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.
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.
