Temperature rise of transformers
• The losses developed in the transformer cores and windings during conversion are converted into thermal energy and cause heating of corresponding transformer parts.
• From its source the heat is directed, due to thermal gradients, to the places where it may be transferred to a cooling medium i.e. to air, or water , depending upon the method of transformers cooling.
• Heat dissipation occurs in the same way as in electrical machines, i.e. by way of radiation and convection.
The path of heat flow is :
(i) From the internal most heated spots of a given part (of core or winding) to their outer surfaces in contact with the oil.
(ii) From the outer surface of a given transformer part to the oil that cools it.
(iii) From the oil to the walls of a cooler, for instance, of the tank.
(iv) From the walls of the cooler to the cooling medium-air or water.
In section (i) the heat is transferred by conduction.
In sections (ii) and (iii) the heat is transferred by convection of the oil.
In section (iv) the heat is dissipated by both convection and radiation.
Methods of cooling of Transformers
The various methods of cooling transformers are
• Air natural
• Forced circulation of oil
• Air blast
• Oil forced-air natural
• Oil natural
• Oil forced-air forced
• Oil natural-air forced
• Oil forced-water forced
• Oil natural-water forced
• The choice of cooling method depends upon the size, type of application and type of conditions obtaining at the site where the transformer in installed.
• Air natural is used for transformers up to 1.5 MVA. Since cooling by air is not so effective and proves insufficient for transformers of medium sizes, oil is used as a coolant.
• In oil natural-air forced method the oil circulating under natural head transfers heat to tank walls. The air is blown through the hollow space to cool the transformer.
• In oil natural-water forced method, copper cooling coils are mounted above the transformer core but below the surface of oil. Water is circulated through the cooling coils to cool the transformer.
• In oil forced-air natural method of cooling, oil is circulated through the transformer with the help of a pump and cooled in a heat exchanger by natural circulation of air.
• Oil is used for almost all transformers except for the transformers used for special applications.
• Both plain walled and corrugated walled tanks are used in oil cooled transformer.
• In oil forced-air forced method, oil is cooled in external heat exchanger using air blast produced by fans.
• In oil forced-water forced method, heated oil is cooled in a water heat exchanger. In this method pressure of oil is kept higher than that of water to avoid leakage of oil.
• Natural cooling is suitable up to 10 MVA. The forced oil and air circulation are employed for transformers of capacities 30 MVA and upwards.
•The forced oil and water is used for transformers designed for power plants.