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Kvar Ratings And Voltage


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#1 kens

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Posted 23 July 2014 - 01:21 AM

Hi all, its been a while.

 

I am coming across instances at the moment where clients are obtaining prices for automatic power factor correction units and they are receiving quotations which have capacitor ratings based on the wrong voltage, i.e. 440V or 415V ratings for 400V systems. Can someone tell me a simple method to calculate the actual rating (in kVAr) of a given capacitor at different voltages?

 

Cheers

 

Ken 


An expert is one who knows more and more about less and less until he knows absolutely everything about nothing

#2 marke

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Posted 23 July 2014 - 09:12 AM

Hi Ken

 

Yes, that is a major issue, too many experts who's expertise is collecting orders rather than solving problems.

 

For a given capacitor on a given frequency, the KVAR varies with the square of the voltage.

i.e. the 400 volt KVAR rating is equal to the 440 volt rating divided by 1.1 squared  = 440V rating / 1.21

 

The other issue is that if you have a capacitor of a given rating at 400 volts, and then add a detuning reactor in series, the voltage across the capacitor will climb considerably and so the net KVAR will increase also.

You need to match the reactor to the capacitor, (the tuned frequency is critical) and the capacitor needs to be rated for at least 525V AC continuous operation.

 

Best regards,

Mark.



#3 kens

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Posted 25 July 2014 - 12:10 AM

Thanks Mark, out of interest how much higher can the voltage be after the reactors? It is usually hard to gain access to suitable measuring points.

 

Cheers


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#4 marke

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Posted 27 July 2014 - 07:16 PM

Hi Ken

 

It depends on the current flowing through them which is of course harmonic dependent.






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