #4 marke

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Posted 23 September 2005 - 08:46 PM

Hello Steven

Assuming both ratings are at 50Hz,

KVAR = 100 x (380/440) x (380/440)

Best regards,

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