Failure -Tertiary Winding 1

[ghostbuster7]

2 MVA Dry Type Tx.
27.6 kv./480 v. wye/wye
2 years old -30% loaded

Note:The transformer manufacturer completed the delta by using a #6 jumper wire instead of the interconnecting busbar they had used for the rest of this delta tertiary.
The wire melted and caused a phase to phase flashover.
We suspect the utility lost an incoming phase,when this failure occurred.
We will be looking at the upstream 27.6 kv. breaker protection settings and single phasing protection.

 

[waross]

If the primary neutral point is connected to the system neutral as is common with wye:wye transformers the delta tertiary becomes problematic. When the primary voltage unbalance reaches 3 x %Z (9% voltage unbalance versus ((Z = 3%) x 3) full load current will circulate in the delta.
If a supply phase goes missing, then the transformer will backfeed to supply the loads on the missing phase to the best of its ability.
However, if the primary neutral is floated the delta tertiary will maintain a good voltage balance on the wye secondary. You will have better luck with phase loss detection with the primary neutral floating.
If the primary wye point remains connected to the system neutral your transformer will attempt to balance any unequal primary voltages to neutral on the supply system. This may defeat phase loss protection in some instances.
A little outside the envelope but a CT on the delta jumper may be advantageous. Primary voltage unbalances will create a circulating current in the delta which is proportional to the amount of primary voltage unbalance. This may used to generate a trip signal when the primary voltage unbalance causes an overload on the transformer. As a phase loss is a 100% voltage unbalance this will also provide phase loss protection.
Another advantage is that if there is so little load connected to the primary phase between the break and your transformer that your transformer may safely supply it by back feeding, this arrangement may allow you to stay online.
There are a lot of unpleasant things that may happen when the voltages to a Yn/d are unbalanced or a primary phase is lost.
There are negative implications for both your transformer and for customers upstream who are back-fed by your transformer on the open phase.

Bill
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"Why not the best?"
Jimmy Carter

 

[ghostbuster7]

It was found at site, the tx. primary neutral was directly connected to the system neutral.
Also,a phase to ground flashover in the tx.had also occurred, tripping this tx.feeder breaker on ground fault.
The loss of phase detection circuitry was installed on the main board but did not trip the breaker.
The tx. mfr.is unsure if they have to design this tertiary(and associated interwiring connections) for this abnormal high current condition and for what time duration.They do not want to honour any warranty.Estimated cost to repair $20k-$30k