Why it is needed to cool down the 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.
Transformers can be divided in two types as
- Dry type transformers.
- Oil immersed transformers.
Different cooling methods of transformers are
For dry type transformers
- Air Natural (AN)
- Air Blast
For oil immersed Transformers
- Oil Natural Air Natural (ONAN)
- Oil Natural Air Forced (ONAF)
- Oil Forced Air Forced (OFAF)
- 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 3 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
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 convection, the heated oil flows in the upward
direction and then in the radiator. The vacant place is filled up by cooled oil
from the radiator. The heat from the oil will dissipate in the atmosphere due
to the natural air flow around the transformer. In this way, the oil in
transformer keeps circulating due to natural convection and dissipating heat in
atmosphere due to natural conduction. This method can be used for transformers
upto about 30 MVA.
Oil Natural Air Forced (ONAF): The heat dissipation can be improved further by applying
forced air on the dissipating surface. Forced air provides faster heat
dissipation than natural air flow. In this method, fans are mounted near the
radiator and may be provided with an automatic starting 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 circulation is forced through the heat exchangers. Then compressed air
is forced to flow 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 bottom as shown in the figure. This type of cooling is
provided for higher rating transformers at substations or power stations.
Oil Forced Water Forced (OFWF): This method is similar to OFAF method, but here forced water
flow is used to dissipate hear from the heat exchangers. The oil is forced to
flow through the heat exchanger with the help of a pump, where the heat is
dissipated in the water which is also forced to flow. The heated water is taken
away to cool in separate coolers. This type of cooling is used in very large
transformers having rating of several hundred MVA.
