Question

Why tertiary winding is used in transformers?

Answer 1

Tertiary winding is provided in electrical power transformer to meet one or more of the following requirements-

It reduces the unbalancing in the primary due to unbalancing in three phase load.It redistributes the flow of fault current.Sometime it is required to supply an auxiliary load in different voltage level in addition to its main secondary load. This secondary load can be taken from tertiary winding of three winding transformer.As the tertiary winding is connected in delta formation in 3 winding transformer, it assists in limitation of fault current in the event of a short circuit from line to neutral.

In a star/star connection unbalanced load may result in neutral displacement and third-harmonic currents may circulate between lines and earth. These difficulties may be overcome by providing a delta connected stabilising (tertiary) winding with a rating sufficient to take short-circuit fault currents.