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Unbalanced load in a 5 MVA Delta-Wye transformer (24940V-4160V)

Unbalanced load in a 5 MVA Delta-Wye transformer (24940V-4160V)

Unbalanced load in a 5 MVA Delta-Wye transformer (24940V-4160V)

(OP)

Our main distribution system is multigrounded Y ( 24940GrdY/14400V) but to feed our older 4160GrdY/2400V downtown system we typically use some 5MVA Delta-Wye transformers (24940D-4160Y). The 4160V system is typically well balanced. However, we need to remove some of the loads from it, and cannot re-balance the load without considerable capital expenditures; as a consequence, there is some significant current unbalance on the Wye LV ( eg phase A: 400A Phase B:390A Phase C: 180A).

Can anyone indicate what potential problems could show up if we keep it significantly unbalanced? Thanks!



RE: Unbalanced load in a 5 MVA Delta-Wye transformer (24940V-4160V)

Unequal currents will develop uneven voltage drops. That may be a problem for motor loads.
A wye:delta bank will do a lot to equalize the voltages. The neutral current will probably cause a voltage drop that will result in some phase angle errors. The motors won't like that either. The length and impedance of the distribution circuit will be a big factor.
I have not done this in practice but I am aware of an instance where a circuit with an unused wye:delta bank was extremely well balanced at the substation until the bank was de-energized and a multitude of unbalances showed up. I have seen quite a few instances of a wye:delta bank attempting to equalize voltage unbalances and phase angle errors.
If you have a set of transformers that may be borrowed for this purpose, I look forward to some interesting discussions as to a protection scheme.

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

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