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.

In recent years, the C₄F₇N mixed gas has attracted considerable attention for its outstanding insulation and arc-extinguishing capabilities, positioning it as a potential substitute for sulfur hexafluoride, SF₆. However, there remains a limited understanding of the arc-extinguishing and insulation performance of C₄F₇N/CO₂ mixed gas. In addition, there is limited research on high-current breaking in circuit breakers. Therefore, this study aims to investigate the arc characteristics and breaking behavior of 10%C₄F₇N/90%CO₂ and 15%C₄F₇N/85%CO₂ mixed gases using a magnetohydrodynamic model based on the 252kV air pressure circuit breaker. The dynamic characteristics of this mixed gas are compared with pure SF₆ under short-circuit current breaking conditions, while analyzing different parameters of the C₄F₇N configuration. The results indicate that the mixed gas exhibits lower levels in terms of arc temperature, axial diffusion distance and pressure difference at the moment of arc initiation compared to pure SF₆. Furthermore, increasing the inflating pressure can effectively enhance the breaking performance of the circuit breaker with 0.6 MPa, making it more suitable. Additionally, increasing the proportion of C₄F₇N in the mixed gases will cause the arc temperature to rise slightly at the initial arc and current crossing zero, but decrease at the peak current. The core pressure also rises significantly, with a greater pressure difference established in the compressor at moment of arc initiation. This study provides a reference for the design of an environmentally friendly circuit breaker and the selection of the mixed gas ratio.

To comprehensively evaluate the feasibility of using C ₄ F ₇ N‐based gas mixture as arc extinguishing medium, a comparative study is performed on the variations of the multi‐physical fields, gas flow environment and decomposition characteristics of the gas mixtures. A modified theoretical basis of the arc model developed for the C ₄ F ₇ N‐based gas mixture is presented in the work with particular emphasis on the non‐recombination features of C ₄ F ₇ N. This work also experimentally studies the electrical characteristics of the gas medium and the monitored parameters are also used to calibrate the arc model. The amount change of the seven dominant decomposing by‐products is analysed as well. The results present that the C ₄ F ₇ N/CO ₂ /O ₂ ternary gas mixture exhibits a more promising interrupting performance. Increasing the O ₂ concentration from 5% to 10% has less effects on the arc characteristics, but it could suppress the formation of CO. The arc behaviour of C ₄ F ₇ N/CO ₂ /O ₂ gas mixture is also improved with increasing the velocity of the moving components. The findings provide important insights into the potential of C ₄ F ₇ N‐based gas mixtures as one of the sustainable and effective solutions for high‐voltage interrupting applications.

Here, the transient surface charge distribution of a basin‐type insulator is investigated under thermal–electric coupled fields. Horizontally installed ±200 kV direct current gas‐insulated 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. Non‐monotonic 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.

This paper presents a simplified equivalent circuit model (ECM) for the post‐arc sheath stage in DC vacuum circuit breakers and outlines the method for calculating circuit parameters using the particle‐in‐cell Monte Carlo collision (PIC‐MCC) method. The post‐arc sheath is modelled as a parallel combination of a variable resistor and a variable capacitor. Based on a specific DC experimental case, the PIC‐MCC simulation provides essential parameters of charged particles and electric field data for calculating equivalent resistance and capacitance. The total current through the circuit components is calculated by applying the transient recovery voltage (TRV), and the resulting post‐arc current (PAC) waveform is compared with experimental results to validate the simplified ECM and the proposed parameter calculation approach. Further simulations study the impact of the initial drift velocity of charged particles on equivalent circuit parameters and PAC traces, showing that higher drift velocities increase capacitance and decrease resistance, leading to higher PAC peaks. The comparison with experimental data indicates that considering the drift velocity contributes to the simulation accuracy.

A plasma jet-triggered gas switch (PJT-GS) has been developed as an important piece of equipment to operate in an $\pm$800 kV ultra-high voltage direct current transmission system (UHV DC) to achieve grid system protection and control. The crucial factors that would affect its operational performance, such as the current level the PJT-GS could withstand and the gas gap distance between the two rotating electrodes, are comparatively studied in the present work by analysing the arc dynamic characteristics. The rotating electrode used in the PJT-GS is designed with a helical-slotted structure, and the arc can be rotated circularly driven by the produced transverse magnetic field (TMF) along the electrode edge. The objective of such research is to provide a thorough study of the arc dynamic behaviour during the current flowing process of the PJT-GS and also to characterise the physical mechanism that affects the arc rotation and the PJT-GS operation performance. The magnetohydrodynamic-based (MHD) approach is applied by establishing a 3D arc model. Following such a study, the variation of arc characteristics under different operation conditions could be thoroughly determined and it also could provide the guidance for the PJT-GS optimum design reasonably to support its corresponding engineering applications.