Importance and Types of Transformer Cooling SystemsImportance and Types of Transformer Cooling SystemsImportance and Types of Transformer Cooling Systems

300 W. Antelope Road, Medford, OR 97503-1089, Ph. No. 541 826 2

The load that a transformer carries without heat damage can be increased by using an adequate cooling

system. This is due to the fact that a transformer's loading capacity is partly decided by its ability to

dissipate heat. If the winding hot spot temperature reaches critical levels, the excess heat can cause the

transformer to fail prematurely by accelerating the ageing process of the transformer's insulation. A

cooling system increases the load capacity of a transformer by improving its ability to dissipate the heat

generated by electric current. In other words, good cooling systems allow a transformer to carry more of a

load than it otherwise could without reaching critical hot spot temperatures.

One of the more common types of transformer cooling equipment is auxiliary fans. These can be used to

keep the radiator tubes cool, thereby increasing the transformer's ratings. Fans should not be used

constantly, but rather only when temperatures are such that extra cooling is needed. Automatic controls

can be set up so that fans are turned on when the transformer's oil or winding temperature grows too

high.

Maintenance of Cooling Systems

Dry-Type Transformers: For dry-type transformers, the area in which the transformer is to be

installed should have proper ventilation. This ventilation should be checked prior to installation to

make sure it is adequate. Additionally, the transformer's radiator vents should be kept clear of

obstructions that could impede heat dissipation.

Forced Air: If the transformer's temperature is being kept at acceptable levels by forced air from a

fan, the fan's motors should be checked periodically to make sure they are properly lubricated

and operate well. The thermostat that ensures the motors are activated within the preset

temperature ranges should be tested as well.

Water cooled systems: Systems that are cooled by water should be tested periodically to make

sure they operate properly and do not leak. Leaks can be checked by raising the pressure within

the cooling system, which can be done in various ways. If the cooling coils can be removed from

the transformer, internal pressure can be applied by adding water. Otherwise, pressure checks

can also be made using air or coolant oil, if the coils need to be checked within the transformer

itself.

If the cooling coils are taken out of the transformer, the water cooling system as a whole can be

tested. Here, the coils are filled up with water until the pressure reaches 80 to 100 psi, and left

under that pressure for at least an hour. Any drop in pressure could be a sign of a leak. The

other equipment linked to a water-cooled system can be tested at the same time, such as the

alarm system, water pump and pressure gauges. Also, the water source should be tested to

make sure it has sufficient flow and pressure.

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Types of Cooling Systems

Oil Immersed Natural Cooled

Oil Immersed Air Blast

Oil Immersed Water Cooled

Forced Oil Air Blast Cooled Forced Oil Natural Air Cooled

Forced Oil Water Cooled

Forced Directed Oil and Forced Air Cooling

Liquid coolants: When oil coolants are prepared they are dehydrated, and processed to be free of

acids, alkalis, and sulfur. They should also have a low viscosity if they are to circulate easily. If a

transformer is cooled by oil, the dielectric strength of the oil should always be tested before the

transformer is put into service.

Types of Cooling Systems

For oil immersed transformers, the options for cooling systems are as follows:

Oil Immersed Natural Cooled (ONAN): Here, both the core and the windings are kept immersed

in oil. The transformer is cooled by the natural circulation of this oil. Additional cooling can be

provided by radiators, which increase the surface area over which a large transformer can

dissipate heat.

Oil Immersed Air Blast (ONAF): In this case air is

circulated and the transformer cooled with the

help of fans. Fans allow one to have a smaller

transformer for a given rating, since not as much

surface area is needed for heat dissipation. This

in turn can cut costs.

Oil Immersed Water Cooled (ONWN): Here the

transformer is cooled by an internal coil through

which water flows. This method is feasible so

long as there is a readily available source of a substantial amount of water, which is not always

the case. This kind of cooling has become less common in recent years, abandoned in favor of

Forced Oil Water Cooled (OFWF).

Forced Oil Air Blast Cooled (OFAF): In this case, cooling is accomplished in two ways. Oil

circulation is facilitated by a pump, and fans are added to the radiators to provide blasts of air.

Forced Oil Natural Air Cooled (OFAN): For this type of cooling, a pump is included within the oil

circuit to aid in oil circulation.

Forced Oil Water Cooled (OFWF): Here, a pump within the oil circuit forces the oil to circulate out

through a separate heat exchanger in which water flows.

The most dependable type of cooling system for a transformer is the oil immersed naturally cooled

(ONAN), which also produces the least noise. A forced-air cooled transformer (OFAF) is more efficient,

but it is also noisier and less reliable on account of the possibility of fan malfunction.

The method of forced cooling has been used for many years now to increase the loading capacities of

transformers. A transformer's thermal performance can be directly improved by the implementation of

cooling systems. Consequently, it makes sense to avoid excess heating and accelerated aging within a

transformer by using the appropriate cooling system.