In case of the delta-star connection of a 3-phase transformer, the primary winding is connected in delta while the secondary winding is connected in star (see the figure).
Primary Winding − As the primary winding is connected in delta. Hence, the line voltage and the phase voltage on the primary side are the same and the line current is √3 times of the phase current.
Secondary Winding − The secondary winding is connected in star fashion. Thus, the secondary line voltage is √3 times of the phase voltage and the line current is equal to the phase current.
The delta-star connected 3-phase transformers are mainly used to step-up the voltage, i.e., at the beginning of the high voltage transmission system.
In case of the delta-star connection, the secondary phase voltages lead the primary phase voltages by +30° and this is also the phase relationship between the line voltages.
The following are principle advantages of the delta-star connection of the transformers −
The delta connected primary winding requires less area of cross-section.
The secondary winding is star connected and the neutral is present. Thus it can be used for 3-phase, 4-wire system.
There is no distortion in the secondary voltage due to third harmonic component.
This connection can handle unbalanced loads.
The relaying of the ground fault protection is very easy in case of delta-star connection.
The main disadvantage of the delta-star connection is that the secondary voltage and the primary voltage are not in phase with each other. Therefore, it is not possible to operate the delta-star connected transformer in parallel with the delta-delta or star-star connected transformers.
The delta-star connection of the transformers is mainly used in the following applications −
The delta-star connection is used in step-up transformers, where neutral at the secondary side is required.
The delta-star connected transformers are mainly used as generator transformer for connecting the generators to the transmission system.
They are also used in industrial, commercial and high-density residential distribution systems.