| Based on the principle of artificial current zero,the vacuum interrupter is usually used as breaking element in mechanical HVDC circuit breaker.Because the frequency of oscillating current is usually at k Hz level,the actual current to be interrupted by vacuum interrupter is a medium frequency current with a certain DC component.However,for the complex physical process of vacuum arc interruption,the current understanding is limited to the field of power frequency application.This paper mainly focuses on the breakdown mechanism of vacuum gap when interrupting medium frequency current.On the basis of theoretical analysis,the breaking process of vacuum interrupter is divided into preparation stage of the sheath,development stage of the sheath and metal vapor decay stage.According to the dielectric recovery mechanism of different stages,corresponding physical models are established respectively,focusing on the effects of the magnitude and frequency of interrupting current on these physical processes.The effects of frequency as well as DC component on dielectric recovery process are analyzed.Based on but not limited to the existing research results and the characteristics of both conductivity and dielectric properties of vacuum arc,the concept preparation stage of the sheath is proposed,and the method of calculating the specific value of duration is introduced.It takes time for plasma to complete reversal of the polarization in this stage,and this duration will affect the initial state of development stage of the sheath.By means of experimental observation,the duration of preparation stage of the sheath was measured.The experimental results are in good agreement with the theoretical values obtained by the proposed method in terms of changing trend and order of magnitude.Thus,the theoretical hypothesis about the duration of preparation stage of the sheath is validated.The experimental data obtained can be used to determine the parameters of the physical model.Based on the characteristics of intermediate frequency interruption and the analysis of continuous transition model,the main factors affecting the micro-process of development stage of the sheath are summarized as three parameters,i.e.initial ion density,decay time constant of ion,and arc diameter at current zero crossing.The effects of current frequency on these parameters and sheath development process are studied.By simulation and calculation,the trend of how frequency impacts on limit interruption current of the vacuum in different stages are obtained,and the limit interruption capacity of vacuum interrupters under different interruption conditions is deduced.It is pointed out that the existence of DC component increases the possibility of thermal breakdown and decreases the possibility of electric breakdown of vacuum interrupter.Finally,in the application of DC circuit breaker in flexible HVDC transmission system,the protection strategy is studied in this paper.The model of HVDC flexible transmission system and DC circuit breaker considering medium recovery are build in PSCAD to carry out simulation tests of different commutation frequencies.It is found that breaking capacity of current circuit breaker are mainly affected by the commutation circuit parameters in the system through the change rate of current at current zero.The research conclusions provide certain guidance for the selection of commutation frequency and capacity of vacuum interrupter in mechanical HVDC circuit breaker. |
