power factor capacitors 1

[bigheadted]

power factor correction capacitors are rated in kvar
what is the formula that relates this to their value in farads
1/2.pi.fc gives xc but how is this related to kvar?

 

[mpparent]

1/(2*pi*f*C) = kV^2*1000/kVAR

Mike

 

[rbulsara]

mparent:

Is multiplier of 1000 correct?

If you have a 3 phase kVAR values, you need to use per phase voltage and divide the 3-phase KVAR value by 3 for per phase KVAR.

Or in other words:

Per phase Xc=Vph/Iph

Iph=KVAR/kV , use per phase values.

Xc= kV/(KVAR/kV)= kV^2/KVAR.

This turns out to be the same if you use 3 phase KVAR value and line to line voltage, as 3's cancel out.

Am I missing something?

 

[mpparent]

Sorry..

My bad...you can either account it to a typo or stupidity.

xc=kV^2*1000/MVAR=kV^2/kVAR


Mike

 

[rbulsara]

mparent:

Thanks. We will go with the typo!!!
Cheers!

 

[mpparent]

Thanks for that. LOL.

 

[jghrist]

Per phase Xc=Vph/Iph

Iph=KVAR/kV , use per phase values.

Xc= kV/(KVAR/kV)= kV^2/KVAR.

Iph is in Amps (k's cancel), then you went from Volts/Amps in the Xc=Vph/Iph to kV/Amps in the Xc=kV/(kVAR/kV). Your first effort was correct.

Xc = V/(kVAR/kV) = kV·1000/(kVAR/kV) = kV^2·1000/kVAR = kV^2/MVAR

 

[mpparent]

Doh!

Mike

 

[rbulsara]

jghrist..

thanks..I suspected i missed something..

Sorry , mparent and all...for creating confusion..but still worth the excercise..

 

[bigheadted]

ok lads thanks for the help
now the three capacitors are around 450uF each, connected in delta and rated at 48 KVAr
this doesnt work out unless i multiply the calculated KVAR per phase by root 3 is this right what is the formula for capacitance across each phase in a delta connection?
sorry to be a pain id like to get my head around it though

 

[waross]

When you calculate the current taken by each single phase bank of capacitors, This is the phase current. With a load from "A" phase to "B" phase and a load from "B" phase to "C" phase the two currents are out of phase and have to be added vectorilly. If the currents are equal on each phase the line current will be phase current times root 3. as you have already observed a discrepancy of root 3, it looks like your on the right track.
Would you like an example?
A 3 phase 48 KVAR capacitor bank is connected delta on a 480 volt circuit. The load on each phase is 48 KVAR / 3 = 16 KVAR.
16 KVAR /480 = 33.3 amps.
The load on each line is 33.3 amps x root 3 = 57.7 amps.

When the capacitors are connected in wye (This will be a different set of capacitors with considerably more micro farads because of the reduced voltage )
Now 48 KVAR / 3 = 16 KVAR per phase.
16 KVAR / 277 = 57.8 amps. (In this case, the root 3 factor was used to convert 480 volts to 277 volts.)
We have about 0.17% rounding error. 277 should be 277.128
Hope this helps.
yours