Electric Field Relaxation of Basin Spacer under Variable Temperature Gradient in DC-GIL/GIS

In real applications of the DC gas insulated transmission line (GIL) and gas insulated switchgear (GIS), there are large temperature gradients between the HV conductor and the grounding shell of the spacer, which leads to a completely different electric field distribution compared to that at room temperature. The DC electric field distribution is mainly related to the conductivity parameter of the insulating materials. By applying the coating with conductivity gradient, the surface functionally graded material (SFGM) spacer is considered to be an effective way to uniform the electric field distribution in DC-GIL/GIS. Meanwhile, due to the change of power generation and load, the temperature of the HV conductor also fluctuates with time, which brings difficulties to the optimal design of SFGM spacers under complex working conditions. To optimize the surface electric field distribution of the DC-GIL/GIS basin spacer under variable temperature gradient, based on the surface conductivity graded materials (σ-SFGM), the RT-SFGM spacer with the goal of regulating the surface electric field distribution at room temperature (RT) and the GT-SFGM spacer with consideration of different gradients of temperature (GT) are designed in this paper. Firstly, an electric-heat coupling model of a ±500 kV DC GIL/GIS basin-type spacer is modelled to calculate the electric field distributions under temperature gradients. Considering the gas flow inside the DC-GIL/GIS, the temperature distribution of the spacer is simulated. Experimental results show that the conductivity of the epoxy composites has a strong temperature dependence. As the temperature increases from 30°C to 90°C, the conductivity of the spacer bulk increases by more than 100 times. Compared to the spacer bulk, the conductivity of the coating has a smaller variation with the temperature. Different reference electric fields are selected as optimization goals for the RT-SFGM spacer and the GT-SFGM spacer. Based on the iterative optimization method, the thickness of the RT-SFGM spacer and the GT-SFGM spacer are designed. After iterative optimization, the coating layer thickness of the optimized RT-SFGM spacer decreases from the conductor to the shell, while the coating thickness on the convex surface of the GT-SFGM spacer presents a U-shaped distribution. Simulation results show that, at room temperature, the electric field at the high-voltage triple junction of the uniform spacer is seriously distorted, and the electric field strength of the RT-SFGM spacer and the GT-SFGM spacer at the same position decreases by 53.3% and 49.5%, respectively. With the increasing temperature of the high voltage conductor, the maximum electric field of the uniform spacer gradually transfers to the grounding shell. Under the temperature gradient of 40℃, the GT-SFGM spacer has a better electric field relaxation effect than the RT-SFGM spacer, and the maximum electric field strength decreases by 59.2%. Under current loading and fluctuation conditions, the electric field distribution of the uniform spacer presents a wide range of fluctuation with the current, and the maximum electric field position transfers between the conductor and the shell. The electric field of the GT-SFGM spacer does not change drastically with the current changing, and the electric field change rate is only 7% and 13.1% under current loading and fluctuation conditions, which achieves the stable control of electric field under the variable temperature gradient condition. © 2024 China Machine Press. All rights reserved.