Thermal characteristics and thermal processes in a transformer

 During the operation of the transformer, part of the electrical energy is spent on losses, turns into heat and is consumed in the environment. The main sources of heat are the windings (losses in them are approximately 80% of all losses), the magnetic system and structural elements.
  When heat is released, the transformer heats up, and the temperature of its individual parts can significantly exceed the ambient temperature. Heating of the transformer is the main reason for limiting its power under load. Indeed, the elements of the transformer metal structures can withstand rather high temperatures without damage, in contrast to insulation, especially paper (class A), which is widely used in transformers. Paper insulation, being exposed to high temperatures for a long time, loses its elasticity, becomes brittle and collapses even from insignificant mechanical forces arising during operation, which leads to loss of electrical strength and failure of the transformer. The higher the temperature of the windings, the more intensive the aging of its insulation occurs. Increasing the temperature of the windings by 8 ° C will approximately halve the life of the insulation. If at a long-term winding temperature of 95 ° C the service life of the transformer is 20-25 years. then at a temperature of 95 + 8 = 103 ° C - only 10-12 years, and at 105 ° C - about 8 years.

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Heating rates for oil transformers

Transformer elements Temperature rise, deg. C Method of measurement
Windings (insulation heat class A):
- with natural circulation or forced with non-directional oil flow
- with forced circulation and with a directed oil flow


 By changing the resistance to direct current
 Oil or other liquid dielectric in the upper layers:
-sealed execution or with an expander;
-performance leaking without an expander; 
 By thermometer or thermocouple
 Surfaces of the magnetic system and elements of metal structures  75  By thermometer or thermocouple









The temperature rise of individual elements of an oil transformer or a transformer with a liquid dielectric over the temperature of the cooling medium, air or water, during heating tests on the main branch, should not exceed the values indicated in the table ♦.

   Exceeding the temperature of individual elements dry transformer above the temperature of the cooling medium during heating tests on the main branch must not exceed those indicated in the table ♦♦. 

Heating rates for dry transformers

Elements of transformers Temperature rise, deg. C Measurement methods
Heat resistance class windings


Surface of the magnetic system and elements of metal structures


No more than permissible for insulating materials in contact with them

By changing the resistance to direct current

Thermometer or thermocouple


However, it is not necessary to constantly force the load, maintaining these calculated temperatures in the transformer, since they are set taking into account the inevitable daily (and annual) fluctuations in the ambient air temperature and load, i.e., taking into account the alternation in operation of the periods of the highest and lowest heating temperatures of the transformer.
     Thermally, the transformer is an inhomogeneous body: steel sheets of the magnetic system, which have high thermal conductivity, alternate with insulating layers, the thermal conductivity of which is low. Likewise, a transformer winding is a complex combination of a highly conductive conductive material (copper and aluminum) with an insulating material that serves as both electrical and thermal insulation.
   During the operation of the transformer, the sheets of the magnetic system and the winding wire serve as constant sources of thermal energy, therefore, in the magnetic system and windings, there is a constant transfer of heat from the inner, more heated parts to the outer surfaces that remove heat. The transformers are manufactured so that the dimensions of these surfaces are sufficient.
    For transformers with a capacity of several kilovolt-amperes, the outer surface of the windings and the magnetic circuit is sufficient to remove the small amount of heat that is released during their operation. The transformers are cooled by the colder ambient air by natural heat radiation. Special devices for their cooling are usually not required. Transformers in which the main cooling and insulating medium is atmospheric air are called air. As the power increases, the losses in the transformer grow in proportion to its mass, that is, approximately proportional to the cube of its linear dimensions. The cooling surface increases in proportion to the square of the linear dimensions, i.e., the losses in the transformer grow faster than the surface that removes heat (according to the law of power growth).
    Starting from a certain power, this surface turns out to be insufficient, and to increase it, channels are made between the coils of the windings and the windings themselves, opening up free access to the cooling air. However, these measures are sufficient only for transformers up to 2500 kVA. A more effective means for removing heat is the use of mineral (transformer) oil, which combines the properties of insulating and heat-dissipating materials. A transformer in which transformer oil serves as the main insulating medium and heat carrier is called oil. Particles of oil filling an oil-filled transformer come into contact with hot surfaces, heat up, rise up and give off their heat through the walls and the tank lid to the ambient air. As the oil cools at the walls, the oil particles move downward, giving way to others, hotter. This method of heat transfer is called convection.
   The temperature of individual parts of the transformer is not the same, it changes along the height of the tank and in the cross section of the transformer. The use of transformer oil as a heat carrier is very effective. Heat transfer per unit surface with oil cooling is 6-8 times higher than with air cooling, therefore, the surfaces of the windings and the magnetic circuit in oil transformers is much smaller than in air transformers of the same power. However, the surface of the tank should be such that the oil temperature does not reach the limit. The easiest way to increase the cooling surface is to increase the size of the tank, but it is not economical, therefore, the surface is increased cooling due to tubular heat exchangers (radiators) installed on the tank. Heat transfer from the surface of the tank occurs both through heated air particles and radiation.
   Cooling of parts of an oil transformer by natural convection of oil and air cooling the outer surface of the tank with cooling elements installed on it is called natural oil and is used for 6300 kVA transformers. Larger transformers use other systems with forced acceleration of oil and cooling air or water.
    There are several ways to force-cool transformers:
1. Dutyeva. Increasing the speed of air movement, cooling the tank and radiators with fans. The fans create forced air circulation along the outer surfaces of the radiators (blast), increasing their heat transfer by 1.5-2 times. The blast cooling system is effective and has an important advantage: when the fan is turned off, the transformer can continue to operate for a long time with a load of up to 50-60% nominal, with natural oil cooling.
2. Circulating. It consists in a forced increase in the speed of both oil and air. Usually, for such systems, special coolers-heat exchangers are used, in which heat is transferred from oil to air, while the movement of oil is forcedly accelerated by an electric pump, and air by fans. The circulation system significantly increases heat transfer. and has another important advantage - due to the compact design of the coolers, the size of the transformer is reduced. However, circulating chillers are only effective when pumps and fans are running at the same time. When it is necessary to reduce cooling (for example, when the load is reduced), one or more chillers are usually turned off entirely.
3. Oil-water. Consists of forced circulation of oil through water-cooled coolers. For this system, special heat exchangers-coolers are used, through the tubes of which heated oil is forcibly pumped; the tubes are in a cavity with cooling water circulating through it. Oil / water cooling is more efficient than other types of cooling due to the increased heat transfer from oil to water. Therefore, such systems are even more compact than circulation systems, since they have an increased heat output. Directional circulation cooling is especially effective, in which oil flows directly through a channel inside the windings, between the windings and into the magnetic system. To ensure the directional movement of oil in the structure, special shields, partitions and other devices are provided.
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