No transformer is truly an 'ideal transformer' and hence each will incur some losses (copper
loss and iron loss), which will be converted into heat. If this heat is not dissipated properly
through convection and radiation, the temperature rise in transformer beyond the
allowable limit may cause serious problems like insulation failure which may lead to
reduction of life. It is obvious that transformer needs a cooling system. Transformers can be
divided in two types as (i) dry type transformers and (ii) oil immersed transformers. In no
case temperature should not exceed allowable limit of temperature rise for a particular
class of insulation used.
Different cooling methods of transformers are
For dry type transformers
1. Air Natural (AN)
2. Air Blast
For oil immersed transformers
1. Oil Natural Air Natural (ONAN)
2. Oil Natural Air Forced (ONAB)
3. Oil Forced Air Forced (OFAB)
4. Oil Natural Water Forced (ONWF)
5. Oil Forced Water Forced (OFWF)
Cooling methods for Dry type Transformers
Air Natural or Self air cooled transformer
This method of transformer cooling is generally used in small transformers (upto 1.5 MVA).
In this method the transformer is allowed to cool by natural air flow surrounding it.
Air Blast
For transformers rated more than 3 MVA, cooling by natural air method is inadequate. In
this method, air is forced on the core and windings with the help of fans or blowers. The air
supply must be filtered to prevent the accumulation of dust particles in ventilation ducts.
This method can be used for transformers upto 15 MVA.
Cooling methods for Oil Immersed Transformers
For better cooling the dissipating area of the transformer is to be increased by artificial
means without increasing the size of the tank. The dissipating area can be increased by
1. fitting fins to the tank walls
2. fitting tubes to the tank and
3. using corrugated tank
4. using auxiliary radiator tanks
Oil Natural Air Natural (ONAN)
as the core and the winding heat up the temperature of the oil in the transform ries .as a result the oil moves upward and flows frome the upper portion of the transform tank due to thermo symphonic effect .this hot oil dissipates heat in the air by natural convection and radiation process the oil gets cools by circulation of natural air and passes through the radiator again for the use of the transform .thes type of cooling is used for the transform rating up to 30mvA
Oil Natural Air Blast (ONAB)
Forced air provides faster heat dissipation than natural air flow in thes method fans are mounted near the radiator and provided with and automatic starting arrngement which turns on when temperature increses beyond certain value this transformer cooling method is genarally used for lafge transformers upto about 60MVA
Oil Forced Air Blast (OFAB)
The oil is forced throught the heat exchangers with the help of a pump and air is forced to flow on the heat exxchanger with the help of fans the heat exchangers is mounted separately from the transform tank and connected through pipes at top and bottom as shown in the figure this type of cooling is provided for higher rating transformers at substation or power station.
Oil Forced Water Forced (OFWF)
This method is similar to OFABmethod but here water is forced for dissipating heat from the heat exchangers. the oil is also forced to flow through th heat exchanger with the help of a pump the heated water is taken away to cool in separate coolers the type of cooling is used in large transformers where ample quantity water is available for cooling particularly in hydro power plant
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