what are the applications of three winding transformers? and what it is exectly ?

Tapan Patel.

Tree winding transformer is a transformer with three windings per phase, thus like in a single trsf with a High Voltage, Medium Voltage and Low Voltage winding.

Most common reason for the addition of a third winding is for the provision of a delta-connected tertiary winding. (connection in delta to provide a low-impedance path for third-harmonic currents)

Other reasons:
To limit the fault level on the LV system by subdividing the infeed - thus double secondary transformers.
Interconnection of several power systems operating at different supply voltages.