ELECTRICAL ENGINEERING
SUBJECT- DC MACHINE
Year/Sem- 2ND year /4th sem
Date-25/04/2020 Time: 10.30-11.20
UNIT-4 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 insulaton failure. 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.
Different cooling methods of transformers are -
1. 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 3 MVA). In this
method the transformer is allowed to cool by natural air fow 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 fltered to prevent the accumulaton of dust partcles in ventlaton ducts. This method
can be used for transformers upto 15 MVA.
2. Cooling methods for Oil Immersed Transformers
Oil Natural Air Natural (ONAN)
This method is used for oil immersed transformers. In this method, the heat generated in the core and
winding is transferred to the oil. According to the principle of convecton, the heated oil fows in the
upward directon and then in the radiator. The vacant place is flled up by cooled oil from the radiator.
The heat from the oil will dissipate in the atmosphere due to the natural air fow around the
transformer. In this way, the oil in transformer keeps circulatng due to natural convecton and
dissipatng heat in atmosphere due to natural conducton. This method can be used for transformers
upto about 30 MVA.
Oil Natural Air Forced (ONAF)
The heat dissipaton can be improved further by applying forced air on the dissipatng surface. Forced
air provides faster heat dissipaton than natural air fow. In this method, fans are mounted near the
radiator and may be provided with an automatc startng arrangement, which turns on when
temperature increases beyond certain value. This transformer cooling method is generally used for large
transformers upto about 60 MVA.
Oil Forced Air Forced (OFAF)
In this method, oil is circulated with the help of a pump. The oil circulaton is forced through the heat
exchangers. Then compressed air is forced to fow on the heat exchanger with the help of fans. The heat
exchangers may be mounted separately from the transformer tank and connected through pipes at top
and botom as shown in the fgure. This type of cooling is provided for higher ratng transformers at
substatons or power statons.
Oil Forced Water Forced (OFWF)
This method is similar to OFAF method, but here forced water fow is used to dissipate hear from the
heat exchangers. The oil is forced to fow through the heat exchanger with the help of a pump, where
the heat is dissipated in the water which is also forced to fow. The heated water is taken away to cool in
separate coolers. This type of cooling is used in very large transformers having ratng of several
hundreds MVA.
