In case of the star-delta connection of a transformer, the primary winding of the transformer is connected in star while the secondary winding is connected in delta (see the figure).
Primary Winding – Since the primary winding is connected star fashion. Therefore, the primary line voltage is equal to the √3 times of the primary phase voltage and the primary line current is same as the primary phase current.
Secondary Winding – The secondary winding is connected in delta manner. Therefore, on the secondary side of the transformer, the line voltage will be the same as the phase voltage whereas the line current is √3 times of the phase current.
In case of the star-delta connection, the secondary phase voltages lead the primary phase voltages by +30° and this is also the phase relationship between the respective line voltages.
The main advantages of the star-delta connection of the transformer are given as follows −
As, the primary is star connected, hence, the neutral is available on the primary side, which can be grounded to avoid the distortion in voltage.
The star-delta connection is free from the problem of the third harmonics, as they circulate in the delta loop on the secondary side.
The star-delta connected transformers can handle large unbalanced loads.
Since the primary winding is star connected, it requires less number of turns. This makes the star-delta connection economical for large high voltage step-down transformers.
The main disadvantage of the star-delta connected transformer is that it cannot be paralleled with the star-star or delta-delta connected transformer because the secondary voltage is shifted by 30° with respect to the primary voltage.
Following are primary applications of the star-delta connection of the transformer −
This type of connection is used, where the primary side requires neutral terminal so that it can be grounded.
The star-delta connection is mainly used in step-down transformers, which are located at the substation end of the transmission line.