Reducing Cpacity of De-tuned PFC Bank
Reducing Cpacity of De-tuned PFC Bank
(OP)
We have a 1000kVAR de-tuned 11kV PFC bank and want to de-rate it to 400kVAr. It consists of air cored reactors in series with five capacitors. It was not designed to correct for specific harmonics. Is there any reason why we cannot simply disconnect three of the capacitors?






RE: Reducing Cpacity of De-tuned PFC Bank
RE: Reducing Cpacity of De-tuned PFC Bank
RE: Reducing Cpacity of De-tuned PFC Bank
You need to worry about a high impedance of the filter in parallel with the utility system. There will be a parallel resonant point with the system at a frequency somewhat below the tuning frequency of the filter. You need to make sure that this resonant point does not coincide with a large harmonic current source. Currents at the parallel resonant point can cause a high harmonic voltage.
RE: Reducing Cpacity of De-tuned PFC Bank
h= Square root (MVAsc / MVARcap)
h is the harmonic order for the parallel resonance of your caps and your system.
MVAsc is the short circuit MVA at the bus where the caps are connected and MVARcap is the capacitor MVAR rating at the operating voltage. If h is near a frequency that might be present in your system, you could have a problem.
By reducing the capacitors, you are raising the harmonic frequency. If it happens to land on the 11th harmonic and you have some VFD's producing 11th harmonic currents, you need to investigate further.
RE: Reducing Cpacity of De-tuned PFC Bank
RE: Reducing Cpacity of De-tuned PFC Bank
RE: Reducing Cpacity of De-tuned PFC Bank
RE: Reducing Cpacity of De-tuned PFC Bank
RE: Reducing Cpacity of De-tuned PFC Bank
RE: Reducing Cpacity of De-tuned PFC Bank
If the reactor is bypassed, you will know the "tuning point". The natural frequency of the capacitor bank will be determined by the reactance (inductance) up to the capacitor bank and the capacitance of the capacitor bank itself. I don't think bypassing the reactor is a big deal because we would normally not have one installed on banks of this size unless there was an identifiable source of harmonics present and none is mentioned, but you seem to think it will cause the capacitors to smoke. Why do you think it will smoke?
GerH,
Seems like an unbalanced capacitor bank to start with. 1000 kvar or 400 kvar is not a balanced rating. What capacitor can size are you using? 200 kvar?
RE: Reducing Cpacity of De-tuned PFC Bank
Alan