Delta Connected Capacitor Bank

[TestBeforeTouch]

I am installing an electronic trip unit on a 480 volt Circuit Breaker that is protecting a 240 KVAR capacitor bank. The cap bank is made up of 12 modules. Each 20KVAR module consists of 3 delta connected capacitors (each cap is 6.66 KVAR). The substation is a typical 1500KVA solidly grounded industrial substation.

Is there any guidance on what my short time delay and instantaneous settings should be to avoid a false trip when the bank is energized?
Also, what amount of current unbalance would you accept before you trip the breaker?

 

[davidbeach]

I'd look to the capacitor manufacturer for guidance in your position. Calculation of back-to-back capacitor switching transients is non-trivial without access to EMTP type software. Hopefully the capacitor manufacturer has done enough studies to have a ready answer to your question.

 

[TestBeforeTouch]

Unfortunatley, the capacitor bank was installed around 1960. So it is unlikely I will get any technical support from the manufacturer. The caps were replaced about 15 years ago to get rid of PCB's. My job is to replace the old oil filled dashpot breaker with a new breaker.

 

[davidbeach]

In that case I'd try to duplicate the existing settings into the new breaker. Depending on how the cap bank is constructed the response to step switching will differ, and the first step is not necessarily the worst. If you have setting that work, use them. If you want to experiment, get a good recording ammeter and put it on the cap bank for a while and see what you get.