Since there is no rotating part in the transformer which could produce a draught, therefore, cooling of transformers is more difficult than other electric machines with rotating parts.
In the transformation of energy from one voltage level to another voltage level some energy is lost in between, in the core and in the windings.
All the energy lost in a transformer must be dissipated as heat. Although the energy loss is a small proportion of the total energy undergoing transformation, but it becomes quite large in magnitude in transformers of higher kva rating.
The greater the rating of a transformer, the more difficult it becomes to dissipate the heat, for the kva rating of the transformer increases much more rapidly than the heat-dissipating surface (losses increase in proportion to the weight of transformers i.e. as the cube of their linear dimensions whereas cooling surface area increases only as the square of their linear dimensions).
There are a number of methods used for cooling of transformers. The choice of method depends upon the size, type of application and the type of conditions available at the site where the transformer is
installed.
In so far as cooling methods are concerned, the transformers are either of dry-type or oil-immersed type. The cooling methods used for dry type transformers are:
1. Air Natural (AN) Cooling:
For smaller output (5-10 kva) the external surface is sufficient to dissipate the heat produced by losses.
Such transformers are cooled by the combination of air convection currents within the enclosing case and by natural radiation from the case itself.
Instrument transformers, except for those for the highest voltages, are ordinarily of dry type. Dry-type transformers are frequently used indoor since it is not necessary to install them in fireproof vaults.
2. Air Blast Cooling:
This type of cooling is also used in dry-type transformers. In this method of cooling, heat dissipation
is improved by a continuous blast of air forced through the core and windings. The air blast is produced by means of external fans or blowers.
This type of cooling is limited to transformers of voltage rating not exceeding 25 kv. It is used in situations located in thickly populated places where oil is considered a fire hazard.
Usually, transformers are placed over an air chamber in which the air is maintained under pressure by blowers. The air is forced through core and windings and is discharged through the top of case. The main disadvantage of this process is that increase in insulation strength, due to immersion in oil, is not obtained.
A filter to remove the dust from the cooling air, thus preventing its deposits in the transformer ducts, is desirable.
The cooling methods used in oil-immersed type transformers are:
1. Oil Natural Air Natual(ONAN) Cooling:
This is the most usual method of cooling. Here the assembly of core and windings is immersed in an insulating oil contained in an iron tank. The heat produced in core and windings is passed on to oil by conduction.
Oil in contact with heated parts rises and its place is taken by cool oil from the bottom. The heated oil transfers its heat to the tank surface which dissipates it to the surroundings.
The advantages of this method of cooling are
1. freedom from dust
2. freedom from atmospheric moisture which would have affected the insulation
3. easy extraction of heat from innermost parts as oil is better conductor of heat
4. good amount of natural convection currents since oil has high coefficient of volume expansion.
In many cases smooth tank walls are not sufficient to dissipate the heat, therefore, the tank dissipating surface area is increased by providing ribs, also called the fins, and corrugations.
A corrugated tank has about 50 % more radiating surface than a smooth tank. For further increasing the heat-dissipating capacity radiators or tubes are provided on all four walls of the transformer.
2. Oil Natural Air Forced (ONAF) Cooling:
Here again, the assembly of core and windings is immersed in insulating oil, and cooling is improved by forced air over the cooling surfaces.
The air is forced over external surfaces such as the case, tubes, and radiators, usually by means of fans mounted external to the transformer. Medium to large capacity transformers are cooled by this method.
3. Oil Natural Water Forced (ONWF) Cooling:
For such cooling, the core and windings are immersed in an insulating oil and the cooling is increased by the circulation of water through copper cooling coils mounted above the transformer core but below the oil surface.
The heated water is cooled in a spray pond or cooling tower. This method will be cheaper where a natural water head is already available.
4. Oil Forced Air Natural (OFAN) Cooling:
In this method of cooling, oil is circulated through the transformer with the help of pump and cooled in a heat exchanger by natural circulation of air.
This method proves very useful where the colors have to be well removed from the transformer. This method is not very common.
5. Oil Forced Air Forced (OFAF) Cooling:
In this method of cooling, as shown in fig, the oil is cooled in an external heat exchanger using air blast produced by fans.
It is not necessary that the oil pump and fans may be used at all times. At light loads, say upto 50% of rated load, (when the losses are small) natural circulation of oil (ONAN cooling) may be sufficient to cool the transformer.
At higher loads, the pump and fans may be switched on by temperature sensing elements. This arrangement results in higher efficiency of the system.
6. Oil Forced Water Forced (OFWF) Cooling:
In this method of cooling, the heated oil is pumped out from the main tank to the radiator (heat exchanger) where the oil is cooled by the water passing through copper tubes.
The pressure of oil is kept higher than that of water and, therefore, any leakage that occurs is from oil to water (not from water to oil).
Also, there is no condensation problem in this method. Power transformers with a capacity of 10 MVA have a cooling radiator system with natural cooling.
The oil forced air forced cooling is the usual one for power transformers of rating 30 MVA and higher. Oil forced water forced cooling is used for transformers designed for hydroelectric power plants.