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Load current unbalance cause of trip?

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sdugre

Electrical
Sep 9, 2005
7
We had an interesting trip the other day. A squirrel got across a normally open tie air break switch between two 13.8kV feeders. Both station breakers tripped correctly, but a downstream recloser on one of the circuits also tripped on ground for some reason. There is no known ground source beyond that recloser. I attached the fault record from the recloser, which also shows the one-line. One thing I notice is that there is a fair amount of load current unbalance on that circuit. Do you think that could have accounted for the ground current shown in the fault record? Unfortunately we don't have PTs on that recloser so I don't know what the voltages looked like.

Thanks,
Sean
 
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You have high current in all three phases and higher neutral current. It looks like you have a grounded wye to delta bank somewhere downstream of the recloser that is acting as a grounding transformer. If this were the case and there was no load current, all three phases would be equal and in phase (zero-sequence). It looks like you have a combination load plus zero-sequence current from a grounding transformer.
 
There is a 3-50kVA, 13.8 kV wye / 600 V delta connected bank on that circuit, but it is before the recloser. The print shows no ground sources beyond the recloser, but I suppose that could be wrong. I'll have to make some field checks

If you're curious, attached is a fault record from another recloser which had a 3-167kVA, 13.8 kV wye / 4.8 kV delta connected bank downstream from it. The fault was before the recloser in that case as well.
 
 http://files.engineering.com/getfile.aspx?folder=a18fcea1-b274-4315-a682-2d597eea0568&file=Acrobat_Document.pdf
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