Effects of Trapping Characteristics on Space Charge and Electric Field Distributions in HVDC Cable under Electrothermal Stress

Abstract

Space charge behavior has a strong impact on the long-term operation reliability of high voltage–direct current (HVDC) cables. This study intended to reveal the effect of trap density and depth on the space charge and electric field evolution behavior in HVDC cable insulation under different load currents and voltages by combined numerical bipolar charge transport (BCT) and thermal field simulation. The results show that when the load current is 1800 A (normal value), the temperature difference between the inside and the outside of the insulation is 20 °C, space charge accumulation and electric field distortion become more serious with the increase in the trap depth (Et) from 0.80 to 1.20 eV for the trap densities (Nt) of 10 × 1019 and 80 × 1019 m−3, and become more serious with the increase in Nt from 10 × 1019 to 1000 × 1019 m−3 for Et = 0.94 eV. Simultaneously decreasing trap depth and trap density (such as Et = 0.80 eV, Nt = 10 × 1019 m−3) or increasing trap depth and trap density (such as Et = 1.20 eV, Nt = 1000 × 1019 m−3), space charge accumulation can be effectively suppressed along with capacitive electric field distribution for different load currents (1800 A, 2100 A and 2600 A) and voltages (320 kV and 592 kV). Furthermore, we can draw the conclusion that increasing bulk conduction current by simultaneously decreasing the trap depth and density or decreasing injection current from conductor by regulating the interface electric field via simultaneously increasing the trap depth and density can both effectively suppress space charge accumulations in HVDC cables. Thus, space charge and electric field can be readily regulated by the trap characteristics.