| Gas insulated switchgear(GIS)has been widely used in the field of high-voltage transmission due to its advantages of small floor space and high reliability.With the continuous increase of grid voltage level and system capacity,the internal faults of GIS equipment also increase accordingly,and operation reliability has aroused widespread concern in the society.In recent years,there have been many sudden flashovers of unknown causes of GIS equipment under normal operating conditions,which have always been an engineering problem that plagued the operation and maintenance department of the power grid.The on-site partial discharge online monitoring system cannot effectively detect the discharge process before the sudden flashover,and the existing partial discharge measurement and analysis technology cannot explain the mechanism.The main reason is that the empirical data about the discharge process of foreign object defects along the GIS were obtained in the laboratory by the short-time step-up method,which was obviously different from the long-term constant voltage environment of the engineering site,making the laboratory research results disconnected from the actual situation of the engineering site.In order to solve this problem,a long-term withstand voltage experiment platform for metal foreign objects on the surface of GIS insulators was built in the laboratory,and the long-term partial discharge characteristics of metal foreign objects on the surface of insulators and the mechanism of flashover caused by this were deeply studied.In this paper,the UHF method,which is widely used in the engineering field,is used to study the partial discharge phenomenon and development process of long-term withstand voltage of metal foreign bodies on the insulator surface.Through continuous 168h long-term constant voltage experiment,based on the full real-time partial discharge UHF signal detection system,the whole process of partial discharge data was completely recorded.The experimental results reveal the phenomenon of intermittent discharge of surface metal foreign objects during the long-term development stage,that is,the distribution of the discharge interval is large,and the interval span varies from a few seconds to several hours.Two evaluation parameters,effective discharge capture probability Pe and effective map capture probability PG,are proposed.Through the Pe and PG curves,the existing partial discharge UHF online monitoring device of GIS equipment is given with a small success rate of discharge event capture(only 3.4%),and the fact that the probability of obtaining a valid spectrum with a charged test is low(only 6.5%).On this basis,the methods to improve discharge detection rate,such as strengthening the popularization of real-time acquisition system and extending the detection time,are proposed.In order to consider the effect of defect size and applied voltage amplitude on the long-term discharge statistical characteristics,the influence law of defect size and applied voltage amplitude was studied.The results of the study show that the size of the defects and the magnitude of the voltage have an effect on the statistical characteristics of long-term discharge.Defect size and voltage amplitude have a greater influence on the discharge characteristics(duration,number of discharges)at the initial stage of discharge,but have a smaller influence on the discharge characteristics(effective discharge interval ratio,Pe and PG values)in the discharge development stage.It shows that the sparseness characteristics of the metal particles on the surface of the insulator during long-term discharge are less affected by the particle size and voltage amplitude.In order to further study the development process of metal foreign objects on the surface of insulators discovered by UHF method during long-term discharge time,the paper comprehensively applied UHF method and photomultiplier tube to observe the long-term discharge phenomenon simultaneously.Through the analysis of the photomultiplier tube data,it is found that there are two types of discharge excitation forms for partial discharge of metal foreign objects on the surface of the insulator:pulse-type discharge form and micro-discharge group form,of which pulse-type discharge form and UHF signal basically correspond at the timing of occurrence.There is no corresponding UHF signal in the form of micro-discharge group.During the development of long-term discharge,the shape of the micro-discharge group has always existed,without intermittent,and the overall amplitude and duration of each cycle have not changed much,while the frequency of pulse-shaped discharge has an initial violent to gentle then to the intermittent changes.In the process of long-term discharge of metallic foreign objects on the surface of the insulator,the shape of the micro-discharge group will cause continuous erosion on the surface of the insulator.In order to study the luminescence process of the metal particles on the surface of the insulator in the long-term constant voltage discharge space,the paper comprehensively used the UHF method and the high-gain industrial ICCD camera(PI MAX-3)to perform simultaneous shooting of the discharge luminescence area.The experiment found that the light-emitting region of the discharge is jointly contributed by two energies of pulse discharge and micro discharge group.When the voltage is low,the light-emitting area is a circular area around the tip of the needle,and during the long-term discharge development,the area of the light-emitting area tends to increase slightly during oscillation.When the voltage level is high,the circular area with the starting needle tip area of the light-emitting area gradually changes into a slender discharge channel with time,and the channel length increases as the applied voltage increases.When the light-emitting area of the discharge becomes an elongated discharge channel,the light-emitting phenomenon of the discharge is very unstable,and the light spot sometimes covers the entire channel,sometimes only stays at the root of the channel,and sometimes even weak and unpredictable.In order to study the mechanism of long-term discharge caused by metal foreign objects on the surface of the insulator,the surface charge of the insulator was measured during the long-term discharge.The measurement results show that the surface charge distribution increases first and then gradually dissipates with time.The result excludes the decisive role of surface charge in the development of discharge.Then the product of the surface of the needle tip and the discharge product of the insulator were analyzed,and it was found that the products were dominated by three elements of Al,C and O.It shows that in the long-term discharge process,the non-metal oxide on the surface of the insulator that has been eroded and decomposed adheres to the surface of the metal needle tip.The needle tip is wrapped by the product,the field emission is blocked and the electrode curvature becomes larger.Affected by this,the pulse-shaped discharge gradually weakens and enters an intermittent phase,while the low-amplitude micro-discharge group has continued to exist.At the same time,the surface of the insulator is gradually cracked under continuous discharge erosion.According to the amplitude and duration of the applied voltage,the cracked area on the surface of the insulator evolves from a circular area near the beginning of the needle tip to an elongated black carbonized discharge channel.It extends to the opposite electrode,and the long-term continuous development of the carbon mark in the form of micro-discharge groups will cause the flashover along the surface... |
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