Bay-o-net & upstream fuse operation - differnet phases

[MarkKW]

The 13.8kV underground distribution system feeds a S&C PHM-9 switchgear. The fuse bay in the switchgear is fused with a 200E Slow Speed fuse. This three phase radial fuse lead then feeds several different transformers. Most of the transformers on this fused lead are delta/grounded wye, a few single phase transformers, and a floating wye/delta transformer bank.

There was a secondary fault on a 1500kVA pad mounted transformer that blew the bay-o-net (353C series)on C phase. The upstream 200E A phase fuse also operated. During the restoration process, the crews did not find any faults or other concerns with A phase.

The SEL relay at the substation recorded several events in the 600-700 amp range. Theses were intermittant, I suspect that this is the secondary fault that the SEL recorded. The SEL then recorded A phase current of 3800 amps. The substation breaker tripped on instantaneous and three seconds later the breaker closed. Once the breaker closed, the SEL recorded 600-700 amps of current for about two cycles, then A phase went to 3800 amps. It looks like the 200E A phase fuse cleared at this point and the current levels recorded by the SEL returned to normal load levels.

The modeled fault current at the 1500kVA transformer location is 3887 amps for a bolted L-G fault and 5052 amps for a three phase fault.

The 1500kVA transfomer is a five-leg core.

Can someone eplain what caused the A phase fuse to blow?

Thank you.

 

[dpc]

How much confidence do you have in the phase identifications at each location?

 

[MarkKW]

I had the crews verify the phasing at the various points and everything was correct.

One thing that I forgot to mention, a TTR was performed on the 1500kVA transformer and that checked out okay.

 

[apowerengr]

200E slow blows hard.....in a PMH there's a lot of gas generated which can flash unfaulted phases. You stated most of the transformers are delta-wye grd so C-A is a served winding. C phase bayonet blows, A phase 200E blows for the same fault. I'd guess gas generated in the PMH caused the fault on reclose. And its not clear that the bayonet coordinates with the 200E slow fuse either however you did not generate 3800 amps from a secondary fault in the transformer IFF the transformer is an ANSI standard impedance. If this is a remanufactured transformer with a non-standard low impedance then you'd have more fault current in the primary than the bayonet is rated to clear. The bayonet is beyond its interrupting rating at probably 2500 amps at 15.5kV rating for a delta-wound primary transfomrer so the 200E as the backup fuse would have to clear the fault.