Research On Fault-tolerant Control Strategy Of Three-level Inverter

The three-level Neutral Point Clamped(NPC)inverter is widely used in motor drive systems and new energy applications due to its high inverter efficiency and low output harmonics.However,harsh operating conditions can cause switching devices to fail,reducing system reliability and even causing damage to equipment and personal safety.Therefore,it is of great significance to adopt an effective and reliable fault-tolerant control strategy when a fault occurs in order to maintain stable operation of the inverter.The traditional fault-tolerant control strategy of NPC three-level inverter can handle limited fault conditions.Besides,it does not consider the problems of system output performance degradation under fault-tolerant operation.The following research is carried out:First,this thesis introduces the topology and space vector pulse width modulation(SVPWM)control principle of NPC three-level inverter.Then,the variation of voltage space vector is analyzed after IGBT open-circuit fault.According to different shapes of voltage vector distribution,all the open-circuit fault conditions are classified into type-I,type-II and type-III.The speed control model of permanent magnet synchronous motor with NPC three-level inverter is constructed.Besides,the system current,torque and speed data are obtained with the inverter under the normal operation and fault operation,which provided the theoretical basis for further research.A four-leg fault-tolerant topology is proposed on the basis of fault degradation reconstruction.The type-III fault problem is solved,which cannot be handled by traditional fault-tolerant methods.Above all,the type-I and type-II fault of inverter are analyzed.The fault-tolerant control strategy of voltage space vector redundancy and three-leg topology reconstruction is studied.Then,combining the advantages of the two fault-tolerant methods,the fourth redundant leg and two sets of triac thyristors are used to realize the fault degradation reconstruction.Finally,the type-III fault is converted into type-I or type-II fault.The fault-tolerant SVPWM algorithm under type-I and type-II fault is used to realize the fault-tolerant control of the inverter.The Matlab/Simulink simulation results show that the proposed method can balance the three-phase current in the fault state.Besides,the motor torque ripple and speed fluctuation are reduced,which effectively improves the output performance of the system.The maximum linear modulation range of the inverter is halved under fault-tolerant control,and the reference vector cannot be actually modulated and output.An over-modulation strategy based on the original small region of SVPWM is proposed in this thesis.Using the four small regions of the SVPWM algorithm of NPC three-level inverter,the correction method of the reference vector in different regions is given.Besides,the calculation process and can be easily converted into digital form with the principle of minimizing phase error.The simulation results show that the proposed method can make the three-phase current waveform smoother and reduce the total harmonic distortion under fault tolerant control.It can effectively reduces the torque ripple of the motor and improves the stability of the system output.Finally,an experimental platform for fault-tolerant control of NPC three-level inverter with TMS320F2812 is built.The fault-tolerant control strategy of IGBT open-circuit fault was experimentally verified.By comparing the waveform changes of the three-phase current,it can be seen that the output performance of the system is significantly improved after the fault-tolerant control.At the same time,the experimental results are consistent with the theoretical analysis,which verifies the effectiveness and feasibility of the proposed method.