Poco draws arc at fuse and cross phase primary.

[11bgrunt]

You almost had to be there but I will do my best to make it clear. POCO was doing a voltage conversion and today was the day to move all transformers from 7200 to 14,400.
The existing 12470/7240Y 3 pase line served was by 3 tap fuses. One customer on the 3 phase line is a 3 phase 120-240 wye/delta transformer bank. 150kVA. Only one primary phase continues on past that load and serves 10 residentail overheadtransformers, all on A phase. Total line is 1 mile from tap fuses to deadend.
POCO is going to kill line at the tap fuses. A phase comes open and there is no arc. The 10 transformers are now supposed to be dead. Less than 5 seconds later the B phase is pulled open and the arc follows the tube like you were breaking 200kVAR. The arc cross phases on hardware and blows 2 line fuses serving this tap fuse location. They pull the third tap fuse and now the mile of three phase is dead plus about a 150 customers who were being served by the high side of tap fuse location because the source side fuses blew.
As line crews had prepared for the upcoming voltage conversion, the three 7200 transformers at the bank were replace and the primary side went from the specified ungrounded wye to a grounded wye because the transformers went from double bushing to single bushing primary.
We are confident that backfeed from the bank to the first phase trying to energize line and 10 transformers is the reason B phase had such a big arc. Remember all the single phase load was on A phase and it cleared with no arc.
What other factors could have contributed? There were no cap banks on the load side of tap fuses.
Would the bank have backfed if the transformer bank had not changed on the high side and stayed ungrounded wye?
We will be discussing this for a while to see what we did wrong and what we can do better.

Thanks

 

[GoldDigger]

When you disconnected the A phase, any motor loads at the three phase customer site (regardless of whether the motors were wye or delta connected) would have acted as phase converters to continue to power the A phase. Then when either the B or C phase was interrupted it would have killed all loads.
If all customer loads, both single phase and three phase, were first turned off, then we have to look instead at transformer effects.

Is the wye delta transformer connected with the X0 open?
If not the combination of wye and delta windings couldn't have created, in effect, an open delta which would have powered the A phase by backfeed from the transformer. I have not yet considered possible interactions on the MV side, and will leave that for others.

Tapatalk!

 

[JoeTroubleShoot]

No load buster? Like GoldDigger said, you probably dropped all the load with the second phase and then you drew a large arc which overloaded the upstream fuses. A load buster would have stopped the arcing unless you couldn't use one because of the type of disconnect. Otherwise, changing the order of opening might have helped or opening the three phase first, then going back and opening the tap fuses, but you would still draw an arc on A since most of your load was single phase. So you would still drop a lot of load whether it was backfeeding through B or on A after three phase was killed.

Maybe the best bet if you couldn't use a load buster would have been to open the three phase load, drop single phase transformers one at a time, then open tap fuses.

I've never had a situation where I had to drop that much load without a loadbuster, but I've seen someone do it by accident and the arc was big enough to track and blow all the lightning arrestors on the pole and open the recloser. Not pretty. You can get away with it on 4kv, but usually not 13kv.

 

[11bgrunt]

In our after action discussion with the entire group, all of these points were brought up.
The X0 bushings at bank were bonded and did turn the bank into open wye-open delta when the first tap fuse came open. Our spec has always been that closed wye-closed delta secondary will have a floated/ ungrounded high side neutral. Lose one leg, lose the bank. There is price to pay for the inexperienced on our 25kV line. Ferro gets bad. We used to blow pot lids every once in a while. Today we add a float/bond switch. When used correctly, we have no problems with ferro.
The bank should have come open first. We do have load busters. They just haven't seen enough arcs to know. My comment was if you don't know the amps, use a load buster. There are no loads in our system that a load buster won't shut down.
Thanks to those that responded.