Star-Delta connection is a transformer winding connection, where primary winding is a star connection and secondary is a delta connection.
The primary star connection and the secondary delta connection is shown in above picture with its phasor diagram.
This kind of connections are used to employe for power supply transmission and commonly used in step down transformer.
Star-delta as grounding transformer:
For power systems, star-delta transformers are used to provide a good ground source. The circuit for grounding system is shown in above picture. If it is required to connect phase-to-ground loads to this system a grounding bank is connected to the system, as shown in the above picture.
Advantages of star-delta connection:
The star connection on the primary side reduces the number of turns required. This makes the connection economical.
Large unbalanced loads can be handled
Allow both single phase and three phase load.
Disadvantages of star-delta connection:
Secondary voltage is not in phase with primary
If secondary of this transformer should be paralleled with secondary of another transformer without phase shift, there would be a problem.
Another problem is the primary voltage is shifted 30 degree with the secondary.