Backfeed on 3P, Wye-Wye Padmount 1

[teachme2009]

Recently we had a three-phase, wye-wye padmount exhibit high voltage on one phase. The transformer secondary is 277/480, two phases measured 286V (typical) and the third measured 311V. The problem ended up being a blown fuse on the primary side of the transformer. I would like to be able to explain it more detail than just, "it backfed," i.e. provide phasor diagrams, more details, etc. Is there any reference material that might be useful in providing a full explanation of this? Or someone who could provide a clear explanation of what happened so that I can better understand the event? Thanks.

 

[opmgr1]

Is this a floating or grounded wye system?

 

[teachme2009]

Sorry, grounded wye system.

 

[ScottyUK]

Are there any other sources feeding the LV side? What are they - transformers or generators?


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If we learn from our mistakes I'm getting a great education!

 

[teachme2009]

There are no generators on the LV side, and this padmount is the only transformer.

 

[opmgr1]

This is an interesting one. For grounded wye- grounded wye system there will only be feedback if the transformer is loaded and the load is not balance L-G loads. There will be feedback for loads connecetd L-L especially motors. What is the nature of the load supplied by this transformer?

 

[pwrengrds]

Was the 311 volts measured phase to ground after the load was removed (the secondary breaker opened)?

 

[waross]

A three legged core acts as if it has a "Phantom" delta winding. A wye delta bank with the primary wye point grounded will transfer power from two healthy phases to a third low or missing phase. You would expect equal voltages on all three phases, within the impedance voltage percentage of the transformer.
But, with a phase shift on the neutral, you may experience above normal voltage on the open phase. You may also expect the circulating currents resulting from a displaced neutral to blow a primary fuse.
By displaced neutral I mean displaced vectorily. A heavy current in a primary neutral will cause a voltage drop in the neutral. This voltage drop causes the neutral to move away from the geometric center of the three phases. The result is unequal phase angles and unequal phase to neutral voltages.
It is possible that an event somewhere else on the system resulted in a heavy neutral current past your plant. The phantom delta would have responded with a heavy current in an attempt to balance the primary voltages and angles. That would have blown the fuse. The improper phase angles could result in the voltage on the open phase being higher than the other phases.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[opmgr1]

Waross,
I am not sure I understand you post. With the neutral tied to earth how is it possible to have a neutral shift? Isnt the unequal voltage as a result of voltage drop along the lines and the neutral?

 

[waross]

Wye point tied to neutral. Most or all of the neutral current travels in the neutral conductor. Even with multiple grounds on the neutral conductor of the distribution circuit I have seen serious voltage drops on the neutral and the associated shift away from the neutral position. We had soft starters tripping offline several times a day due to phase angle unbalance.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[teachme2009]

According to the service man, the customer had shut everything off and opened his breakers to avoid further damage since some lights had already blown. This is getting into he said, he said information so I'm not sure if this makes sense or not. It seems to me that there should not be any backfeeding with the breakers open. Is there something I am overlooking? Could this situation be possible--the open phase reading higher voltage to neutral than the other two phases with no load and breakers open? Is it possible to backfeed through a three-phase meter?

 

[TurbineGen]

If all the breakers are open and no load is on the transformer the phases should be balanced. What was the primary voltage at this time?

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[teachme2009]

The primary is 12470/7200. The phase that exhibited high secondary voltage had an improperly installed fuse feeding the underground from overhead that held under light load but operated under heavier load.It seems, to me at least, that the only source of voltage on this phase would be induced from the other two phases. The feed to the primary of the transformer was de-energized (open fused-cutout) and the load side sources were supposedly de-energized (open breakers), so I can't think of another source. Could an induced voltage read this high? I suppose I would expect a voltage lower than the nominal of the other two, but I have little experience to compare to.

 

[waross]

That is puzzling. Possibly the readings were taken to ground when there was a neutral offset and the neutral was not at ground potential. But we are into second hand he said, she said and can only make guesses at this point.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

Ferroresonance is possible in a grdY-grdY transformer becaused of coupling of the windings.

See Ferroresonant overvoltages in grounded wye-wye padmounttransformers with low-loss silicon steel cores,
Walling, R.A.; Barker, K.D.; Compton, T.M.; Zimmerman, L.E., IEEE Transactions onPower Delivery, Jul 1993, Volume: 8, Issue: 3, pp 1647-1660

Abstract
Reduced no-load losses have resulted in increased susceptibility to ferroresonance of grounded wye-wye three-phase distribution transformers using five-leg silicon steel wound cores. The authors describe the results of an extensive test program which determined that overvoltages are directly related to the ratio of capacitive susceptance divided by core losses and that the conventional use of rated exciting current can be a misleading indicator of ferroresonance susceptibility

Sustained overvoltages over 2 pu were observed.

 

[burnt2x]

Can you post a single line diagram?