Characteristics Of Impulse Partial Discharges At Insulation Defects In SF₆ And The Influence Factors

With a surge of power grid construction,Gas Insulated metal-enclosed Switchgear?GIS?is widely used in power system.Meanwhile,according to recent statistics of the operation faults in power systerm,the faults caused by insulation defects present a increasing trend,including the equipments operating only for a short-term.Practices indicate that some potential defects with abnormal electric field in GIS is undetectable by AC voltage withstanding test and AC-Partial Discharge?PD?test,which make the security trearts for operation of GIS in a long term.As a consequence,the impulse voltage withstanding test has been highly consideration,and PD test under impulse is proposed and proved to have a well effectiveness in insulation state diagnosis.However,the study on impulse PD for the purpose of insulation state diagnosis is still at its initial stage.The general characteristics of impulse PDs in SF₆ gas are not obtained due to the shortage of statistial analysis.The mechanisms underlying the initiation and propagation of PD in different types of defects are still remain elusive.The influence factors on PD behaviors are not obtained in the general.To solve the problems metioned above,a PD experimental platform is designed and implemented in this work,which consists of a IEC60060-3 impulse generator,a SF₆ gas test chamber,a wide-band measurement system for current and light pulses.According the PD positions in GIS and PD incitation mechanisms,the defect types are designed and made for experiment.The main conclusions of this study are summarized as follows:Based on experimental study,the typical current and light pulse sequences are summarized.The relationship between the appearance proportions of the different PD and applied test conditions.By using the theories of metal electron emission,field detachment of negative ion,and thermal emission and detrapping of electron,the theoretical V-t curve of the first PD is proposed and applied to describe the relationships between PD time lag and applied impulse voltage.Experiments investigaed that the PD sequences in different defects are consist of forward discharges occuring within the rising or peak time of applied voltage and reversed ones within the descending or trough time.Their waveforms and sequences have corresponging characteristics in terms of the different rising times and oscillation features for the former and the different charge quantities,time lags?intervals?and PD numbers for the latter.Statistical study shows that though the PD presents obvious randomness,the occurring probabilities of different PD sequences have regular trends with changing of voltage level,gas pressure and electrode dimension.The infulences factors,including the gas pressure,defect structure and applied voltage,on PDs are investigated.It is proved that,the increase of gas pressure improves the PD initial voltage?PDIV?of each defect and limits PD activities of the most of defects;The equivalent radius of fixed metal point,the locations of metal electrode in space or insulator surface impacts the initiation and propagation of PD including time lag,pulse magnitude and appearance frequency;3)The type and value of the applied voltage plays the crucial impacts in the intiation and propagation of discharge and the accumulation and the dissipation of discharge charge.The comparison study on the PD excitation effciency and the PD activities of different applied voltage shows that ligining impulse and oscillating switching impulse have better effectiveness than AC voltage in PD diagnosis.