Study On Flashover Model Of Disc Suspension Insulators Based On Dynamic Variation Of Pollution Layer Resistance

The pollution flashover of insulators is one of the main factors that threaten the safe operation of power systems.Under the effect of Joule heating from leakage current,the wet pollution layer heats up and dries out gradually.The temperature,thickness and surface conductivity of pollution layer on different positions of insulator surface are different,leading to the dynamic variation of pollution layer resistance with time.Although domestic and international scholars conduct a lot of research on the pollution flashover characteristics of insulators,due to the complexity of the flashover mechanism,the existed models are much different from the actual situation.Therefore,the establishment of the flashover model based on dynamic variation of pollution layer resistance contributes to the deep understanding of formation and propagation mechanism of insulator pollution flashover.It possesses great theoretic and practical significance to the external insulation configuration,as well as design and selection of insulators used for transmission lines.This paper first studies the flashover process of the insulator flat plate model.Then based on the analyses of determining factors on surface conductivity,the uneven distribution of leakage current before and after an arc appears,the discrete algorithm of pollution layer resistance,as well as the ignition and propagation criterion of arc,the flashover model of disc suspension insulators based on dynamic variation of pollution layer resistance is established and validated by artificial AC and DC flashover experiments.The established dynamic model is applied to predict the dynamic variation of leakage current,arc length,pollution layer resistance during flashover process.The relationship between arcing voltage and equivalent salt deposit density(ESDD)is analyzed.The effect laws of ambient temperature,wetness of pollution layer and applied voltage on the flashover characteristics are also obtained.Finally,through the flashover experiments of polluted insulators under salt fog and haze fog,respectively,the concept of additional salt deposit density(ASDD)is proposed by analyzing the experiment results,and then the proposed dynamic flashover model can be used to predict and analyze the flashover of insulators under salt fog or haze fog environments.In this paper,the main work and conclusions are as follows:The relationship between wet pollution layer thickness and non-soluble deposit density(NSDD)is studied by experiments,and the relationship among surfaceconductivity,pollution degree of insulators and pollution layer temperature is researched as well.The configurations of actual insulators are expanded to flat plate model to get the distribution of surface leakage current density by finite element method.By analyzing the variation of electric field strength during the drying process of pollution layer heated by leakage current,the electric breakdown theory is proposed to explain the mechanism of arc ignition and propagation.The drying effect of leakage current on the wet pollution layer increases the electric field until it exceeds the electric breakdown strength of air along surface,the arc is able to ignite or propagate.The ignition or propagation speed of an arc depends on the fulfilment speed of air breakdown condition along surface.The flashover model of disc suspension insulators based on dynamic variation of pollution layer resistance is established for the first time,and the effectiveness and accuracy of this dynamic model is validated by a large amount of artificial AC and DC experiments.For XP-160 porcelain insulators,the average relative error of 50%flashover voltage between calculated values and experiment results are 5.2% and 5.0%for DC and AC conditions,respectively.For LXY4-160 glass insulators,the average relative error of 50% flashover voltage between calculated values and experiment results are 4.8% and 2.8% for DC and AC conditions,respectively.This shows that the calculated values by the dynamic flashover model are in good correlation with experiment results,so the proposed dynamic model can be used to predict the flashover characteristics of polluted insulators for both DC and AC conditions.Based on the dynamic flashover model,the variation process of the pollution layer resistance before and after an arc appears is analyzed.It shows that due to the drying effect of leakage current,the pollution layer resistance decreased by about 30% before an arc appears.After an arc appears,the pollution layer resistance is influenced by both the time-variation of pollution layer surface conductivity and movement of the arc root,leading to the decrease of the pollution layer resistance.The pollution layer resistance decreases with the propagation of the arc,and the pollution layer resistance rapidly declines to zero with fast propagation of arc at the critical flashover moment.According to the analysis of factors influencing pollution flashover performance by the dynamic model,it is observed that the arcing voltage decreases with increasing SDD,and can be expressed as a negative exponential function of SDD.However,the critical flashover arc length has not definite relation with SDD,and is about 68% of the creepage distance of the insulator.With the increase of ambient temperature,theinsulator flashover voltage decreases linearly,the effect of ambient temperature is more notable when SDD is relatively larger.The flashover voltage also decreases with increasing wetness of pollution layer.The flashover voltage obtained by the even-rising method is higher than that obtained by the up-and-down method,and the relative difference of the two applied voltage methods increases with increasing SDD.For even-rising method,the flashover voltage increases with the increase of the applied voltage speed.Based on the flashover experiments of insulators in artificial salt fog and haze fog environments,respectively,it is indicated that the flashover in salt fog and haze fog environments is still pollution flashover in essence.The concept of additional salt deposit density is proposed to quantitatively analyze the additional SDD caused by salt fog and haze fog.Then the connections between general pollution flashover and flashover under salt fog and haze fog conditions are established.Therefore,the proposed dynamic model can be applied to analyze the flashover process and mechanism of insulators in salt fog and haze fog environments.