Study On The Thermal Characteristics Of A DC Arc Over An Ice Surface And The Dynamic Numerical Simulation In The Time Domain

Partial discharge and arc discharge on ice-covered insulators may cause flashover.Icing has become one of the leading causes of electrical accidents.The discharge phenomena and characteristics under icing conditions have been studied for many years.However,discharge channels’thermal characteristics（thermal diameter and gas temperature）on ice surfaces are still not understood.The heating phenomenon of the discharge channel will directly heat and melt the ice,then change the dielectric characteristics of the ice,and finally affect the discharge development process.Studying the discharge channel’s thermal diameter,gas temperature characteristics,and related physical processes are essential for revealing the arc discharge mechanisms on the ice surface.In this paper,the spectrum,current,voltage,and thermal characteristics（temperature and thermal diameter）of the discharge channel during the formation and development of an arc over an ice surface are studied by experimental research combined with theoretical modeling.The discharge channel’s dynamic thermal characteristics are numerically modeled based on the observed development process and thermodynamic state.This paper set up an experiment platform to measure the emission spectrum,current,voltage,and thermal characteristics of the discharge channel during the corona turn into an arc over an ice surface.Three kinds of electrodes-ice gap distance（1 cm,1.5 cm,and 2 cm）,four kinds of ice thickness（1 cm,1.5 cm,2 cm,and2.5 cm）,and six kinds of applied water conductivity（40μS/cm,70μS/cm,100μS/cm,150μS/cm,200μS/cm,400μS/cm）under DC voltage are studied.In theoretical research,this paper establishes the thermal expansion model of the discharge channel and the arc’s time-domain dynamic model to realize the time-domain dynamic calculation of temperature and thermal diameter when the discharge propagates over the ice surface.The main achievements of this paper are as follows:（1）A calibrated schlieren platform is designed to measure the time-domain thermal characteristics of the discharge channel on the ice surface.An improved Gauss Legendre algorithm is proposed to improve the accuracy of calculating the discharge channel’s refractive index distribution and temperature distribution according to the schlieren image.（2）It is observed that the DC discharge on the ice surface includes four stages:corona discharge,brush discharge,spark discharge,and arc discharge.There are three development paths of the discharge channels during a spark discharge formation,including the air path,the surface path,and the mixing path.Combined with the calculated relationship among particle density,refractive index,the temperature of the discharge channel,and the measured leakage current change rate,this paper proposed that the critical temperature range of DC arc formation on the ice surface is 2000 K～2882 K.（3）The time-domain thermal characteristics of discharge channels in each discharge stage are obtained.The temperature and thermal diameter of corona and brush discharge channels are unchanged,fluctuating around 500 K～700 K and 0.5 mm～0.7mm,respectively.When the brush discharge develops into spark discharge,there is a jump in the thermal diameter and temperature.The jump range from high to low is the air,mixing,and surface paths.During the transition from spark discharge to arc,the discharge channel’s thermal diameter and temperature increase linearly with time.The law of their growth rate is that the air path is the fastest,the mixing path is the second,and the surface path is the slowest.The model reveals that this phenomenon is that the heat transfer medium around the discharge channel is different under the three paths.After the arc is formed,the growth rate of the temperature and thermal diameter of the discharge channel is gradually saturated.When the arc develops along the ice surface,the thermal characteristics are random,but the thermal diameter and temperature increase with leakage current in the time domain.（4）Based on the thermal balance of a DC arc on the ice surface,a time-domain dynamic model of the arc’s thermal characteristics is established.In order to solve the initial value of arc’s particle density under icing and pollution conditions,an iterative perturbation algorithm is proposed,which overcomes the problem of finding a reasonable initial value.In order to calculate the arc’s particle components characteristics under icing and pollution conditions,an improved quasi-Newton iterative algorithm is proposed to improve the convergence and accuracy of the iterative process.In order to solve the numerical calculation problem of arc particle’s collision integral,a discrete projection algorithm is proposed,which provides a quantitative basis for the numerical truncation of the collision integral.（5）Based on the time-domain dynamic model,the effects of different arc particle components and applied water conductivity on the time-domain thermal characteristics of DC arc on ice surface are simulated and analyzed.Results show that the maximum temperature,average temperature,arc length,thermal radius,average development speed,and average expansion speed of arc increase with increased applied water conductivity.The arc’s thermal diameter,arc temperature,and radial temperature distribution decrease with the increase in H₂O content.The arc’s static volt-ampere characteristics increase with H₂O content and decrease with the increased thermal radius.The time-domain thermal characteristics（thermal diameter and temperature）and static volt-ampere characteristics of the arc measured in the test lie between the upper and lower limits predicted by the model,which verifies the rationality of the model.