Question

Mention any two applications of three-winding transformers.

Answer 1

The tertiary winding provides the path for the triplen harmonic current and zero sequence current. Due to closed delta tertiary winding, induced triplen harmonic component voltages are reduced by cancellation against voltage drops from corresponding triplen harmonic current in delta. Also, the net triplen harmonic flux in the core is reduced due to opposition from the compensating flux generated by the triplen harmonic currents in the delta side. For these reasons, the tertiary winding is also called stabilizing winding.

However, it is not essential that tertiary winding be used only for stabilization purpose only and it can be used for other purposes as well. Tertiary winding can be used to supply auxiliary load which is at a very different voltage than the secondary winding hence eliminating need of a separate transformer to do so. Generally this winding is used for supplying the station auxiliary loads and charging batteries of relay equipment. Sometimes, it is also used with synchronous condensers for power factor correction.