%Vrise on bus due to capacitor installation

[VTer]

Hi All,
I am thinking about installing PF correction Caps on 480V bus and I would like to confirm the voltage raise. I could not find any reference on doing these calculations based on available SC current and X/R at the bus. I have found some estimates that base the Vraise on the transformer size and impedance but I would like to know if there are standards or equations that include the entire source impedance for a more accurate calculation. I would love to see if anyone can verify the calculations below to see if the below method is appropriate for V rise calculation.

Known: Isc = 24,305A with X/R = 3.54, 480V 3-ph, 4-W Bus with proposed PF correction capacitor size = 150kVAR

Calculations:
Zsys = 480V/24,305A = 0.0197ohms
Xsys = 0.0197*sin[(tan^-1)3.54)] = 0.0190ohms
C = (150*1000)/[(480^2)*2*3.14*60] = 0.0017F
Xc = 1/(2*3.14*60*0.0017) = 1.536ohms
%Vrise = (0.0190/1.536)*100 = 1.24%

Your input and comments are greatly appreciated! Thanks!

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[DistCoop]

I use a similar method.

Zsys = 480V/24,305A = 0.0197ohms
Xsys = 0.0197*sin[(tan^-1)3.54)] = 0.0190ohms
Ssys = 480V^2/Xsys = 12.13MVA
%Vrise = 150kVAR/12.13MVA = 1.24%

 

[VTer]

Thank you DistCoop!

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[VTer]

Hi DistCoop,

I think I goofed up, the base impedance should be Zsys = 277V/24,305A = 0.0114ohms.

This puts the voltage raise at 0.71%.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[DistCoop]

Ahh, okay.

When documenting your equations, you may also note that you're using a LL current. Convention is LN... it confused me for a few seconds when I couldn't get my numbers to match yours.

More nit picky than anything, but could save confusion.

 

[Marmite]

There is a formula in IEC60871-1 Shunt Capacitors For AC systems which states that:-

(Delta U)/U approximates to Q/S.
Q is bank reactive power
S is short circuit level at point of connection
U is voltage before cap bank is connected

This gives a similar result.

Regards
Marmite

 

[VTer]

DistCoop and Marmite, thank you very much for your input. It is greatly appreciated.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla