A. Three phase units are available in three ways :-
a) Tap windings are in series with main HT windings in one core frame and in one tank with OLTC.
b) Tap windings are separated through auto transformer in addition to main transformer, but both the transformers are in one common tank.
c) Tap windings are through an auto transformers in separate tank with OLTC and main transformer is in another tank.
B. Single phase units are available in three ways :-
a) Three single phase units having each unit, consisting of Tap windings in series with main HT windings in one core frame and in one tank with OLTC.
b) Three single phase units having each unit consisting of separate Tap winding through Auto Transformer in addition to main transformer to main transformer and in one tank with OLTC.
c) Three single phase units having each unit consisting of only main transformer. The OLTC & three phase auto transformer shall be in separate tank.