Abstract
The X model is a conventional tool of frequency domain spectroscopy (FDS) technology for assessing the insulation state of oil-impregnated paper (OIP) bushings. However, the accuracy and applicability of the X model are questionable because it overlooked the effect of non-uniform thermal ageing, and there are no models and mathematical formulas that can be applied to non-uniform thermal-aged OIP bushings. To solve this issue, an enhanced X model is proposed by considering the radial and axial non-uniform thermal ageing of OIP bushings. The enhanced X model’s mathematical formula was deduced initially. Then, the complex relative permittivity (CRP) of three types of non-uniform thermally aged OIP bushings was tested. Comparing the calculated CRP based on the enhanced X model to the laboratory-measured CRP reveals that the average relative error of the real part and imaginary part of the CRP are 7.24% and 10.94%, respectively, which indicates that the accuracy and practicability of the enhanced X model are good. It is anticipated that the contributions will provide a theoretical foundation for the state assessment of the OIP bushings under non-uniform thermal ageing.
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