Star-Delta Transformer (Y-Δ )
Primary Y (High Voltage side) and Secondary ( Low Voltage side) Δ is generally used at the generating side for stepping up the voltage levels. Power is generated at 11kV to 33kV having current ratings in thousands of kilo amps. This generation voltage has to be stepped up to higher voltage for transmission as stepping up the voltage results in the reduction of losses while transmission
Generation side of the transformer (LV side) voltage applied is low and current flowing through the windings of the transformer is high. At secondary side (LV side), delta connection (Δ) is employed because in delta connection Iphase=ILine/√ 3 and Vphase=VLine
Current carrying capacity of windings depends on the thickness of the conductor. Large current flowing through the conductor means more copper utilised for the windings. Therefore in order to reduce the flow of huge currents by above equation delta winding is employed in the LV side of the transformer
On the other hand star type (Y) of connection is employed on the HV side of the transformer. In star connection Vphase=VLine/√ 3 and Iphase=ILine
Voltage withstand ability of conductor depends on the insulation strength of the conductor. Hence to reduce the phase voltage Star (Y) connection is employed in the high voltage (HV side) of the transformer.