The Tertiary winding with DELTA connection is normally used in Yy connection power & Auto transformers for following purpose.
1. The Tertiary winding used as a path for the Zero sequence current during secondary loaded with unbalanced load. Normally the primary neutral point would not be connected with system ground. Tertiary winding one DELTA point is connected with system ground which is prevent the shifting of secondary neutral point and the voltages become unsymmetrical during unbalanced load condition. Other wise the zero sequence current will flow through the secondary neutral connected with Transformer Tank and tank will get heat up.
2. When an external Earth Fault occurs, the resulting zero sequence currents flow through the 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.
3. Some times a external resistor is used in the path of Delta to ground connection to limit the Current circulation.
4. Some times this winding is used as a station supply and surge arrester are used to prevent the surges from the load side.
The CT in the Grounding conductor will detect ground-faults in the delta winding.