for Delta-Star connection of 3-phase transformer: i) Draw the connection diagram ii) List any two advantages of this connection iii) State its area of application.
2 Answers
Answer :
i) Draw the connection diagram:-
ii) List any two advantages of Delta-Star connection:
1. As primary is connected in delta, distortion due to third harmonic is absent 2. Phase shift of 30 degree elec. is present between the primary and secondary line voltages and line currents 3. Small cross section wire can be used on primary side due to delta connection. 4. Due to availability of neutral on the secondary side, it is possible to use it for three phase four wire system.
iii) State its area of application
1. It is used for step up the voltages at the sending end of high tension transmission lines. 2. Used as distribution transformer 3. Due to star type secondary used for small as well as large loads. 4. As the neutral is available on secondary side it is used for three phase four wire system that is applicable for both single phase as well as three phase load.
Answer :
Disadvantages: i) 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. ii) One problem associated with this connection is that the secondary voltage is shifted by 300 with respect to the primary voltage. This can cause problems when paralleling 3-phase transformers since transformers secondary voltages must be in-phase to be paralleled. Therefore, we must pay attention to these shifts. iii) If secondary of this transformer should be paralleled with secondary of another transformer without phase shift, there would be a problem. iv) As line current is equal to phase current the conductor size of winding should be large.