Surface Charge Dynamics Characteristics And DC Flashover Suppression Of Insulation Materials Based On The Dielectric Barrier Discharge Plasma Treatments

The development of high voltage direct current(HVDC)transmission technology necessitates the higher voltage levels and transmission power,which again puts back the focus on the insulation issues.The surface charge can be easily accumulated on the insulation surface under the high electric field application.Moreover,the probability of surface flashover is higher than the breakdown of insulation material.Thus,the surface insulation properties of these materials have restricted the development and wide-scale implementation of HVDC.The silicone rubber(SiR)is widely used as outdoor insulation,whereas epoxy resin(EP)with AlN nanocomposites possesses the suitable properties for the indoor insulation application.This thesis compares the influence of dielectric barrier discharge(DBD)plasma treatment on the surface electrical and physicochemical properties of SiR and the EP/AlN nanocomposite materials.The major findings are as follows:An equivalent circuit model of DBD used for the material treatment is proposed and the calculation method of the relevant electrical parameters is derived.A developed DBD experimental system is used in the present study.The EP sample is treated by DBD plasma,and the electrical parameters of the discharge during treatment are calculated using the proposed circuit model.It is shown that the areal ratio of the filament discharge on the EP sample surface increases with the plasma treatment time.The surface conductivity and amount of shallow traps are both increased on the EP sample surface after the DBD plasma treatment.The improvement of surface properties makes it easier for the surface charge to evenly spread over on the EP surface.This enhances the electric field intensity inside the gas gap of the DBD generator and it becomes easier for the generation of new micro-discharge channels.It has been demonstrated that plasma treatment improves the surface properties of the treated material and the altered properties can influence the DBD discharge parameters in return.The influence of surface charge dynamics on the surface insulation properties of insulation materials is analyzed.It is found that the surface charge dissipation can improve the threshold voltage of surface flashover.However,the flashover voltage would decrease due to the leakage current and partial discharge when the surface charge moves rapidly.The influence of corona charging voltage,charging time and surface conductivity on the rate of surface potential decay is studied.It is shown that the initial surface potential increases with the charging voltage and charging time.The surface potential would have abnormal variations if the charging voltage is too high.Moreover,the increase in surface conductivity can accelerate the surface potential decay.It is demonstrated that the rate of surface potential decay of the double surface plasmatreated sample is higher than the single surface plasma-treated sample.Meanwhile,the surface potential of the sample decays fastest when the top surface conductivity is higher than the bottom surface conductivity.The mechanism of flashover voltage improvement on the EP/AlN nanocomposite surface after plasma treatment is illustrated.It is found that with the increase of plasma treatment time,the surface conductivity of the EP/AlN sample is improved.Moreover,the trap energy level of the hole trap and electron trap on the surface is further reduced,and the density of shallow traps increase.In the same case,the trap energy level of the EP/AlN sample is lower than that of the electron trap.Furthermore,the density of the shallow hole trap increases more obviously after the plasma treatment.The effect of plasma treatment on the accelerated positive surface charge dissipation and the improvement of positive surface flashover voltage is more evident.The analysis of physicochemical characteristics shows that after plasma treatment,the content of polar groups and the surface roughness of EP/AlN samples increase significantly.The increased surface roughness suppresses the development of discharge channels,thereby increasing the surface flashover voltage.The regulation mechanism of plasma treatment on the dynamic characteristics of different polar charge on the SiR surface is determined.Moreover,the aging properties of the plasma-treated layer on the SiR surface are analyzed.It is found that the plasma treatment can increase the shallow trap density and surface conductivity of the SiR sample,however,it does not affect the volume conductivity.Besides,plasma treatment can increase the content of polar groups on the surface of SiR.The water contact angle decreases continuously with the increase of plasma treatment time.However,the content of the hydroxyl group-OH on the surface of SiR after the plasma treatment decreases significantly and the hydrophobicity is recovered after the plasma-treated sample is exposed in the air for 30 days.The results show that plasma treatment can maintain the hydrophobicity of SiR samples and improve the surface insulation properties.Partial discharge magnitude and surface flashover voltage measurement show that the prolonged plasma treatment can promote the surface charge movement and dissipation on the SiR surface.However,the surface flashover voltage is decreased due to the rapid charge movement.It is found that an optimized plasma treatment time can result in the highest surface flashover voltage of SiR.