Research On Vacuum Arc Diffusion Characteristics Of New Type Of Contacts With Ferromagnetic Material

The vacuum switch,which uses vacuum as the insulating medium,relies on the contact material as the plasma emission source to maintain the combustion of vacuum arc during the breaking process.When it works under the condition which has short arcing time,like DC breaking,the requirements for the dielectric recovery of the post-arc process and the diffusion of vacuum arc during the arcing process are further improved.The movement of vacuum arc during the arcing process directly affects the initial plasma distribution of the post-arc process,and greatly affects the breaking performance of the vacuum switch.Therefore,research on the mechanism and method of rapid and uniform diffusion of vacuum arc during the arcing process,especially in the initial diffusing stage,has always been a hot topic for domestic and foreign scholars,and one of the feasible methods is to optimize the design of the contact structure.This article mainly focuses on the structure of contacts with ferromagnetic material applied and the movement of vacuum arc in the initial diffusing stage.Based on the theory that saddle-shaped axial magnetic field is beneficial to the rapid and uniform distribution of vacuum arc,this article makes use of the magnetic field generated by the traditional cup-shaped contacts to magnetize the ferromagnetic material,enhancing the strength of axial magnetic field at the edge of the contacts.Finite element analysis was used to optimize the design of the contacts with ferromagnetic material.In the initial diffusion stage of vacuum arc,the axial magnetic field of the new type of contacts has a large positive gradient along the radial direction,which is conducive to vacuum arc’s diffusing rapidly and uniformly.And in the stable arcing stage of vacuum arc,high strength of axial magnetic field is remained in a large range of the contacts,which can inhibit the contraction of vacuum arc.Furtherly,by means of a power frequency breaking experimental circuit,the vacuum arc images of the new type of contacts in the arcing stage was taken,which were compared with the butt contacts and the traditional cup-shaped contacts.The distribution and velocity of cathode spots as well as the movement and shape of arc column in the initial diffusing stage of vacuum arc were mainly studied.According to the experimental results,the butt contacts showed higher speed of cathode spots than the other two contacts in the initial diffusing stage when the current was small,but anode spots started to appear when the current amplitude increased to more than 7 k A.The arcing process of the other two contacts was relatively stable within the range of experimental current.Although the initial diffusing speed of cathode spots of the new type of contacts was lower than that of the traditional cup-shaped contact when the distance of the contacts was short,the cathode spots of the new type of contacts could be more evenly distributed in a larger range of the contact surface once the cathode spots were fully diffused and occupied the entire contact surface.What’s more,the arc column showed better uniformity during the stable arcing stage of vacuum arc.As the distance of the contacts decreased,the initial diffusing speed of the cathode spots of the new type of contacts increased.Above all,the new type of contacts with axial magnetic field enhanced in the edge area can not only effectively restrict the contraction of vacuum arc,but also improve the utilization of the contact surface during the arcing process and reduce the loss of plasma.The arc energy is more evenly distributed in the contact gap.This article is mainly to consider some of the requirements of short-arcing-time conditions selectively,which is expected to provide certain references for the design of contacts used in vacuum circuit breakers working under a condition with short arcing time.