| In practical applications,the problem of bridge arm faults in three-level inverters often occurs,which can lead to serious consequences such as unstable output voltage and equipment damage.Therefore,designing a fault-tolerant structure for inverters is crucial.Among the various control strategies for inverters,model predictive control strategy has the characteristics of fast dynamic response,convenient handling of constraint conditions,and good robustness.However,conventional model predictive control methods still have problems such as large fluctuations in the DC-side midpoint voltage of the three-level inverter and large output current harmonics.Therefore,this article conducts in-depth analysis of the bridge arm fault problem in the three-level T-type inverter and researches the fault reconstruction fault-tolerant inverter and corresponding fault-tolerant control.The main research contents are as follows:Firstly,this article analyzes the working principle and mathematical model of the three-level T-type inverter.Analyze the possible faults of vertical,horizontal,and hybrid bridge arms that may occur in any phase bridge arm of the inverter,and propose corresponding solutions.For the most serious hybrid bridge arm fault,a fault-tolerant topology of the three-level T-type inverter is proposed.When a bridge arm fault occurs,the hardware circuit is reconfigured as a three-phase eight-switch inverter.The working principle,voltage vector,and DC-side midpoint voltage ripple imbalance of the three-phase eight-switch inverter are analyzed in detail,providing theoretical basis for designing control strategies.Secondly,this paper elaborates on the basic principles of model predictive control,establishes a mathematical model of a three-phase eight-switch inverter,and analyzes the conventional multi-objective optimization model predictive fault-tolerant control strategy.However,this control strategy uses a single vector action in each switching period,which leads to large tracking reference vector errors,DC side midpoint voltage fluctuations,and large output current and power fluctuations.Therefore,this paper proposes a double-vector modulation MPC-based fault-tolerant control strategy and proposes a bias current injection scheme to solve the balance control of DC side midpoint voltage.Through a comparison of simulation results under two control strategies,the proposed double-vector MPC-based fault-tolerant control can effectively reduce the output grid current harmonic,reduce DC side midpoint voltage fluctuations and make them more balanced.Then,to verify the reliability of the proposed control strategy for fault-tolerant operation of the inverter,this paper developed a loss model and junction temperature model for the Si C-Mosfet power device used in the inverter.To further validate the effectiveness of the proposed control strategy,a method based on power device multiphysics analysis and Simulink co-simulation was adopted.Based on the data sheet of the Si C-Mosfet,an internal model of the Si C-Mosfet was built in the Comsol multiphysics simulation software,and the junction temperature of the inverter power device was observed in real-time through simulation.The simulation results show that under the proposed control strategy,the Si C-Mosfet of the inverter has small loss and junction temperature,which can realize the stable fault-tolerant operation of the inverter.Finally,a three-phase eight-switch inverter experimental platform was established to compare and verify the conventional multi-objective optimization model predictive fault-tolerant control and the proposed dual-vector modulation model predictive fault-tolerant control.The experimental results showed that the proposed control strategy not only could output good power quality after a fault occurred,maintain a low harmonic distortion rate of the grid current,but also could suppress the midpoint voltage fluctuation in a lower range,which met the requirements of improving system reliability and stability. |
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