The physical arrangement of the bushings to match system phase arrangement standards.
Matching existing transformers.
What does the supplier have in stock?
Bill
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"Why not the best?"
Jimmy Carter
ANSI standard is that the low side lags the high side by 30 degrees. Exceptions are old transformers or special circumstances. IEC practice (as I read about it here) seems to be wide open. But any odd numbered vector group can become any other odd numbered vector group with a bit of creative reconnection just as any even numbered vector group can become any other even numbered vector group.
There are unique connections and vector group for various applications.For large generator step up transformers it is Yd1(USA) or Yd11(IEC). Lv is connected in delta to prevent the zero sequence currents (with a LG fault on HV) from entering in to generators as rotating machines cannot stand zero sequence currents.Then auxialry transformers will be with Dy11(US) or Dy1 (IEC) connections.
Distrubution transformers are with Dy1 (IEC) or Dy11(understand US has many alternatives)so as to get the same vector at consumer end and to avoid third harmonic currents entering in to grid.
Solar or wind turbine transformers are with connections Dyn5 or Dyn11 connections to suit inverters.This will be conneced to grid by Yd1 tarnsformers.
Rectifiers have unique connections and vector groups -See C57.18.10-1998 Converter Transformers.Pumped storage hydro plants have started to use a novel connection recently.
To get a earthing neutral point in unearthed systen zig-zag connection is used. Traction supply systems have different connections in various countries.
