Fault Diagnosis And System Reconfiguration Strategy Of ANPC Three Level Cascaded Inverter

The advanced co-phase traction power supply system has wide application prospects as a new type of traction transmission system that can eliminate the electric phase separation device completely,improve the traction capacity and improve power quality significantly.The cascaded inverter,as a device connected with the traction network in the traction transmission system.Its performance affects the power quality of the traction network directly.Due to the use of a large number of power switches with poor shock resistance in the advanced co-phase traction power supply system,the failure rate is high and the safe operation of the train is affected.Therefore,it is of great significance to improve the safety and reliability of traction power supply to study the on-line fast fault diagnosis and system reconfiguration of cascaded inverter power switches.The ANPC three-level cascaded inverter is regard as the research object for this thesis.First,the structure of the advanced co-phase traction power supply system is introduced.Then,the mathematical model of the ANPC three-level inverter is established,and the problem of capacitor voltage balance is analyzed.The four working modes and three typical working state types of the inductor-assisted voltage balance circuit are analyzed.The working principle and control strategy of ANPC three-level cascaded inverter are studied.By analyzing the carrier disposition modulation and carrier phase shift modulation,the combination modulation strategy of carrier disposition and carrier phase shift is adopted in this thesis.In this thesis,the fault types of the power switches of ANPC three-level cascaded inverter are studied with the bridge arm as the unit,and the fault characteristics of each fault type are analyzed.By analyzing the mechanism of the zero-crossing distortion for the bridge arm voltage under the open-circuit fault state of the power switches,the analysis of bridge arm fault characteristics under the power switch fault state is further improved.Based on the analysis of the fault characteristics for each fault type,the fault diagnosis method based on switching sequences is proposed.According to locking the voltage of the faulty bridge arm by the switching sequences,and comparing and analyzing the locked bridge voltage combination with the fault characteristics stored in the system,the open-circuit faults diagnosis of power switches are realized.In order to improve the reliability of ANPC three-level cascaded inverter,the system reconfiguration strategy of cascaded inverter is studied on the basis of online fault diagnosis.First,the voltage vectors of ANPC three-level inverter module under various fault types are analyzed.Then,based on the analysis of the voltage vector of the fault module,the reconfiguration strategy of cascaded system is proposed.The faulty bridge arm is bypassed by using the remaining normal power switches of the faulty bridge arm,and the topology reconfiguration of the cascaded system is realized.Subsequently,according to the structural characteristics of the reconstructed topology and the idea of carrier uniform modulation,the carrier of the cascaded system is reconstructed to ensure that the output voltage has good harmonic characteristics.Finally,the simulation model and low-power experimental platform of the ANPC three-level cascaded inverter are built.The correctness of the fault diagnosis method and the system reconfiguration strategy are verified by the simulation and low-power experiments.The simulation and experimental results show that the fault diagnosis method based on the switching sequences can locate the fault power switches quickly and accurately,and the system reconfiguration strategy can achieve system reconfiguration within a modulation wave period after the fault characteristics occurred.