This is Power Transformer tertiary winding Bushing. The connection of tertiary winding is open delta in YNynd1. If tertiary winding voltage not used in site and it should be solidly ground as like pig. Should not ground only one bushing of open delta. Because at closed delta point the induced EMF in tertiary winding three phases are zero and the leakage current developed due to unbalance voltage will flow to ground. In open delta point the potential difference is very high and it will make heavy current flow ( Direct short) to the ground and protection will operate.
