Arc Formation Process Mechanism And Experimental Research Of Vacuum Switch With Rotating Contact

Vacuum metal vapor arc is generated in vacuum switch at the initial moment of contact separation.Due to the strong material transfer,arc root agglomeration and stagnation of contacts,ablation of contacts occur and melting crater formed further during the formation of vacuum metal vapor arc.Characteristics of the arc and the state of the contacts in the initial stage of arc generation are the key factors that affect the transformation of the arc mode into diffusion or contraction in arc development stage.By changing the rotation mode of the contacts during arc formation process,the micro ionization conditions are modified,and the formation conditions of vacuum metal vapor arc are further affected.Thereby the surface conditions of the contacts are improved and the energy of the steady-state arc is reduced,which provides the beneficial supplement for the research on improving the interrupting capacity of vacuum switch.The metal vapor which is necessary for vacuum arc generation is the main research focused in this dissertation.By means of numerical simulation and experiments,the mechanism of liquid bridge connection,vacuum gap breakdown and column arc formation of jet bridge in the process of contact separation are investigated,and the influence of contact rotation on the initial arc formation of vacuum switch is also studied here.At the moment of contact separation,the line current acts on the final separating electrical contact which has a great electrothermal effect.Firstly,based on fluid dynamics theory,a three-phase transformation model of electrical contact is established to describe the temperature field,current density distribution and airflow field spatial distribution in the process of contact phase transformation.The quantitative analysis of the whole airflow and fracture process of metal liquid bridge is obtained.Due to the extremely high electrothermal action at the fracture of metal liquid bridge,the mathematical model of metal vapor ejection at the contact surface with micro-protrude at different final separation positions is established.Moreover,a method for analyzing the distribution of metal vapor near the micro-protrude is proposed here.Characteristic quantity of contact rotation based on the location of the micro-protrude and rotation speed of the contact is introduced and the influence of the rotating motion of the contact on the thermal process of different micro-protrude direction is analyzed.When the micro-protrude is located in the center of the contact during contact separation,the rotating motion of the contact rarely affects the phase transformation volume of the micro-protrude and the distribution of metal vapor.The farther the micro-protrude from the center of the contact,the faster the rotation speed of the contact is.The spatial distribution of metal vapor is shown as the expansion in high-density area.Since the maximum density of the metal vapor decreases,the overall homogenization degree of high-density area increases.During the contact separation,the diffusion pressure which drives molecules is determined by the density of metal vapor near the cathode.The electric contact thermal process and gap breakdown process are unified by taking the non-uniform distribution of metal vapor in the Knudsen layer near the cathode as boundary parameter.Based on the drift-diffusion principle,the fluid model is established.Coupled with the electron emission process and electric field effect on the cathode surface,the initial discharge path of the metal vapor arc forming when the contact separating is obtained.The influence of contact rotation on the formation of initial conductive path in vacuum gap is also studied.Then,the thermal breakdown process of vacuum gap generates after the cathodic jet process,which promotes the arc development.A vacuum metal vapor arc jet model is established to describe the thermal breakdown process based on the double temperature magnetohydrodynamics model.Simulation results indicate that the restricted arc has a cylindrical shape.When the jet generates at the non-central position of the contact,the contact rotation makes the restricted arc spread and radiate.When the jet generates at the edge of the contact,the faster the rotating velocity of the contact is,the instability in the restricted arc increases more.The formation process of the crater on cathode surface is also described at the function of restricted arc by comparation on the effects of contact rotation with the formation of crater at different jet positions.When the jet flow generates at the edge of the contact,the faster the rotating velocity of the contact is,the flatter the crater morphology of the cathode surface is,and the unstabler the restricted arc is.Finally,a new experimental system of vacuum arc pulling with rotating contact is designed,and the formation process of vacuum metal vapor arc in small gap is filmed with a high-speed camera.The characteristics of restricted arc during arc formation are obtained,which verifies the rationality of the simulation model in this dissertation.The influence of contact rotation on restricted arc characteristics is studied experimentally.The results show that the contact rotation makes the diffusing state of the restricted arc from the cylinder shape to diffusion.Subsequently,the arc splintered and the restricted arc motion transition time are advanced in the initial stage.It provides a research basis for further exploring the application of arc rotary interruption in vacuum switch apparatus.