Direct current (DC) gas insulated transmission lines (GILs) have been widely used in power transmission but might be threatened by partial discharge due to the presence of floating impurities (e.g., dust and metal particles) inside the sealed chamber. In this Letter, by using a 2D fluid model, we characterize the microscopic properties of the partial discharge induced by a floating linear metal particle in SF6 (both the discharge propagation and interaction between space charge and metal particle) under negative high voltage direct current conditions. Due to the strong electronegativity of SF6, the spatiotemporal distributions of the charged species (electrons, positive ions, and negative ions), space charge, and reduced electric field are rather different from those in air. Notably, a negative ion region is observed around the top tip of the metal particle, and it plays an important role in the generation and propagation of primary and secondary streamers in SF6, which may lead to severe motion characteristics of the particle and aliasing of partial discharge signals. Additionally, we analyze the charging process and electric force reversal phenomenon, which may provide a more precise understanding of the underlying mechanisms of the firefly motion previously reported for DC GILs.

In this study, the impacts of humidity on a parallel-rod dielectric barrier discharge (DBD) in atmospheric humid air were numerically examined. Electron transport parameters across varying humidity levels were determined using the Boltzmann equation solver. The modulation effects of photoionization, dielectric permittivity, and secondary emission coefficient on the discharge characteristics under varying humidity levels were analyzed. For all studied conditions, higher humidity delays the streamer onset and reduces the streamer velocity and electron density. The streamer radius was found to initially increase and then decrease during propagation as humidity increases, a phenomenon attributed to the lateral distribution characteristics of the net ionization rate. For a typical condition, the streamer breakdown voltage increases linearly with humidity, with the breakdown field rising by approximately 1% at 100% relative humidity (RH) compared to 0% RH (at 300 K). Photoionization can advance the streamer onset and accelerate its propagation but has little influence on the breakdown voltage, and the impacts of humidity remain consistent with or without photoionization. A higher dielectric permittivity (εr = 4.2–8) can mitigate the impact of humidity, which is attributed to the decreased relative difference in the effective ionization coefficient (αeff) between high- and low-humidity conditions. The influence of humidity is slightly less significant as the secondary electron emission coefficient (γ) increases, primarily attributed to the enhancement of αeff under a higher γ. The above findings provide deeper insights into how humidity regulates streamer dynamics in DBD systems.

Photoionization plays a crucial role in achieving accurate quantitative predictions in SF ₆ streamer simulations, but accurate models for SF ₆ photoionization remain limited, motivating this paper. First, we develop a computational model for SF ₆ photoionization and provide the detailed theoretical modeling process, as well as the comparison between experiment and simulation. A concise summary of model parameters within the comprehensive pressure range of 1–15 atm is provided for direct reference. Then, we perform comparative studies against simplified approaches. The results demonstrate that the proposed model effectively captures the non-local effects of SF ₆ photoionization, enhancing both the spatial numerical convergence and the accuracy of the streamer structure. Finally, we perform comparative studies by artificially increasing the photoionization intensity through multiplying the photoionization source term S _ph by a factor of 50 ( 50 × S ph ) relative to the baseline intensity. Regarding breakdown voltage prediction, 50 × S ph leads to a significant underestimate of the breakdown voltage for positive streamers, introducing errors greater than 0.5 kV, while exerting a small impact on negative streamers. Regarding streamer propagation dynamics, the radius of the positive streamer head exhibits pronounced shrinking, and 50 × S ph reduces this shrinking and significantly lowers the head field by more than 700 Td. In contrast, 50 × S ph has little impact on the morphology of the negative streamers and slightly enhances the head field by less than 30 Td.

A recently proposed Following Streamer mechanism (Feng et al 2025 Phys. Rev. Appl. 23 064039) seeks to explain how floating metal particles induce SF 6 streamer propagation in the combined gap. This mechanism is derived from a two-dimensional (2D) axisymmetric fluid model, which has limitations in describing multiple streamer events in real-world three-dimensional (3D) scenarios. To further study the Following Streamer mechanism, we experimentally investigate the discharge morphology of SF 6 streamers induced by a floating linear metal particle under negative applied voltage. The results are then compared with those from 2D axisymmetric fluid simulations. The comparison reveals both consistencies and discrepancies. Regarding consistencies, experimentally observed features-such as streamer inception at both ends of the metal particle and the formation of subsequent luminous phenomena within the SF 6 streamer channel-are qualitatively consistent with the general idea of the Following Streamer mechanism. Regarding discrepancies, the experiments show a larger number of following streamers and off-axis propagation paths, which cannot be described in the 2D simulation. For scientific rigor, an extended conceptual model was proposed to improve the description of the Following Streamer mechanism, using 3D schematic for visualization.