COOLING METHODS OF A TRANSFORMER.
No transformer is truly an 'ideal transformer' and hence
each will incur some losses, most of which get converted into heat. If this heat is not dissipated
properly, the excess temperature in transformer may cause serious problems like insulation
failure. It is obvious that transformer needs a cooling system.
Cooling of Transformer is the process by which heat generated in the transformer is dissipated or
treated to the safe value. This is achieved by various cooling methods of transformer available.
The major factor for the generation of heat in the transformer is the various losses like hysteresis,
eddy current, iron, and copper loss. Among all the various losses the major contributor of the
heat generation is the copper loss or I2R loss.
Transformers can be divided in two types as (i) dry type transformers and (ii) oil immersed
transformers. Different cooling methods of transformers are –
For dry type transformers
Air Natural (AN)
By Air Natural method the generated heat in the transformer is cooled by the circulation of
natural air. When the temperature of the transformer becomes higher as compared to the
temperature of the surrounding air, thus by the process of natural convection heated air is
replaced by the cool air. This method is also known as a self-cooled method. This method is used
for cooling the smaller output transformer rating that is up to 1.5 MVA
Air Forced (AF) or Air Blast
In this method, the heat generated is cooled by the forced air circulation method. With the help
of fans and blowers, high velocity of air is forced on the core and the windings of the transformer.
As the temperature inside the transformer goes beyond the standard safe level, an alarm is
activated, and the fans and blowers are switched ON automatically. this method is used for
transformer rating up to 15MVA.
Oil immersed type transformer is cooled by the oil air cooling method and oil water cooling method.
Oil Natural Air Natural (ONAN)
Natural convection process is used for this type of cooling. The assembly of the core and windings
are placed in the oil immersed tank. As the core and the windings heat up the temperature of the
oil in the transformer rises. As a result, the oil moves upward and flows from the upper portion
of the transformer tank. This hot oil dissipates heat in the air by natural convection and
conduction process, the oil gets cooled by the circulation of natural air and passes through the
radiator again for the use of the transformer. This type of cooling is used for the transformer
rating up to 30 MVA.
Oil Natural Air Forced (ONAF)
ONAF method is used for the cooling of the transformer of rating up to 60 Mega volts ampere.
As discussed above that in ONAN method, the dissipation of heat is taking place by the convection
process in which air is naturally circulated to cool down, but in this type, the forced air is used for
the purpose of cooling the transformer.
The cooling of oil will be faster if the area of the tank of the transformer is increased finally which
result in the increase in heat dissipation level. As the fans and blowers are installed, a high velocity
of air is forcefully applied to the radiator and cooling towers which will help in cooling oil more
quickly and efficiently. Cost is higher as compared to another process where the circulation of oil
and air is done naturally because a fan and blowers are attached as an extra cooling equipment
in this method.
Oil Forced Air Forced (OFAF)
As the name itself says that both the oil and the air are applied by force for cooling of a
transformer. The Heat Exchanger is installed through which hot oil is circulated with the help of
a pump. Air is forced to pass on the heat exchanger with the help of high-speed fans. This method
is similar to ONAN, as when there is low load on the transformer the cooling is done by a simple
ONAN method, and as soon as the load is increased, the generated heat will also be more and
therefore the sensor gave an alarm that the dissipation of heat exceed the safe value and as a
result, the fans and pumps are switched on automatically and thus the cooling takes place by
OFAF method.
Oil Natural Water Forced (ONWF)
In Oil Natural Water Force cooling method the transformer core and the windings are immersed
in the oil tank. A radiator is installed outside the tank aa the temperature rises and the oil heats
up and moves upward the heat is dissipated by the natural process of convection and oil is passed
through the radiator, but the water is pumped and passed through the heat exchanger for cooling
of the oil.
Oil Forced Water Forced (OFWF)
A heat exchanger is installed through which both oil and water are passed with the help of a
pump. The level and pressure of the oil are always kept higher than that of water so that if any
leakage occurs in the system the oil mixes with the water but water does not get mixed up with
the oil. This type of method is suitable for large capacity of the transformer having rating as
several hundred MVA or where banks of transformers are installed. Mainly this type of cooling is
done for the transformer installed at the hydro power plant.
