%Vrise on bus due to capacitor installation
%Vrise on bus due to capacitor installation
(OP)
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!
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






RE: %Vrise on bus due to capacitor installation
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%
RE: %Vrise on bus due to capacitor installation
"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
RE: %Vrise on bus due to capacitor installation
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
RE: %Vrise on bus due to capacitor installation
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.
RE: %Vrise on bus due to capacitor installation
(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
RE: %Vrise on bus due to capacitor installation
"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