how to test delta star transformer?

To test a delta-star transformer, first ensure the power supply is disconnected. Use a megohmmeter to check insulation resistance between windings and ground. Perform a turns ratio test using a transformer turns ratio tester to verify the correct ratio between primary and secondary windings. Additionally, conduct a polarity test to confirm correct phase relationships. Finally, check for any unusual sounds or heating during operation under load conditions to ensure proper functionality.