When the winding temperature of transformer is in the range of 80 ~ 130 ℃, the insulation aging speed will be doubled for every 6 ℃ rise in temperature, that is, the insulation life will be reduced by 1 / 2 for every 6 ℃ rise in temperature. This is the “six degree principle” or “six degree rule” of transformer insulation aging. For example, the service life of transformers in high latitudes of Central Asia can reach 50 years, while the service life of transformers in the Middle East is generally less than 5 years.
What is insulation aging?
Insulation aging refers to the long-term effect of electric field, temperature, mechanical force, surrounding environment and other factors, which makes the quality of electrical equipment insulation gradually decline and the structure gradually damaged in the process of operation.
What is the allowable temperature and temperature rise for transformer operation?
During the operation of transformer, when the ambient temperature drops a lot, the heat dissipation capacity of the transformer shell will be greatly increased, but the heat dissipation capacity inside the transformer will be little improved. Although sometimes the upper oil temperature of the transformer has not exceeded the rated value, the temperature rise is very high, and the winding will be overheated. Therefore, the allowable temperature rise should be specified during the operation of transformer. Chinese national standard (GB 1094.2): the limit working temperature of transformer winding is 105 ℃（ That is, when the ambient temperature is 40 ℃, Middle East Africa (45-50 ℃), the upper layer temperature should not exceed 95 ℃. Generally, the monitoring temperature (upper layer oil temperature) should be set at 85 ℃ or below. Generally, the temperature of dry-type transformer under rated working condition is between 60-80 ℃. Due to the excessive harmonic current or overload of the system, the working temperature of transformer may reach 130 ℃, which is the temperature measured at the customer’s site. The working temperature of oil type transformer is relatively low, generally less than 60 ℃.