What is Star-Delta connection?

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.

  • The voltage at the primary side= Root 3 times phasor voltage

  • There will -30 degree or +30 degree phase shift between primary voltage and secondary phase voltage.

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.