Fractional transmission line model of oil-immersed transformer windings considering the frequency-dependent parameters

摘要

The frequency-dependent characteristics of the impedance parameter and the admittance parameter in the transmission line model of the transformer windings (TWs) are mainly result of the skin effect of the conductor and the complex permittivity's variation with the frequency of oil-paper, respectively. In this study, an accurate fractional transmission line model of TWs considering the frequency-dependent parameters is proposed. The measured permittivity of oil-paper samples is fitted based on the Cole–Cole model with single dielectric relaxation process, and then simplify the fitting model. The structure coefficient per unit length (PUL) corresponding to the complex capacitance of the TWs can be obtained based on the finite element software FEMM. Combining the simplified model with the structure coefficient, the fractional admittance PUL of TWs is derived. Then the fractional transmission line model of the TWs is determined. Finally, the calculated results of fractional transmission line model considering the frequency-dependent parameters are compared with the experimental measurements, and the results demonstrate the accuracy of the proposed model.