Due to the growing demand for electricity, the need for quality, reliability, safety and efficiency of power transformers is increasing. Operators and users of electricity networks want transformers to operate as reliably as possible for as long as possible. This thesis investigates the aging processes and methods for determining the remaining life of power transformers. The research is based on the most commonly used method, which calculates the remaining service life from the annual decline in the degree of polymerization of paper insulation. The degree of polymerization was calculated from the measured concentration of the decomposition product in 2-FAL oil. The method of decreasing the degree of polymerization in the diploma thesis serves as a reference method for the validation of a newer thermal method. Faster ageing is caused by higher temperatures, higher concentrations of water in the paper insulation and oxygen in the oil inside the transformer. These factors are taken into account by the equation for calculating the residual life by the thermal method. The validation of the thermal method is performed on the example of three transformers, where the remaining lifetimes are calculated and compared using both methods. Two of the transformers, A and B, which are still in operation, are used to calibrate the thermal method and to determine the weighted aging temperature. The factor represents the ratio between the calculated weighted and typical operating temperature calculated from the oil samples of transformers A and B. This factor is used for the third, younger transformer C to calculate the weighted aging temperature throughout the service life. In the case of transformer C, check the accuracy and reliability of the thermal method. The thermal method allows calculations of life expectancy under different aging conditions. The aging conditions of the transformer depend on the temperatures, the humidity of the paper insulation and the oxygen content in the oil. Because we do not know the exact conditions of aging, the results of the remaining life spans can be divided into three categories. The categorization of the results is performed by taking into account the standard deviation of the relative humidity measurements of the paper. By analyzing and comparing the categorized results of the remaining life by the thermal method and the results of the reference method of decreasing the degree of polymerization, we estimate which category gives the most reliable and accurate results. The main advantage of the thermal method compared to the reference method of DP decline is the possibility of predicting aging and estimating the remaining lifespan of younger transformers.
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