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.
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The voltage at the primary side= Root 3 times phasor voltage
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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:
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The star connection on the primary side reduces the number of turns required. This makes the connection economical.
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Large unbalanced loads can be handled
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Allow both single phase and three phase load.
Disadvantages of star-delta connection:
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Secondary voltage is not in phase with primary
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If secondary of this transformer should be paralleled with secondary of another transformer without phase shift, there would be a problem.
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Another problem is the primary voltage is shifted 30 degree with the secondary.