In this article, the surface charge characteristics of the direct current gas-insulated transmission lines (DC-GIL) insulator under thermal–electric coupled fields are investigated. A horizontally installed GIL with a basin-type insulator is employed. The 3-D charge distribution under different ambient temperatures, load currents, and gas pressures is obtained based on the finite-element method. It is found that the charge density increases with increasing ambient temperature since the bulk conduction is promoted due to the increasing insulator temperature. At lower ambient temperature, the surface charge distribution shows a more obvious difference in different radial directions along the insulator due to the increasing difference in electric conductivity in different directions. With increasing load current, the charge density increases, and the difference in charge density in different radial directions becomes more obvious. The charge density increases with increasing gas pressure although the temperature decreases, which is attributed to the domination of decreasing ion mobility over decreasing bulk conduction. It can be concluded that attention should be paid to these conditions when dealing with the insulation property of DC-GIL considering the effect of thermal gradient on the charge accumulation. Besides, simulation based on a 3-D model is necessary, especially for horizontal GILs.

Surface charge accumulation on the insulator of direct current gas-insulated transmission lines (DC-GILs) under thermal-electric multiphysics fields is investigated using the finite element method (FEM). To obtain the accurate temperature and charge distributions in the horizontally installed gas-insulated transmission lines (GILs) with a basin-type insulator, a three-dimensional (3-D) geometric model is constructed. First, the equations and parameters are optimized to improve the computing efficiency. The thermal gradient (TG) negligibly affects the surface electrical conductivity. The recombination and diffusion terms in the ion transport equation can also be removed. After omitting these terms, the calculation efficiency is improved by approximately 70% without affecting the calculation accuracy. The surface charge density increases under TG but varies slightly in different radial directions along the insulator surface. In contrast, the electric field distribution obviously depends on the radial direction. The calculations confirm that the improved method properly analyzes the surface charge characteristics under thermal-electric fields and can thus evaluate the insulation performance of DC-GILs.

Flashover at gas/insulator interface acts as a vital challenge to the safe and stable operation of gas-insulated system especially when involved with surface metallic particle. In this paper, flashover characteristics at SF₆/epoxy insulator interface were investigated concerning on the tracking property based on image analysis. DC flashover experiments were operated for basin-type spacer with metallic particle at various locations. Flashover voltage was obtained and tracking on the surface after successive flashover was captured. Improved image analysis method based on fractal theory was carried out to evaluate tracking pattern quantitatively. The relation between the fractal dimension of tracking and field distribution at particle tip was revealed. Obtained results showed that flashover voltage decreased due to severe field distortion with moving particle from ground to HV electrode. Meanwhile, fractal dimension of tracking covering the area from particle to ground electrode indicated centralized tendency. Simulation result confirmed this variation as the consequence of centralized field distribution at the tip of particle. The quantitative relation between fractal dimension and field distribution suggests new sight to estimate the influence of metallic particle on insulation property in the gas-insulated system.

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.

The temperature properties of real-type direct-current gas-insulated transmission lines (DC-GIL) with a basin-type spacer were investigated by the finite element method in this paper. A horizontally installed model was established and the temperature distribution was obtained with a 3D model. The specific heat capacity and thermal conductivity of the spacer were measured and applied in the simulation. The results show that the temperature of the convex surface was slightly higher than that of the concave surface. With an increase in the SF₆ pressure, the temperature of the spacer decreased, which can be attributed to the improvement of convection due to increases in the heat capacity per unit volume. With an increase in the ambient temperature, the temperature of the spacer increased linearly. The temperature difference between the inner and outer parts of the spacer increased with increases in the load current. Besides, an obvious increase in the surface electric field strength appeared under the influence of the thermal gradient compared to the results without the thermal gradient. Thus, special attention should be paid to the insulation properties of the spacer considering the influence of temperature distribution. This study evaluates both the thermal and insulation characteristics of the GIL along with the spacer under various conditions.