Traditionally, conductivity, a key parameter for evaluating the probability of electrostatic discharge, has been subscribed to the belief that it is a constant value. However, in actual working conditions, such as the low earth orbit environment, the conductivity value of dielectrics is not constant due to the variation in temperature. Therefore, it is of great practical significance to investigate the conductivity value of dielectrics in the time-varying field and explore the hidden mechanism of the charging phenomenon. In this paper, we investigated the dynamic conductivity value using polyimide during four temperature cycles. The result shows that there is a positive correlation between conductivity and temperature above the turning point temperature. However, when the temperature is lower than the turning point temperature, the conductivity value hardly changes with temperature. In the four cycles, the conductivity value in the same temperature decreases with the cycles. The space charge distribution test and the thermally stimulated current experiment are carried out to explain the charging phenomenon. The results show that the heterocharge accumulates continually and is captured by deep traps above 0.55 eV. It could be concluded that the thermally stimulated detrapping effect makes the charge trapping difficult, which results in the conductivity value in the same cycle changing with the temperature above the turning point temperature and the heterocharge accumulating. The trap-filling effect inhibits the charge conduction process, which makes the conductivity value in the same temperature to decrease with temperature cycles.

Lightning-induced breaking accidents of medium-voltage insulated conductors pose a serious threat to the safety of distribution networks, and the key cause lies in the establishment and sustained combustion of the power-frequency follow-current arc after lightning overvoltage breakdown. This paper systematically investigates the formation mechanism and critical conditions of power-frequency follow-current arcs using combined simulation and experimental approaches. Based on the streamer discharge theory, a lightning breakdown model was established and combined with the arc energy balance equation, revealing that the establishment of power-frequency follow-current arcs is essentially determined by the post-breakdown energy competition process. The simulation results show that the required anode electric field strength for lightning breakdown is not less than 3 kV/mm. When the power-frequency voltage reaches 10 kV, Joule heating of the arc continuously exceeds heat dissipation loss, enabling restrike after zero-crossing and sustaining stable burning. Experiments verified this voltage threshold and further revealed that the arc establishment rate exhibits nonlinear growth with increasing power-frequency voltage, exceeding 90% at power-frequency voltages ≥ 10 kV. The study also reveals that increased gap distance reduces the arc establishment rate, while the introduction of insulators can enhance it by approximately 20%. This study clarifies the energy criterion for power-frequency follow-current arc establishment and the influence patterns of key parameters, providing theoretical basis and engineering reference for lightning protection design and arc suppression in medium-voltage insulated lines.

Arc erosion induced by lightning-triggered power-frequency follow currents is a primary cause of insulated conductor breakage. However, significant thermal–mechanical differences remain between laboratory low-current tests and field high-current faults, and existing models fail to quantitatively characterize the transition of damage mechanisms under different currents. This study compares the fracture morphologies of two types of broken samples using scanning electron microscopy. The laboratory low-current samples exhibit an oxygen content as high as 41.22% at the fracture surface, accompanied by a reef-like progressive melting morphology, indicating thermal erosion dominance. In contrast, the field high-current samples display extremely low oxygen content, numerous cracks and pores, and pronounced mechanical fracture features. To quantitatively reveal these mechanisms, a magnetohydrodynamic arc ablation model coupled with material phase change was established using a JKLYJ-70 insulated conductor, and simulations were conducted for a typical fault current of 5 kA. Results indicate a maximum arc temperature of 12,974 K. The axial temperature distribution exhibits a “higher-at-ends, lower-in-middle” profile, while the radial temperature distribution approximates a Gaussian distribution. The peak electrodynamic force on the conductor surface reaches 3951 N, significantly exceeding other force components. Theoretical predictions indicate that the breakage time under pure thermal effects is 14.8 s, which is shortened to 2.8 s after incorporating the electrodynamic force, representing a reduction of 81.1%. Further calculations identify a theoretically predicted critical current threshold of approximately 1.45 kA. Above this threshold, the electrodynamic force is dominant, and mechanical fracture occurs prior to melting. Below this threshold, thermal accumulation and melting erosion are dominant, requiring the conductor to reach its melting point before breakage. This threshold reveals the critical condition for the transition from thermal-dominated to force-dominated breakage under thermal–mechanical coupling, providing a general modeling approach for high-temperature arc–conductor interaction analysis and a theoretical basis for differentiated protection design in electrical engineering.

Tree contact discharge is a key contributing factor to wildfires caused by medium-voltage insulated conductors. Prolonged abrasion of the insulation layer by branches gradually creates weak points in the insulation. When subjected to lightning strikes, these areas are prone to forming lightning-induced pinholes, which can subsequently trigger partial discharge and even ignition. This study systematically investigates the discharge-induced ignition mechanism for 10 kV overhead insulated conductors in tree contact scenarios by establishing an experimental platform integrated with high-speed imaging, ultraviolet detection, and simulation methods. Three types of typical defects were set up in the experiments: complete insulation abrasion, lightning puncture holes accompanied by localized abrasion, and lightning puncture holes without abrasion. The development process and characteristics of different discharge forms were observed and analyzed. The results indicate that the tree contact discharge ignition mechanism can be categorized into two types: thermal accumulation and direct arcing. The former occurs when insulation abrasion or composite defects exist, where sustained partial discharge or a high-resistance current leads to gradual heat accumulation, resulting in an ignition delay lasting tens of seconds. The latter occurs when only small defects such as lightning puncture holes exist in the insulation layer. A concentrated arc forms due to gap breakdown under high voltage, leading to a millisecond-level ignition process. The study found that different discharge forms produce significantly distinct ablation and carbonization patterns on both the insulation layer and the branch surface, reflecting differences in energy transfer pathways. Simulation analysis further indicated that the thickness of the insulation layer affects the electric field distribution in the tree contact gap, with the initial discharge field strength decreasing as the thickness increases. This study provides experimental evidence and classification guidance for tree contact fault monitoring, insulation condition assessment, and wildfire prevention and control in medium-voltage distribution networks.