I believe, the answer is some where around HERE

This practice of providing a ground on the DELTA winding is common on STAR-STAR Power transformers. The purpose of grounding the OPEN DELTA winding is to stabilize the SECONDARY VOLTAGES of the Power Transformer under normal conditions.

Another reason for gronding the OPEN DELTA winding is to prevent the FLOW OF ZERO SEQUENCE currents through the transformer under fault conditions.

please Mr. Anonymous

what do you main by OPEN DELTA winding .the Tertiary winding (delta winding) in our auto transformer not open .

The tertiary(11kv) winding is called OPEN DELTA because there the 11kv is not feeding any external load. The winding is so designed that the net magnetic balance on it is zero(hence net voltage is also zero). Hence a resistor can be connected across it without any net current flowing at normal system conditions.

However, when an external Earth Fault occurs, the resulting zero sequence currents set up flow through this OPEN DELTA winding to ground.This prevents the zero sequence currents from flowing through the Transformer. This also prevents abnormal DIFFERENTIAL RELAY operation of the Transformer for external faults.

2. Grounding transformers(zigzag-star connected) are used to provide paths for zero sequence currents on delta connected circuits.

At the moment I had no means of sending you a DIAGRAM of how the connections look like. However, the NAMEPLATE DATA on the Transformer at times shows the connection of how the OPEN DELTA looks like.

Please check the nameplate data of your transformer.

Regards

Dear sir

Thank you very much to answer, but why not consider connection OPEN DELTA by resistance to earth short-circuit

thanks for all

Please visit www.i-gard.com and they have the answers to your questions in their technical publications.