Tertiary Winding

Some high-rating transformers use an extra or tertiary winding in addition to their primary and secondary windings. Such transformers are known as tertiary or three-winding types, and have several applications:

  1. If a transformer has to supply an additional load which must be insulated from the secondary winding, the tertiary winding can support this load.
  2. Phase compensating devices can be supplied by a three-winding transformer’s tertiary winding, which operates at a different voltage to the primary and secondary
  3. The tertiary winding can be used as a voltage coil in a test transformer
  4. Three supply systems operating at different voltages can be interconnected using a three-winding transformer.
  5. Three-winding transformers can be used to load large split-winding generators
  6. Substation requirements for a voltage differing from the primary and secondary windings’ can be met using a three-winding transformer
  7. A tertiary winding connected in delta reduces the impedance offered to zero sequence currents so that a larger earth fault current flows for proper operation of protective equipment. For unbalanced loads it limits the voltage imbalance and permits the flow of third harmonic current to reduce harmonic voltage.

The KVA rating of the tertiary winding will be a very small fraction of the main transformer’s. To allow for possible short-circuit conditions, the tertiary is generally designed to have a rating about one-third of the main winding’s rating.