# Non-Uniform Distribution of Contamination on Composite Insulators in HVDC Transmission Lines
## 1 Introduction  
Because of the deterioration of the atmosphere environment in China, the operating HVDC transmission lines are still under high pollution flashover risks [1; 2]. Pollution flashover of insulator is a serious threat to safe operation of the power grid. It risks causing large-scale power outages, and badly impacting development of the social economy [3; 4; 5]. The primary cause of pollution flashover is contaminants accumulating and depositing on the insulator surface. Besides this, it is important to further study insulator contamination rules to better guide the anti-pollution works of transmission line and substation outdoor insulation. Plenty of research [6; 7; 8] has been done through simulation and pollution tests to reveal insulator contamination mechanisms. Literature [6] has put forward a power law relationship between the pollution accumulation rate and wind velocity through a simplified wind tunnel contaminationexperiment. Literature [7] built a pollution deposit model of rail insulator using a gas-solid aerosol flow simulation to show the rules of wind velocity affecting pollution deposits and distribution. Through a simulation and wind tunnel test, literature [8] concluded that the DC voltage results in a larger contaminant deposition rate of particles on the insulator surface and a larger increasing rate than AC voltage and non-energized conditions. Not only does the contamination degree have an effect on flashover voltage, but also the contamination non-uniformity has great influence on flashover voltage, many documents have conducted in-depth research on this [9; 10; 11; 12; 13; 14; 15; 16]. In literature [9], we can find that the uniform distribution of the pollution has given a weaker dielectric rigidity than the one obtained in non-uniform pollution. Besides, the influence of non-uniform pollution distribution on DC composite insulators on their flashover performance was analyzed [10], and a correction formula for the non-uniform pollution flashover voltage was proposed. The test results showed that the flashover voltage U\({}_{50}\) decreases with the increase of T/B, where T/B is ratio of the salt deposit density (SDD) on the top surface to that on the bottom surface. Similar conclusions can be found in references [11; 12]. In literature [13], DC pollution flashover tests of insulators under fan-shaped non-uniform pollution were presented, and the influence of fan-shaped non-uniform pollution on flashover voltage, arc propagation, and critical leakage distance were investigated. Research results indicate that a reduction in the ratio W/L from 1/1 to 1/15 gave a median 35% \(\pm\) 4% decrease in flashover strength, where W/L is ratio of the salt deposit density (SDD) on the windward surface to that on the leeward surface. Thus, in literature [14], artificial pollution tests of 4 types of porcelain and glass insulators with ring-shaped non-uniform pollution were carried out. Experimental results indicate that the non-uniform pollution degree K (the SDD ratio of inner part to outer part) has significant effects on insulator flashover performance. AC (Alternating Current) pollution flashover tests of 7-unit insulators in all three different non-uniform pollution conditions were carried out in reference [15]. The results indicate that the increase of uneven pollution degree \(J\) (\(J=\text{SDD}_{2}/\text{SDD}_{1}\), \(\text{SDD}_{1}\) and \(\text{SDD}_{2}\) represent different parts of the insulator's SDD, specifically, \(\text{SDD}_{2}\) represents SDD of bottom surface, leeward side, inner ring side, \(\text{SDD}_{1}\) represents SDD of top surface, windward side, outer ring side respectively.) from 1 to 15 will give a 39% raise of \(U_{50}\) under non-uniform pollution on top and bottom surfaces, a 23% decrease for fan-shaped non-uniform pollution and a 21% higher \(U_{50}\) with ring-shaped non-uniform pollution.