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zig zag grounding transformer sizing

zig zag grounding transformer sizing

zig zag grounding transformer sizing

(OP)
how are zig zag grounding transformers sized? I have a 3 mW generator parralleling with a utility feeder through a delta xformer and I need to solidly ground the high voltage side. the utility feeder is 12 kV and available fault current is 4000 Amps. Thanks Hank

RE: zig zag grounding transformer sizing

A zig-zag grounding bank is typically used to provide a source of fault current so a phase to ground fault on a delta system can be detected and interrupted.  The bank is sized based on the neutral current and the phase to ground voltage multiplied by a constant "K" which is determined by the length of time required for the relay to open the breaker.  The phase to neutral voltage on a 12 kV system is 7.2kV (12/1.732).  Assuming the phase to ground fault current to be the 4000 amps you mentioned the three phase kva rating of the zig-zag bank is determined as follows:

(7.2)(4000)(K) = kVA

K is determined from the following table:
       time          K
      10 sec       0.064
       1 min       0.104
       2 min       0.139
       3 min       0.170
       4 min       0.196
       5 min       0.220

If your generator is paralleling a three-phase, four wire distribution system (three phases and a grounded neutral), you may want to ignore the above and size the bank to handle the maximum unbalance current you are willing to allow on you generator.  Use the minimum pickup on your ground relay times 7.2 kV as the per phase rating of the zig-zag bank.

This information is taken form the Westinghouse "Electrical Transmission and Distribution Reference Book" published in 1964.

RE: zig zag grounding transformer sizing

6/30/00
There appears to be initial average winding temperature limitations involved, e.g. less than 75 deg C. It is probably better to check with the transformer manufacturer before the application/purchase of the zig-zag grounding transformer is completed, just in case.

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