Effect of Capacitor Banks on ground fault current 1

[rex2704]

I am working on a substation design using SKM power tools software (version 6.0). While working with the software, I have noticed that delta connected capacitor banks used for power factor correction give rise to ground fault currents in an ungrounded delta connected system.

I had used a delta/delta transformer to feed an MCC with motor and non-motor loads along with PFC capacitors. I had used delta connection for all my loads as well as for the capacitors. A 300kVAR capacitor bank on a 480V bus gave me a SLG fault current of 2500A. When I disconnect the capacitors from the system, I get zero ground fault current which is correct.

I do not know how to represent capacitance in the positive, negative and zero sequence impedance diagrams for SLG fault current calculation. So I am not able to understand this phenomena at all.

I would appreciate every help to explain this behavior to me.

Thank you every one in advance!

 

[davidbeach]

Sounds like you should probably contact SKM directly. I've always found them to be very helpful.

 

[rex2704]

Thanks davidbeach for replying to me.

Well, I did contact SKM directly. They told me that they include the capacitive reactance of the capacitor bank in positive, negative and zero sequence networks in series.

In the software, when I had entered the kVAR rating of the capacitor, I had found that the software came up with identical values for positive and zero sequence capacitance which explains why I am getting a large ground fault current with an ungrounded system. But this is strange for me because I was under the impression that the zero sequence capacitance of an ungrounded capacitor bank would be negligible. So, it would provide infinite impedance to the ground fault current.

When I change the zero sequence capacitance value to zero in the software, I get negligible ground fault current.

So, now I need to confirm that the zero sequence capacitance of an ungrounded capacitor bank is indeed negligible (SKM did not confirm this to me). This assumption appears obvious to me since there is no direct connection to earth.

Will anyone from this expert Engineering Community please confirm my assumption???

Thank you everyone in advance!

 

[DanDel]

rex2704, I modeled your Delta connected 300kVAR capacitor in my SKM, and it is the same, positive and negative capacitance of 1151.29443 microfarads. This is not accurate for an ungrounded system, even with the capacitive reactance included.
Using Wye or Wye-ground provides 3.4539e+003 microfarads for both positive and zero sequence capacitance. This should only be possible for Wye-ground, as an ungrounded wye system should be virtually the same as the delta system for zero-sequence.
This appears to be an error in the SKM model. Now I'm not even sure that the positive sequence values are correct.