When a transformer is in operation, heat is generated,
due to I2R losses in the windings and core losses.
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The generated heat raises the temperature of the
transformer which could eventually melt the transformer windings and core and/or cause
a fire in the transformer. Therefore, methods are employed to remove this heat
as quickly as possible.
Transformer Cooling Methods
(1) Air
Cooling :- Air cooling is employed in small transformers (below 50kVA).
The transformer heat is carried away by the
surrounding air naturally or by continuous blast of cool air through the core
and the windings.
(2) Oil Cooling :- Oil cooling is employed in
transformers of medium and large ratings.
In this method, transformer windings and core are
immersed in oilwhich acts as both a cooling medium and an insulating medium.
Oil immersed transformers are enclosed in sheet-steel tank.
The heat produced is taken by the oil, heated oil
becomes lighter and rises to the top and its place is taken by the cooler oil
from the bottom of the tank. The heat of the oil is transferred to the walls of
the tank by natural circulation of the oil. The heat is then transferred to the
surroundings through radiation and convection. The oil gets cooler and falls to
the bottom. Thus, a continuous circulation of oil takes place.
Transformer Oil
Oil has certain advantages over air as cooling medium
:-
(1) Transformer oil has greater specific
heat than air so that it will absorb larger quantities of heat for the same temperature rise.
(2) It has greater thermal conductivity than air so
that heat is transferred out more quickly.
(3) Breakdown strength of oil is about six times that
of air. Thus it provides very good insulation and ensures increased reliability
at high voltages.
(4) The oil has high ignition temperature, therefore,
there is less chance of fire.
“Hydrotreated light Naphthenic distillate” is the chemical name of the transformer oil.
Conservator and Breather
Oil cooled transformers are enclosed in steel tanks. Oil is not allowed to come in contact with the
atmospheric air which may contain moisture. The moisture spoils insulating
property of the oil. Thus, an airtight metal drum called conservator is placed above the level of the top of the tank and
connected with it by a pipe. It is partially filled with oil.
When the oil expands or contracts by change in
temperature, there is displacement of air. When the transformer oil cools, the
oil level goes down and the atmospheric air is drawn in. This is known as
breathing. The air coming in is passed through a device called breather
for the purpose of removing moisture from the air. Silica gel is used as the breather.
A dry silica gel is blue coloured and its color changes to pink when it absorbs moisture. The color change is an indication to
replace it with fresh dry sample of silica gel.
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