It's a 160MVA STAR-STAR POWER TRANSFORMER, 220KV/66KV/11KV.
11kv side is the tertiary and it is used as an Open Delta and is grounded for reducing the harmonic effect. But I don't have knowledge of...
If we use the tertiary winding of a Power Transformer as an Open Delta and ground it then how could that will be helpful in reducing the harmonics of system?