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why we need vector group in three phase transformer?
Why manufacturers don't make all transformers in the same angle between primary
and secondary sides.
Well, it's got a lot to do with how the transformer is going to be used. It would be no good having a delta-delta arrangement if one or both of the windings needed to be star, or if one needed to be 180deg out of phase with the other, for example.
Why manufacturers don't make all transformers in the same angle.... - Well, it is not manufacturers' call actually. It is clients ho dictate vector group of transformer part of their electrical system design.
Having said that, you ill notice most of the distribution transformers are Dyn11 connected and most of the Generator step up transformers are YNd1.
Grid transformers are Auto transformers largely.
Exceptions do exist based on the power system needs, for example grid connected distribution transformers are YNd1 connected with earthing transformers at distribution voltage level and this is to improve the earth fault current magnitudes at the grid level (in case grid happens to be weak).
If distribution transformer is connected to a grid with >132kV, the transformer vector group ill either be YNyn0 or YNd1. Y on grid side is to reduce the cost of transformer as the insulation cost as well as OLTC costs are lower with earthed Y connection at EHV level.
To conclude, a fair level of standardisation with regard to vector group of transformers is already prevalent in the power industry with a few exception dictated by system design requirements and cost optimisation.
The only thing that is not dictated by the design of the transformer that can be affected by any manufacturer making a 3 phase transformer is whether the polarity is additive or subtractive on delta-wye or wye-delta transformers. This could be standardized, but there are reasons for having either type on the same power system when serving non-linear loads.
Also, once one person made an additive polarity transformer, it can only be paralleled with another additive polarity transformer, for example. So with a 50 year life expectancy, the likelihood of standardized construction becomes a very slow process.
The transformer manufacturing rational is mainly for flexibility reason..
Every transformer manufacturers I believe wants their product to be a saleable product. Which means the transformer should readily be adaptable and usable in every possible power requirement configurations and/or combination applications...