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Three Winding Transformer
2

Three Winding Transformer

Three Winding Transformer

(OP)
We are intending to order a three winding transformer (72kV-13.8kV) and since it is on a remote facility, we also intend to use a delta tertiary rated for 600V to cater for normal small power requirements (a UPS and portable generator will provided to maintain vital power during transformer shutdown).

The transformer will be rated 50 MVA. The small power will only be rated at 250kVA. Will we able to specify this KVA rating for tertiary? I am just concern that there may be standard ratings for the tertiary depending on how big the transformer is.

Thanks,

RE: Three Winding Transformer

I'd be surprised if you could get a tertiary rating that low. Might be that 5-10 MVA is the smallest you can get. It won't be sized for your station load, but rather for withstanding through fault currents.

RE: Three Winding Transformer

(OP)
Thanks for the assistance David.

RE: Three Winding Transformer

A better alternative might be to look at a tank mounted auxiliary transformer rather than a tertiary winding.
Regards
Marmite

RE: Three Winding Transformer

There are designs with such low tertiary ratings where the winding will be put on yoke and not concentric to main windings.Then the impedance drop will be very high, not suitable to the purpose that you are planning.In case you are looking for delta tertiary better to have 30 % of main winding rating.With star connection it is possible to have slightly lesser kVA,may be 5-10 MVA.

Marmite's suggestion will be a better solution.

RE: Three Winding Transformer

Under some conditions the delta winding of a wye/delta transformer or bank will support a fault current. The current in the delta may be limited by 3 times the PU impedance of the delta winding.
This supports the posts by davidbeach and prc.

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

RE: Three Winding Transformer

(OP)
Thank you for all your suggestions.

I have not seen a tank mounted auxiliary transformer as Marmite have mentioned. I would appreciate if you can provide me a single line representation of the that type of transformer for my undertsanding and a link as to where I can find an illutration of it.

I am intending to specify: Delta Primary - Wye Secondary and Delta Tertiary. Is this configuration standard as I have been used seeing a Wye-Primary, wye-secondary and delta-tertiary configuration.

Thanks.

RE: Three Winding Transformer

2
Why don't you just get a 50 MVA 2-winder 72/13.8 kV delta wye. Then you could get a standard padmount transformer 250 kVA or 500 kVA.

This approach has the advantage of more standardized, readily available and less expensive transformers in case anything fails.

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