This heat energy leads to the rise the overall temparature of the transformer, that is the temparature of core, temparature of windings and oil all are increased. And the generated heat must be dissipated evenly, or the overheating may dmage the paper insulation of the copper windings. Any damage in core insulation turns into a short circuit fault. These all necessitates the proper cooling arrangement for the transformer. As the size and capacity of the transformer increased, the associated cooling arrangement become more powerful and sophisticated. So, by definition, the transformer cooling system is such a arrangement for power transformers, which limits the generated heat into a safe value by means of proper dissipitation of generated heat. Different cooling system is used for different types of transformers, and they are discussed as follows.
Transformer Cooling Methods
Generally, two types of transformers are there according to the use of insulating oil, namely Dry Type Transformers and Oil Immersed Type Transformers. In oil immersed type, the transformer core is immersed into the transformer oil. Different types of cooling is needed for these two category. In dry type transformers, air is used as the coolent medium but in oil immersed transformer (as the size and ratings both are high), both air and transformer oil are used as the coolent medium.
| Dry Type Transformers
||Oil Immersed Transformers|
|Air Natural Type|| Oil Natural Air Natural type (ONAN)
|Air forced Type||Oil Natural Air forced Type (ONAf)|
|Oil forced Air forced Type (OfAf)|
|Oil Direted Air forced Type (ODAf)|
|Oil forced Water forced Type (OfWf)|
Now the details of each cooling methods are described as follows.
Air Natural Type: This cooling method is used in dry type transformer with smaller ratings. As the name implies, the natural circulation of atmospheric air is used in this technique. This type of transformers are also referred as self-cooled transformer. When the transformer is operated in full load, then the temparature of the transformer becomes greater than the ambient air temparature. So, by convection process, the light and heated air get replaced by the heavy and comparatively cool surrounding air. In this way, the generated heat is dissipited via the natural air circulation process. But this type of cooling arrangement provides satisfactory opeartion for low voltage transformers only.
Air forced type: This cooling method is also used in dry type transformer but the application is also implemented in oil immersed transformers. As the name implies, in addition with natural air circulation, cool air with high velocity is provided to the core. High speed fans are provided with the transformer, and by the rotation of this fan high velocity of air is subjected to the transformer. This additional air force ensures quicker heat dissipation of the transformers. The fans are automatically controlled, that is when the temparature of the transformer core goes beyond the safe limit than all the fans are switched ON. Air forced cooling method provides better performance than natural air cooling, but additional cost is associated for the fans.
Oil Natural Air Natural (ONAN) Cooling Of Transformer: This cooling system is used in oil immersed type tranformers. This is the most simple way of cooling of oil immersed transformers. We know that, the transformer core is immeresed in transformer oil. When the transformer is heated up, then temparature of oil near to the transformer core is also raised. So, the light and heated oil flows in upward direction and comparatively cool and heavy oil takes the vaccum places by natural convection process. And the heated oil releases its temparature into the atmosphere. In this way, a natural oil circulation cycle is generated and this cycle continues until the transformer temparature is tapped into a safe limit. In this method, the surface area of the oil tank is usually larger, as more surface area provides more quick heat dissipation process. But to provide more surface areas, several hollows tubular plates are attached with the transformer and they are termed as radiator. When the hot oils are are circulating through the radiators, they get more surface area so the cooling rate is much higher. Also the light and heated air get replaced by the heavy and comparatively cool surrounding air by natural process. In this way, the generated heat is dissipited via the natural air circulation process as well as natural oil circulation.
Oil Natural Air forced (ONAF) Cooling Of Transformer: The word ‘forced’ signifies that, air is forcefully applied to the transformer. High speed fans are provided with this type of cooling system. In larger rated oil immersed transformers, natural air and oil cooling is not sufficient. So, additional air force is applied to the radiator by means of those fans and this method provides quicker heat dissipation of the transformers as compared to natural oil and air cooling. All fans are automatically controlled, whenever the temparature of the transformer goes beyond the safe limit than all the fans are switched ON. But here the oil circulation process made by natural convection, that is no oil pumps are provided for this type of cooling. This method provides better performance than natural oil and air cooling, but there is an additional cost due to the fans.
Oil forced Air forced ( OfAf) Cooling Of Transformer: Actually, for very large rated oil immersed transformers, heat generated is quite high. Therefore, some special cooling techniques are applied in order to provide suffiecient heat dissipation. In Oil forced Air forced cooling system, both oil and air are circulated at high speed by some additional configuration. High speed fans are connected to provide additional air flow of high velocity and oil pumps are provided to circulate the oil at high velocity. So, hot oil is circulated inside the main transformer tank at larger velocity, so the rate of cooling is further increased. Therefore, in oil forced air forced cooling system, both the oil and air are forcefully applied to achieve more fast cooling process.
Oil Directed Air forced Cooling Of Transformer: This is the updated version of Oil forced Air forced cooling method. Here the Oil and Air both are applied forcefully, but the hot oil follows a specific route for flowing. Convection channels are made closer to the windings of the transformer and the transformer oil is passed through those channels. In this way, superior heat dissipation is occured.
Oil forced Water forced Cooling Of Transformer: Water is far better coolent than atmospheric air. So, in this method water is used as the oil coolent instead of natural air. Here, the flow of hot oil is directed to a heat exchanger where water shower is applied. So, here the oil is cooled at faster rate than natural air cooling.
Therefore, from the above discussion, we understood the necessity of transformer cooling and also learned about the various types of transformer cooling.