Why do we take Vector Group of Generator Step UP Transformer as YNd1 or YNd11. Somewhere I have seen YNyn0 also.
The small letters of the Vector group refer to the lower voltage, in the case of a GSU Transformer, this is the winding connected to the Generator. For a GSU application one would normally expect the Transformer winding that connects to the Generator to be Delta (d) as this would limit phase un-balance and potential fault currents. To have a GSU Transformer with a Star (y) connection on the Transformer LV would be an exception, but could be possible is certain circumstances. The actual requirements / Vector group specification is driven by the EPC / system engineering.
Large and medium power transformers
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