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TRANSFORMERS COOLING METHODS

NAME:RUSHANTH.C

INDEX NO:090444U

PER DATE:14/09/2010

SUB DATE:29/09/2010


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IMPORTANCE OF TRANSFORMERS COOLING

The core and cooper losses cause heating of transformer . Iron losses : -These are constant losses and takes place in the magnetic core of the transformer.Copper losses: - These are variable losses and takes place in winding of transformer.These losses vary as the voltage varies. Humming losses: -This type of losses is present in larger tr ansformer only . Th ese are du e t o loose wi n di ng c onne cti ons . . This is due to the fact that a transformer's loading capacity is partly decided by itsability to dissipate heat . If the winding hot spot temperature reaches critical levels, the excessheat can cause the transformer to fail prematurely by accelerating the ageing process of thetransformer's insulation . A cooling system increases the load capacity of a transformer byimproving its ability to dissipate the heat generated by electric current . In other words, goodcooling systems allow a transformer to carry more of a load than it otherwise could withoutreaching critical hot spot temperatures .

One of the more common types of transformer cooling equipment is auxiliary fans . Thesecan be used to keep the radiator tubes cool, thereby increasing the transformer's ratings . Fansshould not be used constantly, but rather only when temperatures are such that extra cooling isneeded . Automatic controls can be set up so that fans are turned on when the transformer's oilor winding temperature grows too high .

In rotating machines the heat dissipation is easier due to the turbulent air flow . Transformers require an elaborate cooling system . Small transformers (up to 25 kVA rating) are of dry type . The air convection current andradiation from transformer tank are sufficient to keep the temperature within limits .

A majority of transformers are oil immersed . The core and the windings are completelyimmersed in mineral oil . Oil is a good insulating material and provides better heat dissipationthan air . These transformers can be further classified asunder:

Types of Cooling Systems

For small size transformers up to 5 to 10 KVA, the external surface is sufficient todissipate the heat produced by losses. But for large size of transformers someadditional means of carrying away the heat are provided. M ost common methods of cooling are :

Oil Immersed Natural CooledOil Immersed Air BlastOil Immersed Water CooledForced Oil Air Blast CooledForced Oil Natural Air CooledForced Oil Water CooledForced Directed Oil and Forced

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

Oil Immersed Natural Cooled (ONAN): In this method of cooling of transformer the core and the winding of the transformer is immersedin special type of oil called transformer oil, contained in an iron tank ., both the core and thewindings 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 alarge transformer can dissipate heat .


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Air Natural Cooling :This type of cooling is used for small sized transformers specially of low voltagerating. In this method natural air circulates and no additional means are used. Tomake this method more effective, the area of the contact area of the tank with airshould be increased.

Oil Immersed Forced Oil Cooled Transformer:

In this type of cooling of transformer, the core and the windings are immersed intransformer oil cooling is further increased by circulation of oil through someexternal cooling means.

Oil Immersed Air Blast:

In this case air is circulated and the transformer cooled with the help of fans . Fans allowone to have a smaller transformer for a given rating, since not as much surface area is neededfor heat dissipation . This in turn can cut costs . Oil Immersed Water Cooled:

This is an artificial method of transformer cooling.

In this method , the core and the windings areimmersed in oil and cooling is further increased by circulation of cold water through the warmtubes immersed in oil . 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 substantialamount of water, which is not always the case . This kind of cooling has become less common inrecent years, abandoned in favor of Forced Oil Water Cooled (OFWF) . Forced Oil Air Blast Cooled:

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:For this type of cooling, a pump is included within the oil circuit to aid in oil circulation .

Forced Oil Water Cooled:Here, a pump within the oil circuit forces the oil to circulate out through a separate heat

exchanger in which water flows .

Maintenance of Cooling SystemsDry-Type Transformers:

In this type transform, the area where transform kept must have proper ventilation and must wehave to make sure this should be adequate .as well we should kept transformers radiator inclear .

Forced Air:If we have used fan to induced forced air to maintain acceptable level of temperatureIf the transformer's temperature is being kept at acceptable levels by forced air from a fan, thefan's motors should be checked periodically to make sure they are properly lubricated andoperate well . The thermostat that ensures the motors are activated within the presettemperature ranges should be tested as well .

Water cooled systems: Systems that are cooled by water should be tested periodically tomake sure they operate properly and do not leak . Leaks can be checked by raising the pressurewithin the cooling system, which can be done in various ways . If the cooling coils can beremoved from the transformer, internal pressure can be applied by adding water . Otherwise,


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pressure checks can also be made using air or coolant oil, if the coils need to be checked withinthe transformer itself . If the cooling coils are taken out of the transformer, the water cooling system as a whole can betested . Here, the coils are filled up with water until the pressure reaches 80 to 100 psi, and leftunder that pressure for at least an hour . Any drop in pressure could be a sign of a leak . Theother equipment linked to a water-cooled system can be tested at the same time, such as thealarm system, water pump and pressure gauges . Also, the water source should be tested tomake 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 Transformer is cooled by oil, the dielectric strength of the oil should always be tested before theTransformer is put into service .

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