Methods of Cooling Transformers

       When transformer supplies a load, two types of losses occur inside the transformer. The iron losses occur in the core while copper losses occur in the windings. The power lost due to these losses appears in the form of heat. This heat increases the temperature of the transformer.
Key Point : To keep the temperature rise of the transformer within limits, it is necessary to dissipate the heat developed to the surroundings.
      A suitable coolant and cooling method is necessary for each transformer to dissipate the heat, effectively to the surroundings.
       Basically there are two types of transformers, dry type transformers and oil immersed transformers. In dry type, the heat is taken to the walls of tank and dissipate to the surrounding air through convection. In oil immersed type, the oil is used as coolant. The entire assembly including core and windings is kept immersed in a suitable oil. The heat developed is transferred to the walls of tank by convection through oil. And finally heat is transferred to the surroundings from the tank walls by radiation.
      The various cooling methods are designated using letter symbols which depend upon :
i) Cooling medium used and ii) Type of circulation employed
      The various coolants used along with their symbols are,
1. Air - A,                    2. Gas - G,            3. Synthetic oil - L,
4. Mineral oil - O,        5- Solid insulation - S   and    6. Water - W
       There are two types of circulations which are,
1. Neutral - N  and  2. Forced - F
       In natural cooling, the coolant circulating inside the transformer transfers entire heat to the tank walls from where it is dissipated to the surroundings and transformers gets cooled by natural air circulating surrounding the tank walls.
      In forced cooling, the coolant circulating inside the transformer gets heated as it comes in contact with windings and core. The coolant partly transfers heat to the tank walls but mainly coolant is taken to the external heat exchanger where air or water is used in order to dissipate heat of the coolant.
1.1 Cooling Methods by Dry Type Transformers
      The cooling methods of dry type transformers are classified as,
1. Air Natural (AN) :
       This method uses atmospheric air as cooling medium. The natural air surrounding the tank walls is used to carry away the heat generated, by natural convection. It is used for small voltage transformers. Due to the available insulating materials like glass and silicon resins now a days, the method can be used for the transformers upto ratings 1.5 MVA.
2. Air Blast (AB) :
       In large transformers, cooling by natural air is inadequate. In such cases, the transformer is located above the air chamber and a blast of compressed air is forced on core and windings with the help of blowers or fans. This improves the heat dissipation and hence higher specific loadings are allowed in dry type transformers. This reduced the size of transformers. The air supply must be property filtered to prevent accumulation of dust particles.
1.2 Cooling Methods for Oil Immersed Transformers
       The oil used as a coolant has following advantages,
1. It is good conductor of heat than air.
2. It has high coefficient of volume expansion. Due to this, adequate circulation is easily obtained.
3. The oil acts as an insulating medium, which increases the insulating strength.
       The only limitation of oil immersed transformers is that these transformers can not be used at places like mines where there are chances of fire hazard.
       The various cooling methods used for such oil immersed transformers are classified as, 
1. Oil natural (ON) :
       The transformer is immersed in oil so heat generated in core and windings is passed on to oil by conduction. The heated oil transfer heat to the tank wall from where it is taken away to the surrounding air. The assembly of oil immersed transformer is shown in the Fig.1.
Fig. 1  Oil immersed transformer

       The tubes are provided on the sides of a transformer tank. The oil in the tank is taken to the tubes. The circulation of oil through tubes causes the cooling.
        The temperature rise of a transformer can be reduced by,
1. Increasing the area of heat dissipation.
2. Decreasing the cooling coefficient.
       As the rating of transformer increases the plain walled tank can not be used. It is necessary to reduce the cooling coefficient. This is achieved by use of some improved methods of cooling.
       The transformers upto 30 KVA use plain walled tanks. But transformer with ratings higher than 30 KVA use corrugations, fins, tubes and radiator tanks. The Fig. 2 shows the fins and corrugations provided on tank walls.
Fig.  2  Tanks with fins and corrugations

       The Fig. 3 shows the tanks with the tubes and the external radiators.
Fig.  3  Tanks with tubes and radiators

       The heat developed inside the transformer is taken outside with the help of oil. The oil is cooled with the help of fins, tubes or external radiations by natural circulation of air.
       Hence these methods are called Oil Natural and Air Natural (ONAN) methods. The tubes are used for transformers upto ratings 5 MVA.
2. Oil Natural Air Forced (ONAF) : 
       In this method, the tank is made hollow and compressed air is blown into the hollow space to cool the transformer. The oil circulating inside takes heat to the tank walls. The method is effective and can be used for large rating transformers. Another way to force air blast is to use elliptical tubes separated from tank walls through which air is forced by fans.
3. Oil Natural Water Forced (ONWF) :
        In this method, the copper cooling coils or pipes are fitted above the core but below the oil surface. The cool water is forced through these coils or pipes which provides the additional cooling where natural water head is available, this method is very cheap. The pipes are provided with fans to increase conduction of heat from oil to pipes. The major disadvantage of this method is, incase of leakage of water. the water can contaminate the oil reducing the dielectric strength of oil.
1.3 Oil Forced Methods With Heat Exchangers
       In these method, forced circulation oil (OF) is the main feature. The motor driven pump is used to force the oil from top of transformer to the external heat exchanger. In the heat exchanger, the oil is cooled with some methods like use of air blast, water blast etc. The cold oil is circulated back to the transformer from the bottom.
      The oil forced methods are classified depending on how the oil is cooled in the heat exchangers. These methods are,
1. Oil Forced Air Natural (OFAN) :
      The oil is circulated with the help of pump and in the heat exchanger it is cooled with the help of natural air. This method is rarely used in practice.
2. Oil Forced Air Forced (OFAF) :
       In the external heat exchanger the compressed air is blasted with the help of fans to cool is the oil. The advantage of this method is at low loads when losses are less there is no need to use the fans to cool the oil. The natural air is sufficient. At higher loads, both fans and pump are switched on by sensing the temperature which improves the cooling. Hence efficiency of this system is higher. The scheme is shown in the Fig. 4.
Fig.  4   Oil forced air forced cooling method

3. Oil Forced Water Forced (OFWF) : 
      In this method, in the heat exchanger instead of air blast, water blast is used to cool the oil. The pressure oil is kept higher than water so oil mixes with water in case of leakage but water dose not mix with oil. Due to this method, smaller transformer size is sufficient as it is not necessary to employ water tubes inside the transformer tank. The method is suitable for transformers having ratings more than 30 MVA. The method is used for the transformers at hydroelectric stations as large water supply with appropriate water head is easily available. The scheme is shown in the Fig. 5
Fig. 5  Oil forced water forced cooling method

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