Electrical Machines, 2nd Edition

Appendix G

Tertiary Windings

There may be the third winding in the transformer which is known as tertiary winding in addition to the primary and secondary windings. These transformers are known as triple-wound transformers. The following are the purposes of tertiary windings:

1. Substation auxiliaries can be supplied at a voltage which are different from those of the primary and secondary windings.
2. Compensating devices can also be supplied.
3. Three supply systems operating at different voltages can be interconnected.
4. It can be used to measure the HV testing transformer.

Figure G.1 shows the equivalent circuit of a three-winding transformer, where Z_p, Z_s and Z_t are the primary, secondary and tertiary windings of the transformer.

Figure G.1 Equivalent ...