Research On Open-Circuit Fault Diagnosis Of NPC Three-level Inverter

In recent years,the NPC three-level inverter has the advantages of flexible control,stable performance and low output voltage harmonics,so it is more and more widely used in industrial occasions.However,due to its own fragility,complexity and harsh working environment,the IGBT in inverter is prone to open-circuit fault,which will lead to equipment damage and personnel loss in severe cases.Therefore,it is of great theoretical and engineering significance to study IGBT open-circuit fault diagnosis of NPC inverter for improving the reliability and safety of the system.In this thesis,the open-circuit fault diagnosis method of NPC inverter is studied,and the specific work is as follows:Firstly,on the basis of analyzing the topology and working principle of the NPC inverter,the mathematical models of inverter based on Newton interpolation method,switching function method and MLD under normal operation are established,and the relationship and difference between the three mathematical models are analyzed.According to the on-off state and actual operation conditions of IGBT,it is divided into four types of faults:no fault,single-tube,in-phase dual-tube and out-of-phase dual-tube,and the fault mathematical model of NPC inverter based on MLD is established.In this thesis,Matlab/Simulink is used to check the fault mathematical models of three normal mathematical models and NPC-MLD model.The results show that the actual results of the three models are corresponding,which shows the precision and usefulness of the three models.Based on the above three mathematical model features and simulation results,the MLD inverter mathematical model is finally used for IGBT open-circuit fault diagnosis.Secondly,aiming at the problem of large chattering in the open-circuit fault diagnosis method of the inverter based on the classic sliding mode observer,an open-circuit fault diagnosis method based on the extended observer is proposed.In this method,a current expansion observer is designed based on the NPC-MLD model of fault and normal conditions.This method uses the difference between the current value of the observer and the practical current value of the motor drive system,the negative twofold residual is obtained for fault detection.In order to eliminate the influence of external disturbance,measurement error and noise on IGBT fault diagnosis,a combination of adaptive threshold and multi-level threshold is designed for single-tube and in-phase dual-tube faults.The residual information table and flowchart are used to locate the fault phase and fault tube respectively,which can overcome the problem of traditional threshold misdiagnosis to a certain extent.The system mutation simula-tion verification and classical sliding mode observer comparison simulation verification were carried out through Matlab/Simulink.The simulation results show that the method does not need to add additional sensors,and it still maintains excellent robustness and diagnostics when the load changes suddenly.Finally,this thesis constructed a semi-physical platform for IGBT open-circuit fault detection of NPC inverters.After applying the above detection method to the platform,and the experimental waveform and data are analyzed to illustrate the viability and practicability of the MLD-based NPC inverter fault model and fault diagnosis method.