A simulation on space charge distribution caused by nonlinear conductivity in HVDC cable insulation

The main objective of this work was to study the characteristics of the space charge induced by nonlinear conductivity of high voltage direct current (HVDC) cable insulation under inhomogeneous field. A simplified model of 320kV HVDC cable was built and the space charge polarization and depolarization in cable insulation formed by nonlinear conductivity were studied by simulation with different temperature gradients (0K-50K) under 320kV step voltage. The results show that a space charge density overshoot phenomenon, which the space charge density reaches a peak and then declines gradually to a steady state over time, occurs near the inner electrode during polarization process. This overshoot phenomenon fades by degree from inner electrode vicinity to outer electrode. The greater the temperature gradient is, not only the more obvious the overshoot phenomenon is and the earlier the peak appears, but also the more quickly the polarization reaches the steady. The similar variation of space charge density also exists in depolarization process, but the difference from the polarization is that the charge density peak is opposite polarity. It is also found that enlarging the temperature gradient makes the current increase in both polarization and depolarization. However, the influence of temperature gradient on the rate of current decay during polarization and depolarization is different.