zig zag grounding transformer sizing

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

 

[Gibbs]

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.&nbsp;&nbsp;The bank is sized based on the neutral current and the phase to ground voltage multiplied by a constant &quot;K&quot; which is determined by the length of time required for the relay to open the breaker.&nbsp;&nbsp;The phase to neutral voltage on a 12 kV system is 7.2kV (12/1.732).&nbsp;&nbsp;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:<br><br>(7.2)(4000)(K) = kVA<br><br>K is determined from the following table:<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;time&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;K<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;10 sec&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;0.064<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;1 min&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;0.104<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;2 min&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;0.139<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;3 min&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;0.170<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;4 min&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;0.196<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;5 min&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;0.220<br><br>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.&nbsp;&nbsp;Use the minimum pickup on your ground relay times 7.2 kV as the per phase rating of the zig-zag bank.<br><br>This information is taken form the Westinghouse &quot;Electrical Transmission and Distribution Reference Book&quot; published in 1964.

 

[bartos]

6/30/00<br>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.