Why we use three winding transformers?

In this area of Canada, three winding transformers are used for several reasons. They are used to supply two secondary voltage levels [such as 13.8kV and 27.6kV]. They are also used to reduce fault levels at the secondary bus level by effectively doubling the transformer per unit impedance on large station transformers up to 75/125 MVA ratings.

Another reason for using a three winding transformer is to feed two redundant busses on the secondary side. This has been a common practice in older nuclear power plants. Both the auxiliary transformer and the startup transformer have three windings with two identical secondary windings. One winding typically feeds the "A" three phase bus and the other feeds the "B" three phase bus. In these older nuclear plants there is typically a third smaller transformer with two secondary windings that is sized to carry the Div. 1 and Div. 2 safety related busses from a second offsite power source. Each of these safety related busses are backed up by their own diesel generator. This configuration complies with the appropriate General Design Criteria at the time that the units were originally constructed. Newer nuclear plants typically have opted for redundant two winding transformers, such that if one transformer fails you can still operate at least one of the two redundant Balance of Plant systems or one of the two redundant safety related transformers. This provides for much better reliability and flexibility to be able to continue to operate with a single failed transformer.

Many transmission level auto-transformers have a delta connected winding to circulate the triplen harmonic currents that are created by the auto-transformer. Substations with a large auto-transformer, say 230kV to 115kV, will utilize the delta-connected winding to provide station power at 13.2kV for example. Proper transformer selection can also be used to magnetically annihilate selected harmonics with the use of phase-shifted secondary windings with zig-zag windings that ameliorate triplen harmonics.