Ground Fault on Ungrounded Secondary Delta transformer

[pwyane]

We found two ground faults (960V and 615V) on our Y-delta 4160/480Volt transformer. I have never seen faults of this magnitude on this system. Does any one have any idea of what could cause such large faults besides voltage transients in the line?

 

[hmchi]

We have a communication problem.

A single phase to ground fault on an ungrounded system does not have the same ramification as a ground fault in a grounded system. The fault current is almost negligible thus circuit interruption is mostly deemed unnecessary, and most operators of ungrounded system will install a detection system so that it can be found and removed without service interruption.

You described the fault condition in terms of voltages, which is different from most 'concerns' about faults or short circuits, where currents cause damages. You may want to explain yourselves again.

When s ungrounded delta system has a single phase to ground fault, the other 2 phase voltage with respect to ground rise to the full line to line voltage.

 

[busbar]

pwyane, lacking information, it is hard to understand the nature of your problem. Can you describe exactly how the two odd voltages were obtained? In what manner is the 480V systems grounded? Assuming the reported measurements are from the 480V system, it is likely there is severe neutral inversion, like described in Chapter 6 of Donald Beeman’s 1955 Industrial Power Systems Handbook.

Search this site using keywords “ ungrounded system “ or “ ungrounded distribution “ for a lot of related discussion.

A short excerpt of the Beeman text is

http://64.146.180.232/dat/beemaIPSH6z.doc