A New Method to Solve Relative Dielectric Constant of 10kV Three-Core Cables Based on Two-Terminal Zero Sequence Voltage

The relative dielectric constant of insulating layer in the cable is an important parameter characterizing insulation levels. However, the structure of three core cable is complex, whose cores, metal shielding layer and sheath are coupled with each other, so it is difficult to solve the relative dielectric constant. To realize online monitoring of the insulation of three-core cables, this paper proposes a method for solving the relative dielectric constant of 10kV three-core cables based on zero sequence voltage at both ends. Firstly, frequency dependent parameter model was constructed for three-core cables. In combination with zero sequence network, the relationship between the specific value of zero sequence voltage magnitudes at the head and tail ends of the three-core cable and the relative dielectric constant was derived. Then, the least-square parameter identification method is utilized to solve the relative dielectric constant in the relationship. Finally, a cable simulation model is built in PSCAD to obtain the specific value of zero sequence voltage magnitudes at the head and tail ends of the cable in a condition of different frequencies, and the relative dielectric constant is solved by using the least square fitting function in MATLAB. As revealed in simulation results, the derived relationship can accurately reflect variations in the specific value of zero sequence voltage magnitudes at the head and tail ends of the cable along with frequency and the results of the relative dielectric constant solution verify validity of the proposed method.