When a transformer is in operation, heat is generated, due to I2R losses in the windings and core losses.
The generated heat raises the temperature of the transformer which could eventually melt the transformer windings and core and/or cause a fire in the transformer. Therefore, methods are employed to remove this heat as quickly as possible.
Transformer Cooling Methods
(1) Air Cooling:- Air cooling is employed in small transformers (below 50kVA).
The transformer heat is carried away by the surrounding air naturally or by a continuous blast of cool air through the core and the windings.
(2) Oil Cooling:- Oil cooling is employed in transformers of medium and large ratings.
In this method, transformer windings and core are immersed in oil which acts as both a cooling medium and an insulating medium. Oil-immersed transformers are enclosed in a sheet-steel tank.
The heat produced is taken by the oil, heated oil becomes lighter and rises to the top and its place is taken by the cooler oil from the bottom of the tank. The heat of the oil is transferred to the walls of the tank by the natural circulation of the oil. The heat is then transferred to the surroundings through radiation and convection. The oil gets cooler and falls to the bottom. Thus, continuous circulation of oil takes place.
Oil has certain advantages over the air as a cooling medium:-
(1) Transformer oil has greater specific heat than air so it will absorb larger quantities of heat for the same temperature rise.
(2) It has greater thermal conductivity than air so heat is transferred out more quickly.
(3) Breakdown strength of oil is about six times that of air. Thus it provides very good insulation and ensures increased reliability at high voltages.
(4) The oil has a high ignition temperature, therefore, there is less chance of fire.
“Hydrotreated light Naphthenic distillate” is the chemical name of transformer oil.
Conservator and Breather
Oil-cooled transformers are enclosed in steel tanks. Oil is not allowed to come in contact with the atmospheric air which may contain moisture. The moisture spoils the insulating property of the oil. Thus, an airtight metal drum called a conservator is placed above the level of the top of the tank and connected to it by a pipe. It is partially filled with oil.
When the oil expands or contracts by a change in temperature, there is the displacement of air. When the transformer oil cools, the oil level goes down and the atmospheric air is drawn in. This is known as breathing. The air coming in is passed through a device called a breather for the purpose of removing moisture from the air. Silica gel is used as the breather.
A dry silica gel is blue-colored and its color changes to pink when it absorbs moisture. The color change is an indication to replace it with a fresh dry sample of silica gel.
Types of transformer oil
Three basic types of transformer oil.
(1) mineral oil (mostly naphthenic)