Capacitive current contribution - calculation

[nkn5]

Hello,

Can you share your thoughts on calculation of capacitive current calculation in a industrial electrical system.

I see a recommendation in one of our reference documents says like this
"capacitive contribution to the earth fault on the part of the 11 kV cables shall be less than the resistive ground current in order to avoid hazardous overvoltage"

This is when we have HRG limiting the alternator e/f current to ~5 amps.

Can ETAP do this calculation anywhere?

Thanks for your responses in advance

 

[dpc]

My thoughts are that obtaining the necessary data to calculate the capacitve current will be very challenging, based on my past experience. You'll likely have to use typical values for transformers, generators, etc since that data is not provided on test reports.

Cable capacitance may be modeled in ETAP - depends on their cable model. I doubt that they actually calculate the leakage current on an ungrounded system, but you could get the cable data for your hand calculation.

Any result over about 10 amps is highly suspect.

 

[jghrist]

I-Gard has an Application Guide "Ground Fault Protection on Ungrounded and High Resistance Grounded Systems" that has an appendix on calculating system capacitance. See

http://www.i-gard.com/index.php?q=techlib:Default/Login

 

[LiteYear]

You'll likely have to use typical values for transformers, generators, etc since that data is not provided on test reports.

Am I wrong in thinking that transformers/generators contribute negligible capacitance? I would have thought the largest contributor would be power factor correction banks followed by cables, unless there's some other unusual loads on the network.

PF banks will have their capacitance specified and cables can be calculated from cable data, as long as you have lengths and types. Are there other sources I'm missing?

 

[davidbeach]

I think that in an industrial electrical system you will find that all the components have a negligible capacitance, but that the cables will have a slightly less negligible capacitance. When you reenergize following the next plant outage, make sure that every load is turned off and measure the system charging current. You'll get a much better number that way than any calculation will provide.

 

[dpc]

Well, I guess you can ignore everything and assume the current is zero, but that doesn't help you much. Transformer capacitance is fairly negligible for most power system calculations, but probably not this one, unfortunately. If you have a generator, it will have a fairly high capacitance - especially if fitted with surge capacitors.