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Dry-type transformers generally trip at about 130 degrees, and oil-immersed transformers trip at about 90 degrees. The specified ambient temperature for transformers is 40°C, the minimum temperature for outdoor transformers is -25°C, and the minimum temperature for indoor transformers is -5°C. .
The insulation level of the transformer is not the concept of insulation strength, but the standard of allowable temperature rise, that is, the insulation level refers to the heat resistance level of the insulating material used, which is divided into A, E, B, H, and F levels. The specific allowable temperature rise standards are as follows:
Insulation class A, E, B, H, F maximum allowable temperature °C: 105, 120, 130, 155, 180
Winding temperature rise limit ℃: 60, 75, 80, 100, 125
Performance reference temperature °C: 80, 95, 100, 120, 145
The difference between the temperature of the transformer and the temperature of the surrounding air is called the temperature rise of the transformer.
The “rated temperature rise” on the transformer nameplate refers to the temperature higher than the “standard environment” generated by the heat generated by the transformer during operation. For oil-immersed transformers, its value is equal to the upper oil temperature of the transformer minus the “standard environment temperature”. For dry-type transformers, its value is equal to the temperature of the transformer winding minus the “standard ambient temperature”.
The so-called “standard ambient temperature” is stipulated in my country as 40°C. The purpose of specifying the standard ambient temperature is to distinguish between using transformers in different places and in different seasons to have a unified reference standard.
The start-stop and over-temperature alarm value of the temperature controller of the dry-type transformer should be set according to the insulation level of the transformer. The principle is not to exceed the maximum allowable temperature.
Take the most common F-class and H-class as examples: F-class can be set to start the fan at 95°C, stop the fan at 75°C, alarm for ultra-high temperature at 110°C, and trip for over-temperature at 130°C. H level can be set to start the fan at 105°C, stop the fan at 85°C, over-temperature alarm at 130°C, and over-temperature trip at 150°C. Of course, this setting value can be adjusted according to the actual situation.
Reasons for low temperature setting of oil-immersed transformers:
Because oil-immersed transformers are mostly A-level insulation, the maximum heat-resistant temperature is allowed to be 105°C, and the winding temperature of the transformer during operation is 10-15°C higher than the average temperature of the upper layer oil, that is, when the upper layer oil temperature reaches 85-95°C during operation. ℃, the winding has actually reached about 105 ℃. If it is operated under this limit temperature for a long time, the winding insulation will be seriously aged, and the deterioration of the insulating oil will be accelerated, which will affect the service life. Therefore, the upper oil temperature can be set at 65 ℃. , Stop the fan at 55°C, alarm for over-temperature at 80°C, and trip for over-temperature at 90°C.
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