Combinations of Delta and Wye Transformer Connections:

The three-phase transformer might have three separate but identical single-phase (1ø) transformers or a single 3ø unit holding three-phase windings. The transformer windings might be connected to form a 3ø bank within any of four various ways.

Figure: Transformer Connections

Figure  displays the voltages and currents in terms of applied line voltage (V) and line current (I), where  the  turns  ratio  (a)  is  equal  to  one. Current and voltage ratings of the individual transformers depend on the connections that were show in figure and are denoted by Table 1 for convenience of calculations.

*a = N1/N2;  √3=    1.73

Example: If line voltage is 440 V to a 3ø transformer bank, find the voltage across each primary winding for all four types of transformer connections.

?-?  :  primary voltage = V = 440 volts

Y-Y:   primary voltage =     V/√3 =440/1.73 =254.3 volts

Y- ? :   primary voltage =   V/√3 = 440/1.73 =254.3 volts

?-Y:  primary voltage = V = 440 volts