Backfeed on 3P, Wye-Wye Padmount
Backfeed on 3P, Wye-Wye Padmount
(OP)
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.






RE: Backfeed on 3P, Wye-Wye Padmount
RE: Backfeed on 3P, Wye-Wye Padmount
RE: Backfeed on 3P, Wye-Wye Padmount
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If we learn from our mistakes I'm getting a great education!
RE: Backfeed on 3P, Wye-Wye Padmount
RE: Backfeed on 3P, Wye-Wye Padmount
RE: Backfeed on 3P, Wye-Wye Padmount
RE: Backfeed on 3P, Wye-Wye Padmount
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
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"Why not the best?"
Jimmy Carter
RE: Backfeed on 3P, Wye-Wye Padmount
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?
RE: Backfeed on 3P, Wye-Wye Padmount
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Backfeed on 3P, Wye-Wye Padmount
RE: Backfeed on 3P, Wye-Wye Padmount
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Backfeed on 3P, Wye-Wye Padmount
RE: Backfeed on 3P, Wye-Wye Padmount
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Backfeed on 3P, Wye-Wye Padmount
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.
RE: Backfeed on 3P, Wye-Wye Padmount