# Wet Snow Flashover Characteristics of 500-kV AC Insulator Strings with Different Arrangements
## 1 Introduction  
In Winter 2015, the city of Tianjin, in northeast China, was struck by a heavy snowstorm which seriously affected the safe operation of its grid. During the snowstorm, the temperature fluctuated around the freezing point so that the snow melting and freezing processes developed alternately. Moreover, the wind direction and velocity, as well as other atmospheric parameters were all favorable to the formation of wet snow according to the meteorological data. The snow filled the gaps between the sheds of composite insulators and icicles formed on the edge of the sheds, thereby degrading the insulation strength. It led to multiple flashovers on the 500-kV transmission lines and caused significant economic losses. Therefore, it is of great significance to study wet snow flashover characteristics of insulators and propose solutions to minimize the flashover incidents.  
Snow can be divided into two categories, wet snow and dry snow, by the process of its formation [1; 2; 3]. When the wet snow falls on the surface of sheds, water droplets infiltrate to thebottom of the snow and accumulate on the edge of the sheds due to gravity [4]. Water may freeze at the insulator surface if enough potential heat is dissipated and icicles can form when the water film is sufficient. The adhesion of wet snow to the insulator surface is much higher than that of dry snow, and it is easier for the icicles to bridge the gaps. Thus, the influence of wet snow on the electrical performance of insulators is more detrimental than that of dry snow [5; 6; 7; 8; 9; 10].  
Due to mechanical and electrical requirements, double suspension insulators with V-string, \(\Lambda\)-string, and II-string arrangements have been used on some transmission lines [11; 12; 13; 14]. Research has been carried out in recent years on the insulator arrangements for three aspects: force analysis [15], electric field analysis, and flashover performance. [PERSON] investigated the force analysis of \(\Lambda\)-strings for 500-kV transmission line and concluded that the optimum angle of ceramic \(\Lambda\)-string arrangements is 60\({}^{\circ}\)-70\({}^{\circ}\)[13]. If the angle is too large, the grip strength of the suspension clamp may exceed its limit. For electric field analysis, [PERSON] calculated the electric field distribution of \(\Lambda\)-string arrangements used for 1100-kV transmission line by the finite element method, and an angle of 90\({}^{\circ}\) was proposed [16]. As for the flashover performance, it seems that insulators with V-string or \(\Lambda\)-string arrangement are at an advantage in preventing the sheds being bridged by the ice and snow [17; 18]. However, as the arc floating from one insulator string to the other in the discharge process facilitates the flashover, the flashover performance of insulators in II-string configuration may be decreased. Furthermore, from the view of installation costs, compared with V-string configuration, \(\Lambda\)-string configuration is easier to install on existing transmission lines to improve their flashover insulation levels because it is less demanding on the tower structure.  
It is known that, for double suspension insulators, the mutual influence between the two adjacent insulators plays an important role in the flashover process [19; 12]. When snow or ice is present, the air gaps formed make this mutual influence more complex. At present, most ice-covered or wet snow flashover tests are performed with a single insulator, and literature on double strings is rare. As wet snow is a serious threat to the grid safety, it is of great importance to find a proper arrangement to minimize flashover incidents. Thus, the main objective of this study is to investigate the wet snow flashover characteristics of 500-kV AC insulator strings with V-string, \(\Lambda\)-string, and II-string arrangements. The results obtained in this study can provide a reference for external insulation designs in regions where transmission lines are frequently subjected to wet snow.