NCEES #131-Electrical Power Sample Exam

[cbinla]

I'm having a hard time finding explanations for this problem in any of my references. Can anyone help me derive where the relationship (208V/240V)X110kvar comes from to find the relative kvar rating of a capacitor operating at a voltage different from it's rated voltage?

Thank you

 

[DK PE]

I'm having a hard time finding explanations for this problem in any of my references. Can anyone help me derive where the relationship (208V/240V)X110kvar comes from to find the relative kvar rating of a capacitor operating at a voltage different from it's rated voltage?Thank you

Sorry I don't have the NCEES problem book but think back to basics... capacitors are really rated in Farads, not KVAR. As the voltage across a capacitor changes, the capacitance stays fixed (ideally anyway) as does the capacitive reactance but the KVAR changes. For example, find out the value of capacitance you would need to have with 208V across it to have 110kVAR. Then take this value of capacitor and hook it across a 240V source.

P.S. I think you missed typing a "squared" or ^2 on voltage part of equation

In case Flyer's explanation is clearer from when he posted quite a while back, here it is...

Problem 131:
A capacitor is a passive component with an impedance value of Z.

Power is proportional to V2/Z

Changing the applied voltage does not affect the impedance.

VAR1= V12/Z

VAR2= V22/Z

VAR2 = (V2/V1)2

Hope this helps.

 

[cbinla]

I'm having a hard time finding explanations for this problem in any of my references. Can anyone help me derive where the relationship (208V/240V)X110kvar comes from to find the relative kvar rating of a capacitor operating at a voltage different from it's rated voltage?Thank you

Sorry I don't have the NCEES problem book but think back to basics... capacitors are really rated in Farads, not KVAR. As the voltage across a capacitor changes, the capacitance stays fixed (ideally anyway) as does the capacitive reactance but the KVAR changes. For example, find out the value of capacitance you would need to have with 208V across it to have 110kVAR. Then take this value of capacitor and hook it across a 240V source.

P.S. I think you missed typing a "squared" or ^2 on voltage part of equation

In case Flyer's explanation is clearer from when he posted quite a while back, here it is...

Problem 131:
A capacitor is a passive component with an impedance value of Z.

Power is proportional to V2/Z

Changing the applied voltage does not affect the impedance.

VAR1= V12/Z

VAR2= V22/Z

VAR2 = (V2/V1)2

Hope this helps.

Yes I missed the square. Also I'm learning not to study too late at night because I forget the basics.

Thanks for your help.