132/33kV Trafo vector group - Dy or Yd

[RRaghunath]

Is Dy connection common for 132/33kV transformers. Delta on 132kV side means more insulation cost, on the otherhand, the Star on 33kV provides neutral for solid earthing of 33kV system.

Any views.

Thanks in anticipation.

 

[prc]

Yes.Once upon a time 132/33 kV Dy connection was very popular even with a solidly earthed system and may be thought essential for industial loads.Definitely transformer will be costlier, 3 pole ,line end tap changer become necessary.But it is no-longer used with star connected 132 kV system.For industrial loads today Yy or Yd transformers are used.But if 132 kv system is an isolated one(in India one regional 110 kV grid is still unearthed all transformers are Dy connected.)Dy connection is necessary.

 

[RRaghunath]

prc,

How about 132/33kV transformers in Indian power distribution system. Are these Yd connected or Yy. If it is Yy connected, is the reflection of 33kV earth faults on to 132kV system not an issue (and how is it addressed).

Dy connection for 132/33kV - J&P transformer Handbook has some discussion (hope I am right about the reference, remember to have seen long back) and says that if the source end 132kV winding has neutral for solid earthing at receiveing substations, delta connected 132kV windings are used and allowed in British power system.

 

[prc]

Nowadays, in India, most of the utilities go for Yy ( with soild neutral earthing on both sides) connection with out tertiary.A few adopt Yd with earthing transformer on secondary system.

How they are taking care of fault currents.... our protection gurus may take over.

 

[HiSet]

PRC,
One of the ways to reduce fault current of YY grounded transformers could be by specifing high positive and zero seq impedance.

 

[cziebold]

In european system with unearthed 110 KV side, the Y-y with a tertiary delta for alowed the zero seq. current, is frequently used.

 

[RRaghunath]

In Oman, 33kV system is delta with zigzag transformer earthing.
The vendors seem to be deliberately limiting the earth fault current by choosing higher zero sequence impedance for the zigzag transformer.
The spec calls for 5 Ohm impedance but vendors say there is limitation in design (to meet the stipulation).