Abstract
The stable operation of transformers is an important guarantee of power system security. The temperature distribution of main insulation is different in long-term operation. The temperature difference between the local hottest spot temperature and the average oil temperature in the transformer exceeds 30 K. Therefore, the oil-paper insulation inside the transformer will exhibit non-uniform ageing after long-term operation. At present, frequency domain dielectric response (FDS) technology is widely used in transformer ageing assessment. However, the existing assessment methods are only used for overall insulation ageing assessment. The average moisture content and polymerization degree of the overall insulation between each phase winding are obtained. But it still cannot be used to assess the non-uniform ageing of the transformer's main insulation. The radial, axial, and two directions non-uniform ageing models have been made in the laboratory in this study. The FDS and physicochemical properties of non-uniform ageing models were tested. The variation law of FDS for non-uniform ageing models was analysed. Meanwhile, the dielectric relaxation model of non-uniform ageing for main insulation was established. The method of frequency division iterative calculation was proposed. Combined with the Fessler moisture balance formula of ageing oil-paper insulation, the quantitative assessment method for ageing parameters of the non-uniform ageing model can be realized. Compared the calculated results with the chemical analysis results, the relative error of the polymerization degree calculation is less than 4%, and the error of the moisture content calculation is less than 0.3%. The calculation results can meet the needs of the engineering apply. Therefore, this method can realize the quantitative assessment of transformer main insulation ageing, which is of great significance to assess the transformer insulation state.
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