Formation of reference trajectories for transformer oil indicators for 330 kV autotransformers

Abstract

The article describes the procedure of statistical processing of the results of periodic operational tests of transformer oils condition in 330 kV autotransformer tanks with the purpose of forming the reference trajectories of indicators for early diagnosis of transformer oils. The proposed procedure for statistical processing of the results of periodic operational condition tests of transformer oils includes two stages. At the first stage of statistical processing, to reduce the influence of errors characteristic of in-service control, the stationary dependences of transformer oil indicators on the operation time are eliminated according to mathematical expectation. To solve this problem it was proposed to use a mathematical apparatus of single-factor regression analysis of the indicator of oils on the operation time. Practical realisation of such approach has shown that use of the regression analysis allows to reveal not only stationary dependences of oil indicators (that is those dependences for which with growth of operation time no statistically significant change of mathematical expectation of the indicator is observed, that is equivalent to equality to zero of value of angle coefficient of the regression model), but also dependences of oil indicators distorted by a sign of a pair correlation coefficient of oil indicator on operation time. At the second stage of statistical processing the direct formation of reference trajectories of indicators is carried out. To take into account the non-linear nature of the dependences of oil indicators on the operation time and significant differences in the intensity of ageing of transformer oils in individual transformers, due to differences in design and cooling systems of autotransformers, as well as different brands of transformer oils and differences in operating modes of autotransformers, the criterion of maximum correlation of oil indicator on the operation time was used. The use of this criterion allowed to form the reference trajectories of transformer oil indicators in 330 kV autotransformers even in conditions of limited information about operating modes of analysed autotransformers. The given example of practical use of the proposed procedure in the formation of reference trajectories of organic acids content in oil has demonstrated its high efficiency.