| With the development of HV and UHV DC transmission technology,the pattern of a nationwide large-region power grid interconnection is gradually formed in China..In alpine and cold,freezing rain and icing areas,transmission line insulators will suffer from ice and snow disasters,causing huge economics losses.Among them,the harm of icing flashover to transmission lines cannot be ignored.At present,domestic and foreign scholars have systematically studied the flashover characteristics,flashover mechanism and flashover voltage prediction model of short string of ice-covered insulators.However,there is a significant difference between the characteristics of short arc and long arc on ice,and the derived conclusion of short series cannot be directly applied to long series of ice-coated insulators.Therefore,the study of the arc characteristics of long strings of DC ice-coated insulators and the establishment of a flashover voltage prediction model suitable for long strings have important theoretical guiding significance for the design of outer insulation of DC HV and UHV DC transmission lines in ice-coated areas.Based on the National Natural Science Foundation of China,the volt-ampere characteristics,propagation characteristics and temperature characteristics of DC long ice-surface arc are measured in a large-scale multi-function artificial climate chamber.Then,combined with the characteristics of flashover path of long strings of natural icingcovered insulators,the flashover dynamic circuit model of icing insulators is established,and the flashover test of long string under natural conditions is carried out at Xuefeng Mountain field station.The main work and results are as follows:(i).The best test model and test method for measuring long arc volt-ampere characteristics on ice is designed to study the relationship between long arc volt-ampere characteristics of ice surface and arc length in this paper.The volt-ampere characteristics of DC long ice-surface arc(10 cm?60 cm)are measured in the multi-function artificial climate chamber using the cylindrical ice model.The variation law of volt-ampere characteristic constant A and exponent n with arc length is obtained.In addition,the relationship between dynamic volt-ampere characteristics and static volt-ampere characteristics in the process of propagation is analyzed.The results show that the overall volt-ampere characteristic constant A of DC long ice-surface does not change much with the increase of arc length,and there is no obvious change rule,which can be regarded as a constant.Furthermore,no matter how the voltage polarity is,the volt-ampere characteristic index n shows an overall downward trend and gradually approaches 1/3with the increase of arc length,indicating that as the arc length increases,the arc cooling mode gradually changes to lateral cooling..The dynamic volt-ampere characteristic of ice surface arc is related to the ice melting state and external environmental factors,which leads to different dynamic volt-ampere characteristic curves of each ice surface arc.However,the voltage,current and arc length of each point on the dynamic volt-ampere characteristics curve meet the static volt-ampere characteristics when the arc is developing slowly,which is the basis for establishing a dynamic circuit model.(ii).The propagation characteristics of DC long ice-surface arc are studied experimentally.High speed camera is used to capture the generation and propagation process of DC long ice-surface arc.The differences between the shape and propagation process of positive and negative arc are analyzed and compared.The generation mechanism of new discharge channel in the development process of ice surface arc is analyzed by using space charge theory.To calculate the arc development speed and residual ice resistance in the dynamic circuit model,this paper measured the arc development speed and arc diameter of DC ice surface and obtained the mathematical relationship between arc diameter and arc current..The results show that compared with the negative arc,the breakdown voltage of local arc in positive air gap is lower,and the time of local arc generation is longer.The branch of the positive arc foot is obvious,and the arc foot is connected to the ice surface through small branches with fewer branches and larger branch diameters.The average velocity of the first stage in the process of propagation is less than 1 m/s,which is obviously affected by the drifting arc and has no obvious relationship with the arc current.In the second stage,the average speed of ice surface arc increases to 10 ? 25 m/s,and it increases with the increase of arc current.The arc speed reaches the maximum value(about 90 m/s)in the impending flashover.(iii).The temperature characteristics of DC long ice-surface arc are studied by experiment and simulation.On the one hand,the rotational temperature of DC ice-surface arc is measured by spectrum analyzer.On the other hand,the arc temperature characteristics are simulated and analyzed according to the arc MHD model to obtain the temperature distribution and analyze the effects of plasma physical parameters and applied voltage on the arc column temperature.Finally,the relationship between the arc volt-ampere characteristics,diameter and temperature is analyzed by the arc energy equation.The results show that the arc temperature gradually increases with the increase of arc current.When the arc current is within 0.4 A,the arc temperature is between 3500 K and 6400 K.The center temperature of the arc column is the highest,and the center axial temperature of the arc column is bipolar High and low distribution in the center.As the arc temperature increases,the arc conductivity increases,causing decrement in the electric field strength of the arc column,which is the main reason for the decline of the volt-ampere characteristics(iv).A DC flashover dynamic circuit model of long ice-covered insulator string is established.Firstly,the arc voltage is calculated according to the volt-ampere characteristics of DC long ice-surface arc obtained from content(i).Secondly,the residual ice resistance is calculated combining the arc diameter expression obtained from content(ii)and the equivalent surface conductivity which is calculated according to the voltage current relationship measured in content(i).Then,the DC ice surface arc propagation speed model is established and perform fitting based on the data of content(ii)to obtain the expression of DC ice arc speed and determine the arc development criterion.Finally,according to the flashover path characteristics of long ice-covered insulator string under natural conditions,the DC flashover dynamic circuit model is established,and the effectiveness of the model is verified in Xuefeng Mountain field station.The research work of this paper is of great academic significance to promote the research of ice flashover from short arc to long arc and from short string to long string,and provides theoretical basis for the external insulation design of HV DC transmission lines in China. |
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