With the widespread application of composite insulators in transmission lines, exploring the accumulation mechanism of pollution particles on composite insulator surfaces is of importance to ensure the safe and steady operation of the power system. Addressing the current theoretical shortcomings, this study categorises the accumulation process of particles on the insulator surface into three stages, namely ‘spatial motion’, ‘surface collision’, and ‘surface motion’. The motion and rotation velocities in a multi‐physics field are calculated in the spatial motion stage. In the surface collision stage, a parameter called ‘neck height’ is introduced to determine the optimum mechanics theory, and the normal deposition criterion is established. For the surface motion stage, the sliding displacement and rolling displacement on the surface are calculated based on the rotation speed of the particles. A dynamic pollution accumulation model of the composite insulator is established based on the normal deposition criterion and tangential displacement. Finally, numerical simulations are performed by using the finite element method. Simulation results show that the proposed model agrees with the actual insulator pollution accumulation, and the deposition model is still applicable for various types of composite insulators operating in different applied voltages. The deposition probability of particles increases with the increasing particle size. In the surface motion stage, particle displacement increases with particle size and wind velocity.

As the improving of voltage grade for high temperature superconducting (HTS) power devices, the surface flashover phenomenon in liquid nitrogen becomes an important factor that make the HTS devices breakdown. So the study on surface flashover characteristics in liquid nitrogen becomes more important. This paper studied the polarity effect of standard lightning impulse surface flashover under extremely non-uniform electric field in liquid nitrogen by experiment. It is found that flashover voltages under positive impulses is generally higher than that under negative polarity, which is opposite to the results in gas gaps breakdown and on solid-SF6 gas and solid-transformer oil interface. By the analysis of flashover traces and electric field distribution between the triangle-plane electrodes, combining the related surface flashover theory in liquid nitrogen, it is assumed that surface flashover in liquid nitrogen is started from the electron emission from cathode triple-junction. Because of the non-uniformity of electric field distribution, the initial voltages of the electron emission from cathode triple-junction and the forces suffered on the head of flashover streamers are differed. It is the main reason that polarity effect in liquid nitrogen is formed. The results could provide reference for the dielectric design of HTS devices.

As a crucial component of power systems, distribution transformers are indispensable to ensure the sustainability of power supply. In addition, unhealthy transformers can lead to wasted energy and environmental pollution. Thus, accurate assessments and predictions of their health statuses have become a top priority. Unlike assumed ideal environments, however, some complex data distributions in practical scenarios lead to more difficulties in diagnosis. One challenge here is the potential imbalanced distribution of data factors since sparsely occurring factors along with some Unusual High-Risk (UHR) components, whose appearance may also damage transformer operations, can easily be neglected. Another is that the importance weight of data components is simply calculated according to their frequency or proportion, which may not always be reasonable in real nonlinear data scenes. With such motivations, this paper proposes a novel integrated method combining the Two-fold Conditional Connection Pattern Recognition (TCCPR) and Component Significance Diagnostic (CSD) models. Initially, the likely environmental factors that could result in distribution transformer heavy overloads were incorporated into an established comprehensive evaluation database. The TCCPR model included the UHR time series and factors that are associated with heavy overload in both spatial and temporal dimensions. The CSD model was constructed to calculate the risk impact weights of each risky component straightforwardly, in line with the total risk variation levels of the whole system caused by them. Finally, the results of one empirical case study demonstrated their adaptation capability and enhanced performance when applied in complex and imbalanced multi-source data scenes.

The surface discharge of solid dielectric in liquid nitrogen has became one of the key problems in the development of HTS power devices. It is necessary to study the mechanism of surface discharge of the solid dielectric in liquid nitrogen. This paper chose polytetrafluoroethylene(PTFE) and DW-3 epoxy resin as specimens, tested the surface discharge voltages of 2 kinds of specimens with different surface roughness under lightening impulse voltages in liquid nitrogen and discussed the effect mechanism. The experimental results showed that the surface discharge voltage and discharge time of both materials in liquid nitrogen increased as the surface roughness increasing under lightning impulses. The surface roughness affects the surface discharge voltage of the solid dielectric in liquid nitrogen under lightning impulse, mainly in the charge left on the grinding surfaces of solid dielectrics, the heights of projections, and the creepage distance on the dielectric surfaces. The main discharge process is in the gasification path, the effect of solid dielectric on surface discharge in liquid nitrogen is smaller. These reasons make the surface discharge in the liquid nitrogen less affected by the surface roughness. The results further explore the characteristics of surface discharge in liquid nitrogen and could provide methodological guidance to improve the surface discharge voltages in liquid nitrogen.

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.