Shorted secondary cables on 300kVA Padmount transformer.

[11bgrunt]

New install. Parallel runs on the 120/208Y secondaries were ohmed out and connected at the secondary bushings of the 300kVA 3 phase deadfront padmount transformer. Customer was responsible for landing secondaries at his switchgear. Transformer was energized with pole mounted overhead fuse disconnects. Two fuses blew and the substation breaker operated with 2500 amps overcurrent trip on two phases. The high side is 14.4/24.9Y. Substation is 1/2 mile from this customer. Two conductors on the secondary at switchgear were crossed and there was a short circuit on two secondary phases.
I contend that the secondary fault could never create the high side amps recorded and asked for a TTR test. TTR came back within limits.
Does anyone believe the estimated 15K infinite buss fault current could have caused the 2500 amp fault on the high side? Estimated transformer impedance 5%. No customer load on the transformer when energized.
Was the TTR the best call? What other test should should be requested?
Substation operations affected a major customer and we are not prepared to energize transformer again while that customer is online.
Any other things to consider?
Thanks,

 

[Bugman1400]

I calc the per-unit base at 2319 amps (100MVA @ 24.9kV). And, the xfmr imp equates to 1667% on the 100MVA base. So, for an infinite source (no source impedance and no line impedance) and a 3PH bolted fault, 1.0Vpu/1667% equals 0.06 pu which equates to 139 amps on the high side.

I agree with you. Is the 2500 amps from the initial fault or after a reclose attempt?

 

[11bgrunt]

Initial fault. Transformer has not been re-energized.

 

[Bugman1400]

Initial fault. Transformer has not been re-energized.

The dist feeder should have reclosed successfully if the fuses cleared. I think you had a flash over somewheres......either at the fuses or on the line. Could be from the conductors slapping together. Check the line from the fuses back to the sub and see if there is a shiny spot on the conductor on the faulted phases.

I'd love to see the event files.

 

[11bgrunt]

After looking at primary fuse barrels, the pair of fuses trying to clear the secondary fault created a large enough primary flash to crossphase at the pothead fuses. Transformer energized today and all is good... so far.