The core is built-up of thin steel laminations insulated from each other. This helps in reducing
the eddy current losses in the core, and also helps in construction of the transformer. The steel
used for core is of high silicon content, sometimes heat treated to produce a high permeability
and low hysteresis loss. The material commonly used for core is CRGO (Cold Rolled Grain
Oriented) steel. Conductor material used for windings is mostly copper. However, for small
distribution transformer aluminum is also sometimes used. The conductors, core and whole
windings are insulated using various insulating materials depending upon the voltage.
In oil-immersed transformer, the iron core together with windings is immersed in insulating oil.
The insulating oil provides better insulation, protects insulation from moisture and transfers the
heat produced in core and windings to the atmosphere.
The transformer oil should possess the following qualities:
(a) High dielectric strength,
(b) Low viscosity and high purity,
(c) High flash point, and
(d) Free from sludge.
Transformer oil is generally a mineral oil obtained by fractional distillation of crude oil.
Tank and Conservator
The transformer tank contains core wound with windings and the insulating oil. In large
transformers small expansion tank is also connected with main tank is known as conservator.
Conservator provides space when insulating oil expands due to heating. The transformer tank is
provided with tubes on the outside, to permits circulation of oil, which aides in cooling. Some
additional devices like breather and Buchholz relay are connected with main tank. Buchholz
relay is placed between main tank and conservator. It protect the transformer under extreme
heating of transformer winding. Breather protects the insulating oil from moisture when the cool
transformer sucks air inside. The silica gel filled breather absorbs moisture when air enters the
tank. Some other necessary parts are connected with main tank like, Bushings, Cable Boxes,
Temperature gauge, Oil gauge, Tapings, etc.