Automatic Power Factor Correction Faults

[avogel]

This is an original unit circa 1980s. The load has not changed in this time period. So far we have experienced 4 disturbances, each which has popped up after a few months of running great.

The unit is connected to a 4160V circuit with several 6-pulse DC rectifiers. There are 5 steps, each bank is a 300 KVAR delta configuration (effective 708 KVAR / bank). Each bank has 2.9mH reactors for harmonic filtering. Banks protected with fusing and controlled via type CSD oil filled switches.

Looking at the tracking on the wall adjacent to the horizontal bus it looks like the disturbances seem to involve all 3 phases, each time taking out the main 800E fusing. On the bank that was pulled in during the fault, the conductors and cap insulators blew apart.

We found that someone before us had jumpered out the out of balance relay. So the first fault we chalked up to a cap getting a shorted series and then creating an out-of –balance and escalating from there. But we replaced the relay and faulty caps and checked the ESR of each to ensure a proper balance. So after 3 faults within a year, I doubt it is just aging caps.

One of my later suspicions was over voltage due to switching the caps and getting multiple re-ignitions. But we checked the contacts of the switches and saw no pitting or signs of arcing and the oil was clean. And this unit has operated over 25 years without this problem.

I was hoping someone with more experience with PFC banks them can give some insight. There is no surge arrestor on the unit, should there be? Even if we do this, we still have not found the phenomena generating the overvoltages.

 

[REM76]

It could be a harmonic resonance issue where the LC combination of the PFC bank is sinking all the fifth harmonic coming from the drives (5th and 7th harmonics are normally produced by six pulse drives). Usually the reactor is added to detune the PFC to sink something close to the fifth harmonic like 4.7th for example.

I would calculate the resonance frequency of the circuit and see if it matches any of the common harmonics like the 5th, 7th, 11th, 13th...

 

[Marmite]

Shunt capacitors operate continuously at full load. Overstressing and overheating shorten the life of the capacitor, and therefore the operating conditions need to be strictly controlled. Don't rule out the simple explanation. The unit has operated for more than 20 years with no change to the load, so it is unlikely that overvoltage transients have suddenly become a problem. The likely explanation is that one of the capacitors shorted, causing operation of the out of balance relay. Someone then defeated the protection, but the fault will have caused changes to the voltage and current distribution in the bank. The long term overload current is likely to have damaged the remaining capacitors in the bank which are now failing. Element failure is most likely to occur when the voltage across the capacitor is high. The effects may have been exacerbated by the inductances in series with the capacitors causing a voltage rise at the capacitor terminals. Also if the voltage rise at light load is increased by capacitors this can cause saturation of transformer cores and generation of harmonics of abnormal magnitude, which would again exacerbate the problem.
Regards
Marmite