Research On SF₆ Discharge Behaviors Of Electrode-covered With Dielectric Under The Nanosecond Impulse Voltages

When the disconnectors on/off in the GIS which is insulated with SF₆,the very fast transient overvoltage?VFTO?appears and it may damage the power transformer and the secondary equipment.A further research on the discharge characteristics of SF₆ under the nanosecond impulse voltage can improve the insulation coordination so that the insulating property of SF₆ can be fully developed.In this thesis,onedimensional SF₆ discharge model of electrode covered with dielectric under the nanosecond impulse voltage is established based on the convection-diffusion mechanism.As the charged particles accumulate extremely fast on the dielectric layers under nanosecond impulse voltage,it requires the calculation method more stringent in computational stability.In this thesis,the MacCormack differential format is used to discrete equations for solution,this format not only has second order accuracy,but also a non-dissipative stable one,it can guarantee the stability on the premise and takes account of the calculation accuracy,the amount of computation and convergence speed.In this thesis,at the condition of both side of electrodes are covered with dielectric and only single side of electrode is covered dielectric respectively,both the discharge characteristics of SF₆ under the nanosecond impulse voltage and the relationship between the behavior of the charged particles and the electric field are researched theoretically by changing the voltage amplitude and the rise time,the gap dimension,the thickness and the relative dielectric constant of the covered layer and the pressure of the gap.The results showing,when both side of electrodes are covered with dielectric,a bipolar current appear in the gap under a nanosecond impulse voltage,the current pulse with low amplitude during the rise time is causedby the rapid change of the applied voltage,and the external parameters have a significant effect on the severity of the discharge;the process of the charged particles accumulating on the dielectric at the anode side can be divided into three stages:rapid rise,slow rise and rapid rise again,this progress is influenced obviously by the external parameters and it is closely related to the changing of conduction current;the electric field is basically evenly distributed in the gap before discharge while it is mainly concentrated in the anode side of the dielectric.When only the anode is covered with dielectric,the conduction current density and the variation of the charge density on the dielectric covered layer are closed to those of the two sides of electrodes are covered,while the electric field is mainly concentrated on both sides.When only the cathode is covered with dielectric,the slowly rise stage of the conduction current density and the variation of the charge density on the dielectric covered layer doesn't exist,and the electric field is mainly concentrated on the anode.