Looks similar to a wye-delta-wye with one corner of the delta brought out for testing purposes. Those bushings will be tied together, and often grounded.
Well- I know one of the prints show the bushing tied together. David is right, this most likely a wye-delta-wye unit. I will have the actual transformer specs soon.
Two questions:
1. why does the delta have to be grounded?
2. Can the primary remain floating or does it have to be grounded no questions asked?
The H0 and X0 bushings support Davids suggestion of a wye:wye transformer.
If there is a buried delta I wonder why only two corners would be brought out.
David, I am not challenging you or contradicting you. I don't know and I am asking.
Respectfully yours.
Bill
--------------------
"Why not the best?"
Jimmy Carter
One corner, all three windings in series between the two bushings. Grounding the winding reduces stress on the insulation. It's a common wind farm transformer configuration.
Regarding question 2, it looks like you have a choice. Tie the Y bushings together and the closed delta will provide a path for zero sequence to flow, leaving you the option to let H0 float on a three wire system. Or leave the delta open and run it grounded wye-grounded wye.
This is one corner of the delta stabilizing winding provided in Star /star transformers for reducing zero sequence impedance and stabilizing grounding of star neutrals.. The ends will be connected together and earthed. Some times in US this connection is done inside tank and earthed to tank side without bringing it out through bushings. Then it is called buried stabilizing winding. The reason why two terminals are brought out is to test the winding during maintenance. The integrity of winding can be checked,say after a severe LG fault when heavy current will flow through the tertiary. Another reason is in case one feels to avoid this stabilizing winding due to any reason, then this can be kept open with one of the bushing earthed. I had to resort such action when this winding started failing in a group of transformers due to inadequacy in withstanding strength from fault current forces.
