480 V arcing ground fault

[magoo2]

Had a three-phase transformer fail due to an arcing fault in the customer's switchgear. The fault occurred either at or ahead of the customer's 480 V main breaker. There was no other low side protection on the transformer since it was just a short distance between the transformer and the customer's switchgear.

The high side fusing eventually operated - but only after the transformer had failed. Fusing was set at 150% of transformer FLA and plots below (and to the left of) the transformer damage curve.

Customer claims the transformer fusing should have prevented the transformer from failing. My response is that the coordination between the fuse and tranformer damage curve only applies if it were a bolted fault condition. Am I looking at this wrong?

 

[Eleceng01]

magoo2, does the fuse curve fully cover the transformer damage curve or is there a portion, as I suspect, that is not covered? Can you provide the TCC?

 

[davidbeach]

When you say the fuse plots below/left of the damage curve is that before of after the damage curve is shifted left to 57% of its 3-phase value for single phase faults (assuming a delta-wye transformer)?

 

[dpc]

It's difficult, if not impossible, to fully protect a delta-wye transformer from line-to-ground through faults using only primary fuses, especially fuse links or current-limiting fuses. Use of an expulsion-type power fuse improves the protection, but it is still tough to get under the damage curve and still allow energization of the transformer.

Also, the transformer damage curve is not an absolute. Each through-fault takes its toll, even with good transformer protection. The damage is cumulative and eventually a failure can occur.

 

[magoo2]

My apologies on the curve info. The total clear of the primary fuse and the transformer damage curve intersect at about 42% of the bolted fault condition. So at 42% of available fault current and below, you'll get heating of the fuse and damage to the transformer - I think I've answered my own question!

On the connection, it's wye wye, so no shifting is required.