Transformer Failure Resulting in Overvoltage 3

[jmbelectrical]

A 240/120V, three-phase, four-wire, delta high-leg system comprised of three (closed delta) pole-mounted transformers serves two commercial properties. One of the transformers presumably failed, resulting in the opening of its primary-side fuse cutout. Both properties allegedly suffered from damages that are consistent with an overvoltage event.

By what means could the failure of a transformer result in overvoltage on its secondary side?

 

[waross]

Turn to turn shorts in the primary will result in a higher secondary voltage until the protection clears on over current both from the turn to turn shorts and the higher secondary voltage.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[crshears]

Ah! Turn-to-turn shorts effectively reducing the number of turns in the primary winding and altering the ratio of the trafo.

But what would be the underlying cause? Overloading causing overheating causing insulation failure? Loss of insulation integrity due to lightning strikes / surge arrestor failure? Inferior trafo to start with? I wouldn't think there would be wedges in such small trafos . . .

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[davidbeach]

Turn-turn fault is certainly a possibility. Given the available information it is also just a guess.

High-side turn-turn faults can become rather vicious. But it’s also possible that the transformer failed from the same overvoltage condition that damaged the loads. An interaction between overbuilt transmission and the distribution can do the same thing. Absent a more complete picture of the event we can just speculate.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

But it’s also possible that the transformer failed from the same overvoltage condition that damaged the loads.

Good point, David. I remember years ago when our distribution system was built under a 60 kV line.
A car hit a pole and the 60 kV slapped the distribution line. I was watching TV and the set flashed bright white and the lights went out.
When the power came back on, the TV still worked but homes closer to the fault lost a lot of equipment. My inlaws lived about one mile closer and the utility bought them a new TV set.
Your suggestion is probably more likely than my suggestion David.
Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

Not necessarily more likely. Don't know enough about that system. Maybe there's an even more likely cause that's entirely obvious to the casual observer. The damnedest things can happen. Still get surprised with things that happen, but when reviewed and analyzed seem quite obvious in retrospect. Different systems have different weak points.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[RRaghunath]

If the neutral is intact (not affected by loss of one phase supply) on secondary side, overvoltage should not occur just because one of the three single phase transformers is disconnected.
If neutral-to-earth connection or neutral distribution is somehow affected, that could give rise to the appliances getting exposed to overvoltage.

 

[stevenal]

I assume the primary is wye?
If the primary wye point is connected to the system neutral, an open fuse will cause the system to transition to an open wye-open delta system. Three phase loads will run normally, but single phase loads may be affected as RRaghunath indicated.
It is common to float the wye, however. This keeps the voltage balanced and prevents circulating current through the secondary delta. A primary phase loss, however, causes the whole system to go single phase.

 

[waross]

Hi Steven. I have lived through this many times.
I became responsible for the technical aspects of a small isand system with a couple of wye-delta transformer banks.
Our generators could not withstand block loading of the entire system so after an outage at the plant, they would go back on-line one phase at a time.
In the evening we would be running one set. Our very old sets were prone to unexpected stoppages.
I would sit in my room in the dark, waiting for my boys to get another generator started and on-line.
When they connected the first phase, all the lights on that phase would come on. The wye-delta banks would backfeed about 50% voltage to the other two phases and the incandescent lamps would light dim.
When the second phase went in, the lights would jump in brilliance to almost 100% but not quite.
Due to the voltage drop caused by the heavy current in the neutral, and by the regulation of the overloaded wye-delta bank trying to back-feed one phase of the entire system from an open delta source, the voltage would be low.
Then the third phase would be established and all circuits would have full rated voltage.

Aside issue was burned out refrigerators and freezers. During the time that two thirds of the system was on about 50% voltage, all the small refrigeration compressors would stall. They would sit there cooking on about 300% of FLC. Then the voltage would jump up to rated voltage or some high percentage thereof, with the compressor still stalled.
But not to worry, they all have internal over-temperature protection, right.
Yes, but from time to time, the internal protection would not act fast enough and another motor would smoke.
By the way, the boys did the switching with pole top mounted, fused disconnects or cut-outs.
Imagine in the rain and the dark, handling a sagging, telescopic 25 foot hot stick.
Your partner is holding a flashlight.
The system may be at 50% voltage for longer than expected before the operator can get the end of the hot stick disengaged from one cut-out and inserted in the last cut-out.
My hotel room was on the third phase and I am quite familiar with the sequence.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Mbrooke]

Ferroresonance? Just a wild guess.