Heating test of transformers (part 1)

Test task

The heating test is typical, is performed on the first transformer of a newly developed type and is repeated at least once every 2 years or at other times specified by the technical conditions. Typical tests are also repeated, in whole or in part, in case of changes in the design or manufacturing process transformers, as well as when replacing the materials used, if the specified replacement may affect the characteristics of the transformer. The heating of the transformer is mainly determined by its losses and cooling conditions. and short circuit Rk
P = Pn- (P0 + Pk).
The power lost in the transformer during the conversion of electrical energy is released as heat in the windings, magnetic circuit and other parts of the structure. The generated heat leads to heating of the windings, the mains, individual components of the structure and the oil (for oil-filled transformers). Of the insulating materials used in the transformer, the least heat-resistant are oil (for oil-immersed transformers) and paper insulation. Paper insulation at temperatures above 105 ° C - ages quickly and leads to premature failure of the transformer. From this it can be concluded that the power of the transformer is limited mainly by the heating of its windings, which are in direct contact with the paper insulation and oil. The heating test is determined compliance of this type of transformer with its rated power.
The heating of the transformer depends not only on the magnitude of the losses, but also on the intensity of heat removal, which is transferred from the heated parts to the outer surfaces that remove heat.
So the oil particles in contact with the heated magnetic circuit and windings rise and give up their heat to the lid and walls of the tank, which, in turn, give off heat to the environment. The most heated oil (along the height of the tank) is not in its uppermost layers, but somewhat lower. This is due to the fact that in the places of direct contact of the oil with the lid, the transfer of heat from the oil occurs more intensively. The oil temperature in the horizontal direction is also not the same. Layers of oil
in places where it touches the main sources of heat - the magnetic core and windings have a higher temperature, and in terms of distance from them and approaching the walls
In the tank, the oil temperature gradually decreases. V dry transformers Трансформатор сухой силовой 45кВАheat is removed by air. The efficiency of heat removal depends on the size of the cooling surfaces of the transformer, i.e. the size of the surface of the windings and magnetic circuit in contact with the cooling oil (or air), and on the surface area of the tank in contact with the cooling air. For this purpose, depending on, they are made with channels that increase the surface of the winding in contact with oil. In transformers with a capacity of more than 1000 kVA, cooling channels are also made in magnetic circuits. The cooling surface of the tank is increased by welding steel pipes or by connecting radiators to the tank through which the oil circulates. Low power transformers are made with smooth tanks... The ultimate purpose of the heating test is to determine whether the temperatures of the oil (for oil-immersed transformers), windings and magnetic circuit under continuous load of the transformer with rated power meet the requirements of standards and specifications. The temperature of the windings, oil and magnetic circuit is the sum of the temperature of the cooling medium, which is taken as the ambient temperature Tamb, and the temperature rise of the parts of the transformer T (winding, oil, magnetic circuit) above the ambient temperature t = T + Tamb. All transformers must be suitable for operation at ambient temperatures up to + 50 ° С, and the temperature rise over the ambient air must not exceed the following values:
  • windings ... ... 70 ° С - average temperature
  • magnetic circuit. ... ... 75 ° С - on the surface
  • oils …… .60 ° С - in the upper layers
At present, insulating materials are beginning to be used in dry transformers, which are more heat-resistant than paper made of inorganic electrical insulating materials, using organic silicon varnishes and resins for their bonding. These materials include fiberglass, porcelain, mica, asbestos, etc. They have higher heat resistance than electrical insulating materials made of organic materials (paper, cardboard), and can be used at temperatures exceeding 180 ° C. In oil-filled transformers, the use of heat-resistant insulating materials is impractical, since the oil itself limits the possibility of their use by the permissible heating.
When using insulating materials with increased resistance to heat, the permissible temperature rises of the windings and the magnetic core must be specified in special technical conditions. Due to the fact that the heating of the transformer is determined by the magnitude of the losses, the conditions under which the no-load losses and short-circuit losses, reduced to a temperature of 75 ° C, are maintained for a long time during the heating test.

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