Study On Temperature Rise In Transformer And The Influence On Insulation Aging

The transformer is one of the most important and the most key electrical equipments in power system, and the safe and reliable operation directly relates to the security and stability power system. The insulation condition is the decisive factor of transformer life, and the temperature is the key factor of insulation condition, especially the hot-spot temperature. Therefore, the research of transformer internal temperature and influence on the insulation aging is of positive significance for transformer operation management and life assessment. The calculation method of transformer internal temperature, the method of estimate the actual service life, and the influence of the thermal characteristic parameters to the results of the insulation equivalent aging are researched.Based on the thermoelectricity analogy method, the heat transfer process of internal transformer is transformed into the thermal circuit model, which is easy to be solved. The thermal circuit models for the top oil temperature and hot-spot temperature are established, and the top oil temperature and hot-spot temperature expressions are deduced. Based on the exponential equations solution recommended in IEC60076-7, a calculating method for transformer internal temperature is proposed. The calculation results are close to measured values, especially during the step of load increasing, the results are nearly the same to measured values. On the basis of the thermal circuit models for top oil temperature and the hot-spot temperature, the simple thermal circuit model for the hot-spot temperature is established. The direct expression of the hot-spot temperature to environmental temperature and load coefficient is proposed, and the results are similar to measured values.According to the collection of transformer load and environmental temperature data, typical load and typical environmental temperature are formed. Based on the IEEE insulation equivalent aging model, a transformer actual service life prediction method is proposed. The method considers the changes of load and environmental temperature, and 50 times calculations are preformed. Using Weibull distribution on the results, the actual service life maximum probability value is obtained. Considering the influence of load and temperatures expanding gradually, the actual use life is forecasted. When the average load is less than rated load and little overload, the actual service life is far longer than its design service life. If the overload is often happened, although the average load is less than rated load, load overshoot can cause transformer temperature sharp rise. The transformer insulation aging will accelerate, because of the transformer temperature rise overshoot due to overload.The thermal characteristic parameters of the calculations of transformer internal temperature and insulation equivalent aging are elaborated. The influence of each thermal characteristic parameter in calculation results is analyzed. By varying these variables under various load conditions and by Monte-Carlo simulation with uniform distribution and normal distribution of these variables, their influence in calculation accuracy is obtained. The influence is small under normal load, and the accuracy can meet the estimation needs by using recommended value. But it is great under heavy load, and the thermal characteristic parameters have to be obtained accurately to improve the calculation accuracy.