Investigaiton On Current Transfer Theory And Application Of Microgrid Hybrid DC Circuit Breaker

With the development of distributed generation and electric vehicles technologies,DC distribution micro-grid has become one of the future developing trends and directions in power system.And the relevant technologies in DC micro-grid turn to be hot research issues these years,such as power generation system,energy storage system,energy management system,utility grid connection system,as well as DC protection and control system.The intelligent and fast DC circuit breakers with large interruption capacity are the crucial protection equipment in future DC micro-grid,which play key roles in the safety of DC micro-grid's operation and control.And the key technologies of DC circuit breaker,for its application in DC micro-grid,still need to be breakthrough in the field.The hybrid DC circuit breaker,which is composed of paralleled mechanical switch and electronic switch,shows its developing tendency as the protecting switches in DC micro-grid,as it can utilize the advantages of mechanical switch with larger current flowing capacity and less depletion,and the advantages of electronic switch with faster switching speed.Realizing the advantages compensation of two switches and avoiding the disadvantage of two switches.This paper mainly studies the theory and application of current commutation in hybrid DC circuit breaker,providing basis for the engineering application of intelligent,fast,and large interruption capacity DC circuit breaker,promising the operating reliability of DC micro-grid.This paper reviews state-of-the hybrid DC circuit breaker at home and abroad at first;and raising design scheme of the general structure and the hardware composition principle of micro-grid hybrid DC circuit breaker.The arc model of high speed vacuum circuit breaker is built based on Mayr arc model and the measured arc voltage-current characteristics of axial magnetic vacuum switch.According to the corresponding technical parameters,the simulation analysis model of IGBTs and arresters is established,and the parameter model of RCD snubber circuit is obtained.The simulation model of hybrid DC circuit breaker is built based on above models.The operating process of the circuit breaker are analyzed theoretically,especially the current commutation characteristics in the opening process;as the current commutation characteristics affect the interruption capacity of the circuit breaker.Then,the test platforms to simulate the large and small current commutation process in the DC circuit breaker are established.And the current commutation characteristics,including the commutation time and different value of fault current,are intensively studied during the period of fault current transferred from vacuum switch to IGBTs.However,in the large current interrupting condition,the current commutation time from vacuum switch to IGBTs is comparatively longer,limiting the interruption time of the hybrid DC circuit breaker.To enhancing the interruption capacity of the circuit breaker,the influent factors on the current commutation characteristics,such as the value of transfer resistance,transverse magnetic,interrupted current value,are studied based on the established analogic large current commutation characteristic test platform with glassy vacuum interrupter.The transfer process is divided into the inherent transfer process and the trailing current transfer process,the influence of transverse magnetic field on the transfer characteristics is studied,the transfer current transfer criterion and the mathematical description expression are deduced,providing references for the fast interruption of high capacity hybrid DC circuit breaker.Based on above theoretical studies,the designed structure scheme of hybrid DC circuit breaker is proposed.And the prototype of high speed vacuum circuit breaker is designed with the high speed actuator,which is based on fast repulsion driver and permanent magnetic holding mechanism.The parameters in IGBTs branch,RCD circuit and arrester branch are selected.Besides,the designed intelligent control system for the circuit breaker is also present in this paper.The control system can realize intelligent detection and operation of the prototype,with the signal detection and acquisition units to obtain the current and voltage in bus,the circuit breaker's statue detection and fault judgment units,the operating control units of high speed vacuum switch and electronic switch,and the remote communication units to transmiting with outer system.And the test prototype of the 400V/3kA large-capacity hybrid DC circuit breaker is developed based on above designed schemes.And the test circuit to simulate large DC interruption is established finally.The mechanism and interruption characteristics of the prototype are tested,verifying the availability and reliability of the designed scheme.In summary,this paper studies on theory and application of current commutation in hybrid DC circuit breaker,providing theoretical basis for the design of test prototype.With the theoretical analysis and experiments researches on the current commutation characteristics,the mathematic descriptions of the circuit breaker's current commutation characteristics are obtained.And the operating characteristics and control strategies of the hybrid DC circuit breaker is presented based on the analysis of the operating principle.The prototype of micro-grid hybrid DC circuit breaker is developed in this paper;and the opening response time is controlled within 0.6ms;and the circuit breaker can interrupt the fault current within 2.5ms,and the closing time of the prototype is controlled within micro-second level.The developed DC circuit breaker prototype,which' s rate voltage and current is 400V and 2400A,and the interruption capacity is 3kA,can meet the requirements of the protection in DC micro-grid and with less effect on the applied power grid,providing theoretical bases and thchnical supports for the application of the intelligently high speed hybrid DC circuit breaker with larger capacity in China.