4 Limb Wye-Wye Transformer

[Breaker]

We were in need for a zero phase shift transformer with the characteristics of a delta-wye transformer, and after some research we decided that a 3 winding wye-wye with buried delta could provide this (Yyn0d1).

A well known manufacturer, has now provided us with a 4 limb wye-wye (Yyn0)transformer with the HV ungrounded as an equivalent alternative.

I have tried to research this but cannot find anything with enough detail to indicate that a 4 limb wye-wye is equivalent to a 3 winding wye-wye-delta.

Some texts state that wye-wye transformers should have the HV grounded to allow fault currents to flow for a phase to earth fault on the secondary. However if the HV is ungrounded and is a 4 or 5 limb designs would this be different? Some texts such as 'J&P Transformer Book' appear to imply that for a secondary phase-to earth fault fault current can flow.

Does anyone have knowledge/experience or know where I can find more information on this subject?

 

[davidbeach]

Unless all possible load combinations on the secondary of the Yyn0 transformer are totally and completely balanced, you will not be satisfied with the voltage balance on the secondary. Best to have the high side grounded. There will be some phantom delta effects in the magnetics and some zero-sequence current on the the yn side, but it will be small and difficult to quantify.

 

[prc]

Please state the MVA rating of transformer that you are considering and also whether it is 3 phase or single phase bank unit.

 

[Breaker]

Unfortunately to avoid circulating currents the HV cannot be grounded as the connected circuit is already resistively grounded.

This is a 3 phase 1MVA transformer.

 

[unclebob]

Another option is to go with a delta-delta (Dd0) plus a small zig-zag on the secondary for HRG.

But, why don't you use a delta-wye (Dy0)?

 

[Breaker]

I did not know that you can get Dy0 transformers.

 

[jghrist]

A well known manufacturer, has now provided us with a 4 limb wye-wye (Yyn0)transformer with the HV ungrounded as an equivalent alternative.

I have tried to research this but cannot find anything with enough detail to indicate that a 4 limb wye-wye is equivalent to a 3 winding wye-wye-delta.

Some texts state that wye-wye transformers should have the HV grounded to allow fault currents to flow for a phase to earth fault on the secondary. However if the HV is ungrounded and is a 4 or 5 limb designs would this be different? Some texts such as 'J&P Transformer Book' appear to imply that for a secondary phase-to earth fault fault current can flow.

A wye-wye transformer with the HV ungrounded is not equivalent to a wye-delta-wye, no matter what core construction is used. There will be no zero-sequence path. Be very careful about applying a wye-wye transformer with the neutral isolated. See

http://www.geindustrial.com/publibrary/checkout/38652.30055.7762.44019/generic/GET-3388B.pdf

The wye-wye with an embedded delta is also not equivalent to a wye-delta transformer. This is not a ground source unless the primary system is grounded.

 

[davidbeach]

unclebob, no such beast as a Dy0. You can have Dy1 or Dy11 or any other odd number in that range, but never an even number.

 

[unclebob]

Oh, but it's so easy to draw on a piece o' paper...