It depends on its components and maximum temperature rise and hot spot temperature.Usually class F &H Insulators are mostly preferred for winding insulation.But other than this A,B and E class insulators are used to design its various components.

Dear Nithin,

Not any one particular class will be used.

For Example,

Wire enamels are used as inter-turn insulation, I mean the insulation varnish on the winding conductor which falls under Class E. In some cases the impregnated paper is used for the winding which falls under class A.

Glass fibre, mica or porcelain strips/ plates are used for insulation between different layers of windings. The windings will be in different layers for different reasons.

For insulation between the core/windings and the tank, usually the transformer oil will be used which also helps in heat dissipation. 

Porcelain bushings are used for insulation between the terminals and the tank.

The decision about the insulation class depends on the different parts of the transformer in which it 's going to be applied. Thus, the insulation distances, the temperature and the desired heat dissipation play a major role on the final choice. You have to pay attention on the specifications of each and every part of the transformer, i.e. the windings, the core and the tank.

Almost all 400 kV transormers are oil-insulated with Kraft paper, so the most common insulation class is A. Maximum hot spot temperature is kept below 98°C (see IEC 60076-2).

The isulation class depends on several paramters such as:  ambient temperature, desired allowable temperature rise, hot spot, type of load, load fluctuation, overload suitable capability,....

because the insulation systems are classified according to maximum allowable operating temperature which is the sum of ( reference ambient temprature + allowable temperature rise + allowance for hot spot).

But for 400kV transformer, generally,  the insulation class is A (Maximum Operation Temperature Allowed =105°C, Temp. Rise=55 °C) .

Thermal insulation Class = E

Dielectric insulation level Class = II (1425 kV BIL as per  IEEE C57.12.00)