Study On Dynamic Model For Icing With Wet Growth Process And Ice Flashover Voltage Prediction Of Suspension Insulators

Ice accretion on transmission lines is a fairly complicated natural phenomenon,which is related to fluid mechanics,thermodynamics,heat transfer,meteorology and other issues.Many domestic and foreign experts have done lots of researches on conductor icing for several decades and established a variety of icing forecast models.Due to the complexity of the structure and the ice-shaped irregularity of insulator,it is difficult to develop a model for predicting the growth process of ice accumulation on insulators.Among various kinds of icing,glaze-ice with wet growth process is most threatening to the safety operation of power grid.The reasons can be divided into two parts.The one is that the formation of icicle changes the electric field distribution along insulator surface.The other reason can be described as the conductive ion migration which happens in the process of phase transition and leads to the increase of water film conductivity while ice melting.The combined action of these two factors may contribute to the flashover of ice-covered insulator easily.Therefore,the research on dynamic model for icing with wet growth process and ice flashover voltage prediction of suspension insulator can not only realize the effective monitoring of transmission line icing state,but also has important engineering significance for preventing the flashover of iced insulator.The research method is the combination of the artificial icing test,simulation and theoretical analysis in this paper.First of all,the characteristics of the wet growth icing of insulator are studied.Then,the external gas-liquid two phase flow field of insulator is simulated.On this basis,a model for icing with wet growth process of insulator is established according to the energy and mass conservation law.Finally,the change rule of melting water conductivity of the ice layer and icicle with ice melting time is researched through the measurement test in the melting period.Considering the propagation path of the local arc and the change of the residual ice resistance after the arc is generated,the critical flashover voltage of ice-covered insulator is calculated by combining the dynamic icing model and the AC flashover model.The main research work and achievements are as follows:According to the artificial icing test of insulator,icicle growth process and ice accretion on insulator surface are analyzed by using fluid dynamics numerical simulation software.The results show that the windward side of insulator is directly subjected to the collision of super-cooled water droplets,and the icing is relatively serious.While the icing accretion on the leeward side is mainly due to flowing around insulator and recirculation,the icing is lighter.With the increase of icing time,the blocking effect of icicle on airflow result in the reduction of the number of water droplets colliding with insulator surface,which leads to the collision coefficient decreasing.Therefore,ice accumulation on insulator gradually slows down.The higher the wind speed or the lower the temperature is,the faster the ice growth and the shorter time of icicle bridging shed needs.The collision characteristics and freezing characteristics of ice-covered insulator surface are obtained through the simulation and analysis of fluid dynamics software.Taking the effect of super-cooled water quality on icing and energy conservation law into account,a dynamic model for icing with wet growth process of insulator is established by analyzing the physical process of wet growth icing of insulator.Then,the dynamic growth process of ice thickness,icicle length and icicle diameter are studied by using theoretical analysis and numerical simulation.The validity of the model is verified by the artificial icing test in the climate chamber.The results show that the changing trend of ice thickness,icicle length and icicle diameter with icing time is consistent with that of the artificial icing test,and the simulation results are in good agreement with the experimental results.The influence of freezing water conductivity,melting time,electric field and equivalent salt deposit density on melting water conductivity of the ice layer and icicle is analyzed through the measurement test in the ice melting period.The experimental results show that the melting water conductivity of the ice layer and icicle decreased nonlinearly with the increase of the melting time for different freezing water conductivity.The melting conductivity under energized condition is lower than that under non-energized condition.For the polluted insulator icing,the melting water conductivity of the ice layer first increases,and decreases,and then increases slightly at the interface of the ice layer and insulator surface.However,the melting water conductivity of icicle has been declining with the increase of the melting time.Based on the dynamic icing model and the AC flashover model,the critical flashover voltage of ice-covered insulator under different icing degrees is calculated by considering the path of arc development and change of the residual ice resistance after the arc is generated.The results show that the average relative error between the computed values from the model and the experimental value is 11.9%.At the same time,the influence of the water film conductance obtained by the numerical calculation method and the empirical formula on the flashover voltage of ice-covered insulator is compared and analyzed under severe icing condition.