Abstract: Breaking operation of hybrid DC circuit breaker is the core function which reflects the essential features as current breaking equipment.The study of the turn-off stresses derived from the fault-current breaking transient process is the key content of the electrical characteristics research of the breaker,which is the theoretical basis of the electrical,structural and experimental system design and needs to be analyzed in detail.Firstly,based on the analysis of circuit topology and breaking operation,the stress characteristics both of the millisecond overall breaking process and the microsecond commutation process had been studied.The relationship between these two kinds of processes was discussed,which aim to extract the millisecond-level breaking process and the microsecond-level commutation process stress characteristics and influencing factors.Then,in order to verify the validity of these analyses,a device-level simulation model in PSCAD/EMTDC was built.Finally,the implementation of 200 kV hybrid DC breaker、100 kV breaker unit、50 kV breaker unit and single sub-module breaking test results verified the accuracy of the simulation circuit and mathematical analysis.As above,value of this paper mainly focus on electrical,structural design and test system design for hybrid DC circuit breakers with IGBT Series Valve.