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Friday, May 15, 2015

No guarantees!

We got a single call for power trouble from a place with three-phase service, some of their stuff wasn't working but they weren't totally out.  According to the mapping data, they were served from three individual overhead transformers.  Certainly this meant that one of those transformers had failed, especially since we were not getting any other calls.  We sent Bear over in 574 to check it out.

Upon arrival, Bear is a little perplexed to find that all three cutouts to the overhead banks are closed in and holding.  He investigates at the customer's panel and is getting no voltage on a couple of their low-side phases.  Perhaps something is wrong in the secondary from the transformers to the panel?  No, it is all above ground, plainly visible, nothing obviously wrong.  Bear flies up in the bucket and tests for voltage above the banks, and shows good voltage to all three transformers.

Nothing is apparently wrong, yet stuff isn't working.  Rare is the day that the Bear is stumped.  Today is one of those days.  Bear swallows his pride and calls for backup.  577 Kevin heads his way.

Kevin arrives and goes through the same checks, comes to the same conclusions.  While he is up in the air near the transformers, Kevin also load-checks all three phases of the 12kV overhead going down the tap to the 150 or so customers downstream past this place.  They are stumped, and looking for inspiration.  Oddly enough, Kevin gets 30-something amps on A phase, 2 amps on B phase, and 50-something amps on C phase.  Ideally they should be sort of balanced, and the mere 2 amps on B phase is outside of plausible under normal circumstances.

This isn't making any sense.  If there is voltage on B phase, people should be in power, but according to the load check of just 2 amps there is effectively no flow going downstream, yet none of the B phase customers downstream have reported power outages over 90 minutes into this incident.

At this point, if I was out there, I would want to have a cup of tea to think things over.  The urge to break something in frustration would also cross my mind.  Thankfully it isn't me out there, but the dedicated duo of Kevin and the Bear.

Kevin has a hunch, and drives back upstream to a set of line reclosers just a few spans before the problem site, and much to his surprise, finds the B phase recloser is open.  Yet..... B phase has voltage.  Can it get any more confuzzling?

Kevin and Bear pair up in one of the trucks and go patrolling the downstream tap to try to sort out the mystery.  Sure enough, about a half mile down the way, they find that something.... wind?... has caused a span of the B phase primary overhead to lay flopped over C phase.  Suddenly, the flood of comprehension washes over them.

Under normal circumstances, all three phases should be more or less equally loaded.  Something caused B and C phase to come in contact with each other and cause a cross-phase fault.  The single phase reclosers for B and C phase would have been extremely unhappy about this and would have tripped one or two times hoping the fault would clear.  The timing of these reclosers was just ever so much of a smidgen off that one of the reclosers closed back in and held while the other gave up.  The result was B phase load was now being carried not through the recloser as normal, but through where the lines were entangled.  This is why there was no load on B phase at the outage site, they were now electrically at the farthest end of B phase with its source coming through the tangle.  This is why no B phase customers reported an outage, at most they saw a couple of blinks.

And lastly, the original caller was the only 3-phase customer on this tap.  Some 3-phase service relies on magical AC theory stuff having to do with the gap between phases, and when two of the three phases are unexpectedly tied together (instead of A-B-C they were getting A-C-C), anything relying on the difference between AB or BC phases will get no potential, and stuff won't work.

If not for that one and only 3-phase customer reporting a problem, there's no telling how long this might have sat this way until something else brought the problem to our attention.

And another lesson was driven home for everyone.  Despite the B phase recloser being open, the line was backfed and hot.  Even if you have a visual open, a line isn't dead until it is grounded and dead.

Those guys did a good job sleuthing it out.  Stay safe out there.

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