In some transformers extra winding i.e turshury winding is used? where and why it is used?

Answer 1

Transformers with more than 2 windings are fairly common.

In EHV substations, the primary and secondary EHV & HV windings are often configured Wye-Wye (or star-star). In this case, it is common to apply a delta-connected MV tertiary with lower power rating to act as a zero-sequence current trap. Sometimes, these tertiary windings are also used for station service (auxiliary) power as well.

Other multi-winding transformers include 12-pulse rectifier units.
In this case, windings 2 & 3 have the same voltage rating but are connected wye and delta, to introduce a 30 degree phase shift between the two phase groups.
It is common to see a 4th winding that serves the harmonic filters for the rectifier.

Answer 2

Tertiary windings are used to produce separate supply voltages or to provide feedback loops or to act as transrectors.
A transrector winding is wound ir reverse to the secondary and is in series with it. The idea is that high overload currents pass through the tertiary winding and magnetically saturate the core to self limit the amount of current drawn by the primary.

Answer 3

Some power transformers use overvoltage taps to adjust for voltage drop in feeders typically 2.5%
above rating.

Answer 4

sorry, its your bad spelling....

tertiary transformers

yahoo should spew forth high voltage apps

but simply they are big transformers using multiple transformers in series for voltage and in parallel for current