Had a 10yr old 600 volt 10kvar capacitor bank short and shut down both a 2000 ton chiller plant plus a 1400hp boiler plant(critical motors for both are fed from the same MCC).So far Ive found a total of 4 caps open and 1 shorted of the 9 on this MCC.Also I'm finding power factors of .8 to 8.4 once the caps are removed from circuit.I thought that 8.4 was pretty good and checked with an EE Apparently 8.5 is the factory cut off point.These are 30HP to 50HP motors. What is the life expectancy of the typical power factor capacitor?I've seen many of them in service for 15yrs and never a problem? Now i have to do my own power factor correction to make sure it's not over corrected.
I can not answer you question on capacitor life but I can assist in correcting the PF. 0.85 is not good if the customer is billed on KVA demand. You should try to get the PF to 0.95 to 0.99. You can check with the local utility and see if they can tell you the max KW and KVA demand. Did the capacitor bank trip the chiller breakers? Are the caps connected to the chiller compressors or motors? I would think the the mfg could tell you what size to go back with. Are you considering adding additional cap to improve the PF? IF you happen to overcorrect a little nothing bad will happen. The utility will appreciate it. See if you can get the max KW and KVA demands.
[This message has been edited by Bob (edited 06-23-2005).]
Re: power factor capacitors#53409 06/23/0511:36 PM06/23/0511:36 PM
Bob, The caps are wired static connection on the load side of the starters before the O\Ls.The chillers are 4160v and were screaming The draft towers,condenser,chilled and secondary water pumps are fed from a 600v MCC. A secondary water pump cap shorted and tripped the distribution breaker instead of the main on the MCC.So much for the selective coordination.I wonder why so many of the capacitors on this on Mcc have failed?Have you seen this happen? .9 is the max pf allowed by the utility and one concern i have is high transient voltages in the motor circuit from an over corrected power factor.At what point does this occur?I took a look at the tables and the capacitors look about the right size but the tables are too general and every cap is supposed to be tailored to the individual motor. thanks