Fault Tolerant Control Strategy For Open-Circuit Power Tube Faults Of Multilevel Inverter

Compared with two-level inverter,multi-level inverter has been widely used,especially in medium and high voltage applications,because of its high efficiency,low stress of power switch,low electromagnetic interference and high output power quality.Multilevel inverter has more power switches,the probability of failure increases.The fault will make the output waveform of the inverter system distorted and bring some losses to the user department.In order to improve the stability of inverter system,it is necessary to study the fault diagnosis and fault tolerant control technology.In this thesis,the neutral point clamped(NPC)multilevel inverter with stable performance and simple control method is taken as the research object.The fault-tolerant control strategy after the open-circuit failure of the power switching device is studied,so that the inverter can still obtain a continuous and stable output under certain fault conditions and improve the reliability of the system.Firstly,taking NPC three-level inverter as an example,the topology,working principle and space vector pulse width modulation(SVPWM)strategy of NPC inverter are analyzed.The simulation model is built in the MATLAB / Simulink simulation platform.For NPC three-level inverter,the mixed logic dynamic(MLD)model is established and the corresponding current state estimator is built.According to the residual between the state estimator and the actual system,the residual current of three-phase inverter is obtained after the inverter failure.The residual current of three-phase is transformed into current residual vector by Clarke transform.The specific position of the fault is determined according to the phase angle and amplitude of the current residual vector,and the signal can represent the fault information is obtained.Then,the fault-tolerant control structure is switched based on the diagnosis signal by using the fault-tolerant method of redundant bridge arm.The fourth bridge arm is used to replace the bridge arm where the fault power tube is located,realize the fault-tolerant control of inverter.The simulation results show that the method can realize fault diagnosis and fault-tolerant control of inverter quickly,and it will not cause the inverters to run down after fault tolerance.For NPC five-level inverters,the voltage space vector after open-circuit failure of power switch is analyzed.The open-circuit failure of power switch is divided into type I and type II faults according to whether the inverter itself has fault-tolerant capability.After type I failures occur,the redundant space vectors of NPC five-level inverters are used to adopt the redundant space vector fault tolerance strategy,and the SVPWM modulation algorithm is adjusted accordingly.When type II failures occur,a fault-tolerant strategy of topological reconfiguration is adopted.With the help of a bidirectional switch,the fault phase is connected to the DC bus midpoint.According to the space vector distribution after the type II fault,the space vector synthesis method and its SVPWM modulation algorithm are obtained.Simulations in MATLAB/Simulink show that both fault-tolerant strategies can keep the inverters running steadily and continuously after single-pipe failure.Finally,a modular Five-Level Inverter with fault tolerance is composed of two-level inverters and NPC three-level inverters.When the power switch in the NPC three-level inverter module has open-circuit failure,redundant space vector fault tolerance strategy is adopted;when the two-level inverter or the bidirectional switch has open-circuit failure,disconnect the two-level inverter module and the DC bus,and only make the NPC three-level inverter module work.This Five-Level Inverter has fewer power switching devices than NPC five-level inverters.The simulation results show that the proposed Five-Level Inverter Topology can operate normally and has good fault tolerance for power tube open-circuit faults.