Research On The Dynamic Characteristics Of Cathode Spots And Post-arc Process In Transient Vacuum Arc Interrupting

Vacuum switches(VS)are widely used in the fields of AC/DC breaking and aerospace due to their excellent arc extinguishing performance.The design of the existing commercial vacuum switch is mainly for the power frequency AC current breaking condition.When the VS is applied to transient arc breaking such as DC and medium-high frequency current breaking,the cathode spots can’t spread sufficiently and the recovery speed of the post-arc media strength slows down which will increase the probability of post-arc re-breakdown.In this dissertation,the dynamic variation characteristics of the cathode spots and the post-arc current and the microscopic particles in the recovery process of the post-arc medium strength under the transient arc breaking condition are studied.Firstly,the experimental research on the cathode spots dynamic characteristics under the condition of medium-high frequency current breaking was conducted.The cup-shaped axial magnetic field contact was used to study the variation of the initial diffusion velocity of cathode spots under different frequency and di/dt conditions.The result shows that the initial diffusion velocity of the cathode spots is proportional to (?).The effect of magnetic field and metal vapor on the diffusion velocity of cathode spots was studied.Besides,according the difference in velocity and size,the cathode spots were classified.The critical conditions for the conversion of cathode spots types and the corresponding emission mechanism were analyzed.Through the statistics of the experimental results,the quantitative relationship between the diameter of the cathode spots distribution before the current zero crossing and the current frequency and amplitude was established,which provides a basis for the calculation of the post-arc initial plasma density.Secondly,the axial magnetic field contact and butt contact were uesd to complete mediumhigh frequency current breaking experiments.The stability of vacuum arc when the value of di/dt is relatively low was studied.The results show that arc instability phenomenon occurs when the current is low.The quantitative variation of the initial instability current with di/dt was obtained,and the influence of the magnetic field on the instability of the arc was analyzed.According the analysis of arc stability and plasma density,combined with the characteristic of plasma lagging behind the instantaneous value of current,and the calculation model of post-arc initial plasma density was established which can provide initial conditions for the simulation of post-arc process.According to the physical meaning of the gap,the equivalent circuit model of the sheath and the plasma was established.The voltage spike during the arc instability process and the transient recovery voltage were simulated.Compared with the experimental results,the influencing factors of the voltage and current variation were analyzed,and the accuracy of the proposed plasma density calculation model was verified.Thirdly,the post-arc particle distribution characteristics were studied through the experiments of the post-arc current measurement under high di/dt conditions.The variation of the rise rate of post-arc current di/dt with time before the sheath growth was analyzed.By establishing an equivalent circuit model,the relationship between the transient recovery voltage and the post-arc current waveform was also analyzed.And according to the emission mechanism of the electrode surface,the movement and distribution characteristics of the plasma were discussed,and the concept of ‘plasma depletion zone’ was proposed,which reasonably explains the reason for the variation of di/dt.Based on PIC-MCC simulation,the effects of plasma density and metal vapor density on the development speed of plasma depletion zone were analyzed,and the variation of post-arc resistance and its influence on di/dt were discussed.Finally,according to the experiments of the post-arc current measurement under mediumhigh-frequency breaking and DC breaking conditions,the effects of metal vapor on the plasma density during the sheath growth process was studied.By integrating the post-arc current waveform with time,the variation of the post-arc charge with frequency and di/dt was obtained.Combining with the cathode spots distribution and the metal vapor calculation model,the effect of the movement of the cathode spots on the metal vapor density was analyzed.The impact of the collision between metal vapor and charged particles on the duration of the post-arc current was discussed.The higher the density of the metal vapor,the lower the velocity of the charged particles,and the longer the duration of the post-arc current.A calculation model of post-arc plasma density decay considering the ionization of metal vapor was established,and the influence of metal vapor ionization degree on the sheath growth speed was discussed.The study in this dissertation is of great significance for perfecting the vacuum arc theory and expanding its application in the field of transient arc interruption.Also,the study can provide theoretical guidance for the regulation of cathode spot diffusion and the speed of postarc dielectric strength recovery.