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Capacitive charging between windings of a transformer

Capacitive charging between windings of a transformer

Capacitive charging between windings of a transformer

(OP)
Our Utility recently had a 12.5kv to 240/120 volt (connected delta primary with an ungrounded common, 4w delta secondary) transformer bank blow a fuse on one of the "high leg" transformers. When re-fused, it blew again.

Standard procedure at the Utility calls for isolating the tranformer from the load and other transformers, then refusing once more to ascertain whether the problem is transformer or load related.

When this was done, the fuse stayed in. The lineman went up in the bucket to check voltage. Since this was the "high leg" xfmr., the ground strap to the X2 bushing had been removed, and, since the wires had been disconnected at this point, all three load side bushings were un-attached. When the lineman put his voltmeter from the case of the xfrm. (which was grounded) to one of the low side bushings, it arced violently enough to cause minor molten metal burns. The primary fuse blew and imbedded the link in the linemans' arm.

We have not tested the xfmr. yet (this happened yesterday) and we are assuming a winding to winding failure at this point. However, it did bring up this question (yes, I'm finally getting to the question).

I have seen the secondary of a 69 kv to 120v PT go to several thousand volts when all reference to ground was removed from the secondary. It was explained as capacitive coupling between the high and low windings.

Given the scenario that the xfmr in question had no primary ground reference (the high side was a floating delta and the other two fuses were open), and the low side had also been isolated, could a significant enough capacitive effect build up to cause this accident. Since this was only a 50 kva xfmr., my first thought is no, but it does make me want to examine our check-out procedures. Any thoughts?

RE: Capacitive charging between windings of a transformer

If the secondary is unreferenced to earth then there is a possibility that you can have a high voltage induced capacitively to earth - this would be without much 'omph' behind it as the relative impedance would be pretty high limiting any current toa low value.

The case that you describe here I would reckon involve a possible interwinding failure. It sounds like when he went to measure the voltage using a regular lv voltmeter he went across the primary somehow. Lucky guy hope he is alright.

I hate ungrounded systems....

Rugged

RE: Capacitive charging between windings of a transformer

Please explain how closing one fuse to a delta connected transformer bank proves anything. One side of two primary windings is energized with no return path (assuming no fault path to ground). Sounds like a primary to secondary fault which only had a path to ground when grounded loads were connected, or a grounded meter.  

RE: Capacitive charging between windings of a transformer

(OP)
I couldn't agree more. Obviously our "standard" procedure needs some revision.

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