Dry type transformers require minimum maintenance to provide many years of reliable trouble free service. A dry-type transformer has no moving parts and is a completely static solid state device, under normal operating conditions, are usually insure a long and trouble-free life. Unlike liquid fill transformers which are cooled with oil or fire resistant liquid dielectric, dry type units utilize only environmentally safe, CSA and UL recognized high temperature insulation systems. Dry type transformers provide a safe and reliable power source which does not require fire proof vaults, catch basins or the venting of toxic gasses. These important safety factors allow the installation of dry type transformers inside buildings close to the load, which improves overall system regulation and reduces costly secondary line losses.
1. Air Cooled Transformer
Air Natural (AN)
By Air Natural method the generated heat in the transformer is cooled by the circulation of natural air. When the temperature of the transformer becomes higher as compared to the temperature of the surrounding air, thus by the process of natural convection heated air is replaced by the cool air. This method is also known as a self-cooled method. This method is used for cooling the smaller output transformer rating that is up to 1.5 MVA.
Air Forced (AF)
In this method, the heat generated is cooled by the forced air circulation method. With the help of fans and blowers, high velocity of air is forced on the core and the windings of the transformer. As the temperature inside the transformer goes beyond the standard safe level, an alarm is activated, and the fans and blowers are switched ON automatically.this method is used for transformer rating up to 15MVA.
2. Oil Immersed Transformer
Oil Natural Air Natural (ONAN)
Natural convection process is used for this type of cooling. The assembly of the core and windings are placed in the oil immersed tank. As the core and the windings heat up the temperature of the oil in the transformer rises. As a result, the oil moves upward and flows from the upper portion of the transformer tank. This hot oil dissipates heat in the air by natural convection and conduction process, the oil gets cooled by the circulation of natural air and passes through the radiator again for the use of the transformer. This type of cooling is used for the transformer rating up to 30 MVA.
Oil Natural Air Forced (ONAF)
ONAF method is used for the cooling of the transformer of rating up to 60 Mega volts ampere. As discussed above that in ONAN method, the dissipation of heat is taking place by the convection process in which air is naturally circulated to cool down, but in this type, the forced air is used for the purpose of cooling the transformer. Cost is higher as compared to another process where the circulation of oil and air is done naturally because a fan and blowers are attached as an extra cooling equipment in this method.
3. Oil Forced Air Forced (OFAF)
As the name itself says that both the oil and the air are applied by force for cooling of a transformer. The Heat Exchanger is installed through which hot oil is circulated with the help of a pump. Air is forced to pass on the heat exchanger with the help of high-speed fans. This method is similar to ONAN, as when there is low load on the transformer the cooling is done by a simple ONAN method, and as soon as the load is increased, the generated heat will also be more and therefore the sensor gave an alarm that the dissipation of heat exceed the safe value and as a result, the fans and pumps are switched on automatically and thus the cooling takes place by OFAF method.
4. Oil Natural Water Forced (ONWF)
In Oil Natural Water Force cooling method the transformer core and the windings are immersed in the oil tank. A radiator is installed outside the tank aa the temperature rises and the oil heats up and moves upward the heat is dissipated by the natural process of convection and oil is passed through the radiator, but the water is pumped and passed through the heat exchanger for cooling of the oil.
5. Oil Forced Water Forced (OFWF)
A heat exchanger is installed through which both oil and water are passed with the help of a pump. The level and pressure of the oil are always kept higher than that of water so that if any leakage occurs in the system the oil mixes with the water but water does not get mixed up with the oil. This type of method is suitable for large capacity of the transformer having rating as several hundred MVA or where banks of transformers are installed. Mainly this type of cooling is done for the transformer installed at the hydro power plant.