The Transformer is a device used to convert the energy at one voltage level to the energy at another voltage level. During this conversion process, losses occur in the windings and the core of the transformer. These losses appear as heat. The transformer’s output power is less than its input power. The difference is the amount of power converted into heat by core loss and winding losses. The losses and the heat dissipation increases with increase in the capacity of the transformer.
The temperature rise of a transformer can be estimated by the following formula:
ΔT = (PΣ/AT)0.833
ΔT = temperature rise in °C
PΣ = total transformer losses (power lost and dissipated as heat) in mW;
AT = surface area of transformer in cm2.
Cooling of a transformer is the process of dissipation of heat developed in the transformer to the surroundings. The losses occurring in the transformer are converted into heat which increases the temperature of the windings and the core. In order to dissipate the heat generated cooling should be done.
There are two ways of cooling the transformer:
- First, the coolant circulating inside the transformer transfers the heat from the windings and the core entirely to the tank walls and then it is dissipated to the surrounding medium
- Second, along with the first technique, the heat can also be transferred by coolants inside the transformer.
The choice of method used depends on the size, type of applications and the working conditions.
The coolants used in the transformer are air and oil. In dry type transformer air coolant is used and in oil immersed one, oil is user. In the first said, the heat generated is conducted across the core and windings and is dissipated from the outer surface of the core and windings to the surrounding air. In the next, heat is transferred to the oil surrounding the core and windings and it is conducted to the walls of the transformer tank. Finally the heat is transferred to the surround air by radiation and convection.
Based on the coolant used the cooling methods can be classified into:
- Air cooling
- Oil and Air cooling
- Oil and Water cooling
1. Air cooling (Dry type transformers)
- Air Natural(AN)
- Air Blast (AB)
2. Oil cooling (Oil immersed transformers)
- Oil Natural Air Natural (ONAN)
- Oil Natural Air Forced (ONAF)
- Oil Forced Air Natural (OFAN)
- Oil Forced Air Forced (OFAF)
3. Oil and Water cooling (For capacity more than 30MVA)
- Oil Natural Water Forced (ONWF)
- Oil Forced Water Forced (OFWF)
In this method, the heat generated is conducted across the core and windings and is dissipated from the outer surface of the core and windings to the surrounding air.
This method uses the ambient air as the cooling medium. The natural circulation of the air is used for dissipation of heat generated by natural convection. The core and the windings are protected from mechanical damage by providing a metal enclosure. This method is suitable for transformers of rating up to 1.5MVA. This method is adopted in the places where fire is a great hazard.
In this method, the transformer is cooled by circulating continuous blast of cool air through the core and the windings. For this external fans are used. The air supply must be filtered to prevent accumulation of dust particles in the ventilating ducts.
In this method, heat is transferred to the oil surrounding the core and windings and it is conducted to the walls of the transformer tank. Finally, the heat is transferred to the surrounding air by radiation and convection.
Oil coolant has two distinct advantages over the air coolants.
- It provides better conduction than the air
- High coefficient of conduction which results in the natural circulation of the oil.
The transformer is immersed in oil and the heat generated in the cores and the windings is passed on to oil by conduction. Oil in contact with the surface of windings and core gets heated up and moves towards the top and is replaced by the cool oil from the bottom. The heated oil transfers its heat to the transformer tank through convection and which in turn transfers the heat to the surrounding air by convection and radiation.
This method can be used for the transformers having the ratings up to 30MVA. The rate of heat dissipation can be increased by providing fins, tubes and radiator tanks. Here the oil takes the heat from inside the transformer and the surrounding air takes away the heat from the tank. Hence it can also be called as Oil Natural Air natural (ONAN) method.
In this method, the heated oil transfers its heat to the transformer tank. The tank is made hollow, and the air is blown to cool the transformer. This increases the cooling of transformer tank to five to six time its natural means. Normally this method is adopted by externally connecting elliptical tubes or radiator separated from the transformer tank and cooling it by air blast produced by fans. These fans are provided with automatic switching. When the temperature goes beyond the predetermined value, the fans will be automatically switched on.
In this method, copper cooling coils are mounted above the transformer core. The copper coils will be fully immersed in the oil. Along with the oil natural cooling, the heat from the core passes to the copper coils, and the circulating water inside the copper coil takes away the heat. The disadvantage of this method is that since water enters inside the transformer any kind of leakage will contaminate the transformer oil.
In this method, the oil is cooled in the cooling plant using air blast produced by the fans. These fans need not be used all the time. During low loads, fans are turned off. Hence the system will be similar to that of Oil Natural Air natural (ONAN). At higher loads, the pumps and fans are switched on, and the system changes to Oil Forced Air Forced (OFAF). Automated switching methods are used for this conversion such that as soon as the temperature reaches a certain level, the fans are automatically switched on by the sensing elements. This method increases the system efficiency. This is a flexible method of cooling in which up to 50% of rating ONAN can be used, and OFAF can be used for higher loads. This method is used in transformers having ratings above 30MVA.
In this method along with oil cooling, water is circulated through copper tubes which enhance the cooling of transformer. This method is normally adopted in transformers with capacities in the order of several MVA.
In this method, copper cooling coils are mounted above the transformer core. The copper coils will be fully immersed in the oil. Along with the oil natural cooling the heat from the core passes to the copper coils and the circulating water inside the copper coil takes away the heat. The disadvantage in this method is that since water enters inside the transformer any kind of leakage will contaminate the transformer oil. Since heat passes three times as rapidly from copper cooling tube to water as from oil to copper tubes, the tubes are provided with fans to increase the conduction of heat from oil to tubes. The water inlet and outlet pipes are lagged in order to prevent the moisture in the ambient air fro condensing on the pipes and getting into the oil.
In this method, hot oil is passed though a water heat exchanger. The pressure of the oil is kept higher than that of the water. Therefore, there will be leakage from oil to the water alone, and the vise verse is avoided. This method of cooling is employed in the cooling of transformers with very larger capacity in the order of hundreds of MVA. This method is suitable for banks of transformers. Maximum of three transformers can be connected in a single pump circuit. Advantages of this method over ONWF are that the transformer size is smaller and the water does not enter into the transformer. This method is widely used for the transformers designed for hydro electric plants.