Research On Current Commutation Mechanism And Characteristics Of Current And Voltage Sharing Of Hybrid DC Circuit Breakers

Natural commutation hybrid DC circuit breaker(NHCB)is the key equipment for fault interrupting of medium and low voltage DC power grid.It combines the advantages of the mechanical switch and power electronic switch,and has the advantages of fast-breaking speed,strong current capacity and low on-state loss.Hybrid DC circuit breaker has been a research hotspot at home and abroad in recent years.This paper focuses on the research of arc current commutation characteristics,secondary current commutation characteristics and current and voltage sharing characteristics of natural commutation hybrid DC circuit breakers,and designed a hybrid DC circuit breaker prototype using current-sharing impedance.The main contents are as follows:Compared with the forced commutation hybrid DC circuit breaker,the natural commutation hybrid DC circuit breaker has the problem of low reliability of arc current commutation.In order to ensure the reliable completion of arc current commutation,it is necessary to study the arc current commutation process.First of all,a simplified model of arc current commutation is established,and the theoretical derivation is carried out according to the simplified model,the mathematical expression of the influence of key parameters on arc current commutation time in the process of arc current commutation is obtained.Secondly,an experimental platform for arc current commutation was built,and the influence of the number of series-parallel of power electronic devices and the short-circuit current rise rate on the arc current commutation characteristics are studied through experiments.Thirdly,through theoretical derivation and experimental verification,the concept of arc current transfer coefficient is proposed,which is the key parameter affecting arc current commutation time,and the relation curve between arc current commutation coefficient and arc current commutation time is obtained,and it is concluded that the arc current commutation coefficient should preferably be greater than 2.Finally,under the guidance of the theory of improving the arc current commutation coefficient,the methods of promoting arc current commutation are explored by increasing the number of power electronic devices in parallel,external transverse magnetic field and double break.Aiming at the problems of optimal design of the parameters of the buffer energy-absorbing branch and limiting the maximum breaking overvoltage,the secondary current commutation process of the hybrid DC circuit breaker is studied in detail.First,the secondary current commutation process of the hybrid DC circuit breaker is divided into five stages,the equivalent circuit diagram of each stage is established,and the theoretical analysis and derivation are carried out according to the equivalent circuit of each stage.After that,by building the experimental platform of secondary current commutation of hybrid DC circuit breaker,the influence of key parameters such as the breaking current,the buffer capacitor capacity and the stray inductance of each branch on each stage of the secondary current commutation process is studied.And the effect of key parameters on the maximum breaking overvoltage is studied emphatically.As the turn-on time of the power electronic switch in the hybrid DC circuit breaker is extremely short,the series current-sharing impedance in the power electronic switch will not increase the power loss,and the series impedance current-sharing method has obvious advantages over other current-sharing methods.Based on the above mentioned,the influence of series current-sharing impedance on arc current commutation process,dynamic and static current sharing,breaking overvoltage and breaking capacity of hybrid DC circuit breaker are studied through theoretical derivation and experiments.Aiming at the problem of series voltage sharing of power electronic devices in hybrid DC circuit breakers,by establishing a voltage equalization simulation model based on Saber and building a voltage equalization experimental circuit,the influence of RCD passive snubber circuit on the dynamic voltage equalization of power electronic devices in hybrid DC circuit breaker is studied.Based on the above theoretical and experimental research results,a power electronic switch prototype using current-sharing impedance is designed,and the power electronic switch prototype is combined with the fast vacuum switch to form a hybrid DC circuit breaker prototype.A large current breaking experiment is carried out for the prototype of the hybrid DC circuit breaker,the experiment verified the effect of current-sharing impedance on the breaking capacity of the prototype,and it is concluded that the breaking capacity of the prototype can be improved by 12.1% through the current-sharing impedance under the set dynamic and static uneven current conditions;the influence of the snubber capacitor capacity on the breaking process of the prototype is studied,and the optimal capacity of the snubber capacitor is determined;the breaking performance test was carried out on the prototype,and the reliable breaking of 3.4k A current was completed within 2.8ms.