Vacuum Breaker Prestrike Blowing Fuses? 1

[BigJohn1]

Very unusual problem on a 25MVA transformer 34.5kV Y : 13.8kV Δ.

Has been in service for many years. Recent unexplained feeder fault blew a fuse. Now every single time the transformer is energized via the vacuum breaker it blows those same upstream feeder fuses. The transformer has so far always been energized unloaded.

After the initial fault, this sub was extensively tested:
[ul]
[li]Transformer PF, TTR, excitation, winding Ω, leakage X, and oil sampled.[/li]
[li]All bushings PF tested.[/li]
[li]All high and low side arrestors PF tested and AC overpotential.[/li]
[li]Breaker PF tested and resistance tested - No timing test yet.[/li]
[li]Overhead structure hipot.[/li]
[/ul]
No problems whatsoever. But every time the transformer is energized with any of the high side arrestors in place it blows fuses. Remove arrestors and re-energize and it stays energized. Add an arrestor and it blows a fuse on that phase.

Here's the kicker: All these affected arrestors are in the load-side of the VCB. There is an identical set of arrestors on the line side that have never blown any fuses: If the VCB is open the line side will remain happily energized for hours, as soon as the VCB closes, fuses pop.

I am at a lose except maybe to say that a new malfunction in the breaker is causing transformer overvoltage due to prestrike?

Any ideas at all?

 

[BigJohn1]

I figured I'd give some update:

We reenergized an alternate feeder that had OCBs as protection instead of fuses and the customer went online and stayed on despite having all the arrestors in place.

We found a low-side ground fault in the facility that may have accounted for the original fuse failure that single-phased the plant: There was a flashover in a 15kV disconnect.

During a final round of unsuccessful testing prior to trying the alternate feeder we put a PQ meter on the transformer breaker CTs and recorded a serious current imbalance during inrush on the unloaded transformer. One phase was 5A, the other two phases were 2.5A. No arrestors connected during this test. We went back through historic data for excitation tests to see if we could spot a trend and they all looked fine.

There was talk of core magnetization being the root cause due to the through-fault, but I can't swear and the customer chose not to do any more investigation now that they are online.

The only thing I am reasonably sure of at this point is that a problem still exists, but unfortunately it's out of my hands.

Thanks much for the help so far.