Research On Fault Diagnosis And Fault-tolerant Control Of NPC Three-level Inverter In Motor Drive System

Motor drive systems,targeted as globally leading dynamic systems and important mechanisms for electromechanical energy conversion,are urgently needed in areas of transportation,aerospace,renewable energy generation,etc.Neutral Point Clamped(NPC)three-level inverters are gradually becoming important mechanisms of electric energy conversion for motor drive systems with characteristics of middle-high voltage and high power,owing to the superiorities of simple topological structure,high work efficiency,low voltage stress of power components,etc.However,because of complicated environments of motor drive systems,the open-circuit faults of NPC threelevel power components occur frequently.The efficiency and reliability of motor drive systems are severely affected.Therefore,it is of great theoretical significance and practical application value to carry out studies on fault diagnosis and fault-tolerant control of NPC three-level inverters.NPC three-level inverters are chosen as research objects in this thesis.The open-circuit fault diagnosis problems of power components are studied under conditions of uncertainties of analytical model parameters and external disturbances,sheer amount of monitoring data,unstable signals,etc.Since the inverter topology varies during failures occurrence,the functioning time of voltage space vector is anomalous and the driving capability is insufficient.The fault-tolerant control problems are studied under the state of failures occurrence.The research on open-circuit fault diagnosis and the research on fault-tolerant control are carried out for NPC three-level inverters.The main innovative work of this thesis is followed.1.Because of the uncertainties of analytical model parameters and external disturbances,the estimated system values are inaccurate via observers.Hence,the recognition rate of model fault is low and the anti-interference ability is poor.Based on the theory of interval sliding-mode observer,a fault diagnosis method is proposed for NPC three-level inverters.First,the topological structures and the principles of NPC three-level inverters are analyzed.The mixed logical dynamical model of inverters are established.The estimated value of current is calculated on the convex weighting of estimated three-phase current of upper and lower sliding-mode observers.The current residual is derived combining the estimated current and the practical three-phase current.Second,the characteristics of typical open-circuit faults are analyzed.Relational tables are built between threephase current residual and open-circuit faults.The diagnosis and the recognition of open-circuit faults are achieved.Finally,simulations and physical experiments are carried out for verification.Experimental results show the high accuracy and strong robustness of the proposed method.2.For an NPC three-level inverter system,the effective availability of historical monitoring data is low and the feature extraction of unstable fault signals is difficult.Therefore,a feature extraction method is proposed.The method combines Optimized Compressed Sensing(OCS)and Wavelet Packet Decomposition(WPD).Besides,fault diagnosis models are built using feature data driven method.First,the measurement matrix is optimized combining Gram matrix and fast iterative shrinkage thresholding algorithm.The relevance is reduced between measurement matrix and sparse base,and the reconstruction of compressed signals is improved.Second,local anomaly and instability of fault signals are considered.The compressed data is decomposed using wavelet packet.The sub-band signal energy is extracted,and the eigenvectors are established.Finally,classifier models about feature data sets are built.Tests are carried out for verification of OCS-WPD method in the reconstruction of compressed data and the feature extraction of faults.3.For the SVPWM-based fault-tolerant control algorithm about NPC three-level inverters,the functioning time of voltage control vectors is less than zero.As a result,the harmonics of current output are large.Therefore,a fault-tolerant control algorithm is proposed using volt-second balance theory,where the voltage control vectors are reconstructed.First,the fault classification is carried out,according to geometric characteristics of voltage space vectors of NPC three-level inverters under different open-circuit faults.Second,the functioning time of voltage space vectors in 1 st opencircuit fault is analyzed.The SVPWM overmodulation is carried out through the reconstruction of voltage space vectors using volt-second balance theory.The aberration rate of current output is reduced.Finally,the proposed algorithm is verified through simulation platform.Experimental results show that the harmonics of current outputs are significantly reduced,compared with traditional fault-tolerate control.4.For the fault-tolerate strategy of NPC three-level inverters through reconstructing voltage space vectors,the output-voltage amplitude decreases and the drive capability falls down.Hence,a fault-tolerate control strategy is proposed through the double reconstruction of topology-voltage vectors for NPC three-level inverters.First,the main circuit topology of NPC three-level inverters with fault-tolerance ability is reconstructed.The reconstruction relies on distribution characteristics of voltage space vectors and features of alternator loads.Second,the main circuit topology after reconstruction is combined with voltage space vectors.A fault-tolerate control strategy is proposed through the double reconstruction of topology-voltage vectors.The insufficiency of maximum modulation of voltage space vectors is avoided.Finally,the proposed algorithm is verified through simulation platform.Experimental results show that the proposed strategy maintains output voltage and improves current driving ability.