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It is a3 phase transformer.
Here "D" means primary winding with delta connection.
'y' means secondary with start connection.
Secondary currents lags voltage by30degree, which in vector diagram looks like11'o clock on watch.
(Firstly , it should be written like that (Dy11
Hence D (Delta Connection ) is capital letter for the high voltage side of the transformer & y(Star Connection ) is small letter for the low voltage side
Dy11 it means that the angle of the delay of the secondary voltage from the primary11 *30 =330
degrees
Of course it is necessary to know that angle when trying to connect transformers in parallel to prevent short circuits between them
