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 aging 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.
*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.
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
* 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.