Evaluation of the Relative Aging Rate of the Insulation of the Mineral-Oil-Immersed Power Transformer According to the Data of the IEC 60076–7 Standard

The IEC 60076–7 standard contains a table of the relative aging rates of mineral-oil-immersed transformer cellulose insulation as a function of temperature, moisture, and oxygen content. However, these data are provided for a limited range, with a large and variable humidity step, which makes it difficult to use them in the process of monitoring parameters, to modeling insulation degradation processes and to comparing them with the results of known studies of cellulose insulation degradation processes. The purpose of the work is to increase the accuracy of estimates of the relative aging rate of cellulose insulation of a mineral oil power transformer by detailing the data of the IEC 60076–7 standard. To improve the accuracy of estimates of relative aging rates, a method of interpolation and extrapolation of IEC 60076–7 data is proposed and, on this basis, the contribution of individual accelerated aging factors to the overall relative aging rate is assessed. Using the proposed methods, a table of relative insulation aging rates develops, which details Tables A.3 and A.4 of this standard, and extrapolates data from these tables to typical ranges of factors that are used in modeling practice. As a result, we obtain the components of the relative rate of aging from the effects of temperature, moisture, and oxygen, which allows us to synthesize new data on the relative rates of aging with access to oxygen and different humidity levels of the insulation and to combine two approaches to considering the factors of accelerating aging: multiplicative and additive.