Star-Delta Connection of 3-Phase Transformer

       In this type of connection, then primary is connected in star fashion while the secondary is connected in delta fashion as shown in the Fig. 1.

Fig.  1

       The voltages on primary and secondary sides can be represented on the phasor diagram as shown in the Fig.2.
Fig.  2

       The same type of connection can be represented in another way as shown in the Fig. 3.
Fig.  3

       This type of connection is commonly employed at the substation end of the transmission line. The main use with this connection is to step down the voltage. The neutral available on the primary side is grounded. It can be seen that there is phase difference of  30° between primary and secondary line voltages.
Key point : The delta connection on secondary side allows third harmonic current to flow which provides a sinusoidal flux.
Let                                VL1 = Line voltage on primary side.
                                      VL2 = Line voltage on secondary side.
                                      Vph1 = Phase voltage on primary side.
                                      Vph2 = Phase voltage on secondary side.
                                       K = Transformer ratio.
                                       Vph1 = VL1/√3
Now                               Vph2 /Vph1 = K
...                                     Vph2 = K Vph1 = K (VL1/√3)
       Since secondary is connected in delta.
                                        Vph2 = VL2
                                         VL2 = K (VL1/√3) =  ( K/√3) VL1
       The connection suffers no problems due to unbalanced load as secondaries are connected in delta. This  type of transformers are commonly employed at receiving end.

       The advantages and disadvantages of this type of connection can be stated as follows.
  1. The primary side is star connected. Hence fewer number of turns are required. This makes the connection economical for large high voltage step down power transformers.
  2. The neutral available on the primary can be earthed to avoid distortion.
  3. Large unbalanced loads can be handled satisfactory. 
In this type of connection, the secondary voltage is not in phase with the primary. Hence it is not possible to operate this connection in parallel with star-star or delta-delta connected transformer.