The insulator in direct current gas-insulated transmission lines (DC-GIL) would suffer discharge risk due to surface charge accumulation under thermal-electric coupled fields. In this paper, the transient surface charge accumulation characteristics of a basin-type DC-GIL insulator is investigated via finite element method based on a three-dimension horizontally installed GIL model. The stationary temperature distribution of the model is obtained and then applied to the transient simulation of charge. Weak form partial differential equation is employed to deal with the ion transportation equation. Equations and parameters in the simulation are optimized to reduce the computing memory and time. Results indicate that the charge accumulation is accelerated due to the promotion of conduction through the insulator under thermal gradient. Higher charge density is obtained under thermal gradient. And the surface charge density of the convex surface is higher due to the promoted conduction. The highest field strength increases and the corresponding location moves along the convex surface during the transient process. This could attribute to the influence of transient charge behavior under thermal gradient on the electric field distribution. This study indicates that the thermal gradient and transient charge accumulation should be considered when dealing with the insulation characteristics of DC-GIL with insulators.INDEX TERMS Gas insulated transmission lines, insulator, surface charge, thermal-electric coupled fields.

The transient surface charge characteristics of a basin-type insulator in horizontally installed SF 6 -filled direct current gas-insulated transmission lines (DC-GIL) are investigated based on a three-dimension (3D) simulation model under thermal-electric coupled stress. The influence of ambient temperature, load current and gas pressure on the surface charge and electric field characteristics is investigated. Simulation results indicate that the surface charge accumulation process is accelerated due to the promotion of bulk conduction under high ambient temperature, high load current and low gas pressure. The transient field variation is accelerated as the consequence. The highest field strength on the convex surface increases and moves towards the enclosure with increasing time. With increasing load current, the field strength increases due to the influence of charge accumulation and temperature-dependent conductivity of the insulator. It is revealed that the transient surface charge under coupled fields should be focused when dealing with the insulation characteristics of DC-GIL insulators during long-term operation. The mentioned operating conditions should be paid attention. This method introduced in this study can help to evaluate the 3D transient charge and field characteristics under various conditions.

Here, the transient surface charge distribution of a basin-type insulator is investigated under thermal-electric coupled fields. Horizontally installed ±200 kV direct current gasinsulated transmission lines (DC-GIL) are employed, and a 3D geometric model is applied. An improved method is introduced in the transient simulation under coupled fields, which involves simplifying geometric model, decoupling calculation, applying weak form partial differential equation, and simplifying ion transport equation. The influence of volume and surface electric conductivity on the transient surface charge and electric field distribution is discussed. With increasing volume conductivity, the transient charge accumulation is accelerated due to the promotion of conduction through the insulator. With increasing volume conductivity, the polarity of the charge on convex surface changes from negative to positive, while it changes from positive to negative with increasing surface conductivity. This is the consequence of the transition in dominant conduction mechanism. Nonmonotonic variation of charge density is observed attributing to the variation transient field distribution. It can be concluded that the influence of volume and surface conductivity should be focused on when evaluating the insulation characteristics of DC-GIL insulators, and the thermal gradient should be considered in dealing with the long-term operating insulators. INTRODUCTIONThe insulator in gas-insulated equipment, for example gasinsulated transmission lines (GIL), acts as the supporting and insulating medium [1]. Usually, the insulator is made of epoxy resin composites with excellent insulating properties. Inside the enclosure, insulating gas is filled up to 4-7 bars. It is reported that flashover would occur at the gas/insulator interface which may be due to the electric field distortion [2, 3]. With the development of high voltage direct current (HVDC) transmission technology, DC-GIL attracts much attention. More obvious surface charge would accumulate on the insulator surface under DC voltage compared with the condition under alternating current (AC) voltage, and this may cause more serious field distortion in DC-GILs [4].