Transformers for high-voltage systems

The designs of transformers for 220-kv. systems do not represent radical departures from designs which have become standard for lower voltages. The solidly grounded neutral system is used and advantage is taken of this to use graded insulation and reduced voltage test. Auto-transformers are used to connect between the 220-kv. system and existing high voltages and are connected star-star with delta tertiary. The tertiary may or may not be used to supply synchronous condensers but it is not used for the supply of power load.