Surge Capacitor Self Discharge 1

[mls1]

We have a retrofit 15 kV generator breaker with a GSU side TRV capacitor rated 200 nF. After installation we discovered that the capacitor nameplate indicates that the capacitor does NOT have an internal discharge resistor. This is the first time we've seen a medium voltage capacitor that did not include self discharge (i.e. <50 V within 5 minutes) however, we are struggling to find a requirement for it. The NEC specifically excludes surge capacitors that are part of an apparatus "conforming to the requirements of the apparatus". The breaker supplier is balking at the requirement and, though it is standard practice to ground MV systems before they are touched, it does not seem like this is acceptable when there are no automatic means of grounding or installed grounding switches. Is there a requirement we are missing? The supplier will not correct this if we cannot quote a specific requirement.

 

[electricpete]

> This is the first time we've seen a medium voltage capacitor that did not include self discharge (i.e. <50 V within 5 minutes

Among the the rotating equipment surge capacitors at our plant, none of them have discharge resistors. We have many in the same voltage and capacitance range as yours, ours are 0.25 microfarads, 15kv rating (used on 13.2kv system), no discharge resistor.

I'm not familiar with requirements for other capacitors like your breaker capacitor. Maybe it's not as critical for rotating equipment surge capacitors to have bleed resistor, since rotating equipment surge caps can bleed through the connected machine insulation resistance (we could even calculate RC time constant 200 nF, 1Gohm gives 200 sec, although I wouldn't rely on that... as you said electrical safety ground must be hung prior to contact).




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(2B)+(2B)' ?

 

[mls1]

Thanks for the reply. Machines are a special exclusion because it is expected that the capacitor will discharge through them. For example, the NEC excludes self discharge on power factor caps when they are installed with the motor T-leads but not when they are installed on the high side of the contactor. These caps are installed on the GSU side but the GSU is delta connected so the best we could expect is that they would equalize in voltage but not necessarily fully discharge. Yes, they do ground the system but that seems like a secondary protection. For example, if they are planning on power factor testing the GSU they might just tick trace it (wont' detect DC) and then go about testing since they cant' test with grounds. I agree, however, that there is relatively little charge on a 200 nF capacitor and the transformer resistance to ground will probably do most of the work of discharging them. The rest of the system is Isophase bus so that probably will not help.