Study On The Streamer Characteristics Along The Polluted Insulation Surface And The Development Arc Energy Balance Model Under DC Voltage

For the discharge along the polluted surface involved many fields such as light,heat, electric and so on, and it is also restricted by the condition of experiment andtheoretical basis for a long time, the research on the microcosmic mechanism of thepartial arc formation and development have made a slow progress. At the same time,with the vigorous development of UHV transmission technology with the characteristicsof large capacity, high efficiency and long distance, the security requirements to thepower grid became higher. From a micro mechanism angle to investigate the dischargeprocess along the polluted surface further can provide the technical guidance andtheoretical basis to the insulation coordination design in UHV transmission lines andmake the anti-pollution flashover measures, which has important academic andengineering practical application values.However, there is still a big controversial for the mechanism of the pollutionsurface discharge, and no perfect theory can explain all the observed phenomena in theexperiment reasonably. For the discharge of streamer can not be easily observed directly,the effect of the developing characteristic of the arc head stream on the partial arc didnot have much attention. Meanwhile, the partial arc belongs to high density plasma, theenergy can be exchanged by thermal conduction and radiation, and the effect of energyloss could not to be ignored during the development of arc. For the problems mentionedabove and combined the different views proposed by different researchers, theformation and development mechanism of arc were carried out in this paper emphasized,which can provide the technical guidance and theoretical basis to the insulationcoordination design in transmission lines and make the anti-pollution flashovermeasures.The relations of the streamer radius, velocity and length varied with the streamercurrent and the thermal ionization zone temperature during the development of streamerat arc head was investigated. From the angle of plasma physics, the body force boundeffect on the streamer plasma and the force balance at steady state were analyzed, and amathematical model of streamer radius varied with the current and pressure in streamerwas proposed. Then, take the calculated streamer radius into the expression of streamervelocity, the streamer velocity along the polluted surface can be obtained. Based on thegas discharge theory, and considering the charged particle in streamer formed by the thermal ionization at arc head and distributed uniformly in streamer, a mathematicalmodel of streamer length varied with the current and the thermal ionization zonetemperature at arc head was established.Based on the image method and the electron impact ionization theory, therelationship between the minimal electric field needed to keep arc move ahead and thepollution degree was analyzed. Meanwhile, the arc force analysis was carried out byconsidering the effect of ambient airflow; and an arc velocity calculation model whichis the function of the electric field at arc head was proposed, which can calculate the arcvelocity accurately. After the simulation calculation of tangential electric field at archead along the horizontal polluted glass plate was carried out, the relationship betweenthe arc velocity and arc length was obtained. According to the mathematical expressionof arc velocity proposed in this paper, the arc velocity at arbitrary position of thepolluted glass plate can be calculated only need to know the pollution degree and thelength of glass plate.Based on the arc energy model, a new method of arc being equivalent to a fluidresistance which can produce radiation was proposed, the influence law of thermalradiation to the arc along the polluted surface was revealed, and an arc energy balancemodel considering the radiation was established. By using the artificial pollution test,the calculation model was validated. Compared with early models, this model cancharacterize the development law and physical significance of arc more accurately. Theconcept of arc radiation constant is the product of radiation function, gray factor and thequartic of temperature, and the selection basis of test constant was discussed. In addition,based on the gas molecular dynamics, a mathematical expression to characterize theeffect of altitude on the thermal conduction and radiation of arc was established, amodified model of the critical flashover voltage of polluted flat plate at high altitudewas proposed, and was validated by using the experiment.