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Sorry people, I went back a looked at my notes and the transformers were wye-wye grounded not delta-wye. I'm fairly sure that it did not involve ground. The transmission lines also supply houses and no issues with single phase transformers were reported.

They didn't have surge arresters on the primaries, the transformers had to be replaced... 2 were catastrophic.

I don't see where you can edit a post so I will post twice... Edit: The transformers are all pad mount, Delta/Wye(Grounded). The service from the utility overhead until the utility pole and then is routed underground to the transformers.

12.7kV/480V ~200KVA They were all within .25 miles of each other. A reported phase to phase fault occured the night that the transformers were damaged. I checked strikenet and no apparent lighting in the area. The clearing breakers upstream from the fault tripped and then attempted a reset...

dpc, Thanks for the reply.... The fault was upstream about 1/2 mile. The transformers were at the end of the line so to speak as they were step down transformers that serviced buildings.

I am looking into an issue where several Delta / Wye(Grounded) transformers where damaged due to a line to line fault upstream from the transformers. The transformers are step downs for businesses. Can someone explain why the primary fault protection fuses would not protect the transformers...