Study On Model Predictive Fault-Tolerant Control For Open-Circuit Fault Of T-Type Three-Level Converter-Based Systems

In photovoltaic and wind power generation systems,power converters are the core part of realizing energy conversion,and their failure will affect the normal operation of the entire power generation system,bringing security problems and economic losses that cannot be ignored.Therefore,improving the reliability of power converters has become a current research hotspot.This thesis uses three T-type three-level converter-based systems: the T-type three-level inverter,parallel T-type three-level inverter system,and the T-type three-level active power filter(APF),as the research objects.Based on the finite control set model predictive control(FCS-MPC)method,the software fault-tolerant control strategy of the system after an open-circuit fault occurs in a single switching device is studied,which improves the system’s reliability.The application of FCS-MPC in fault-tolerant control is extended.The main work and innovations of this thesis are as follows:Firstly,this thesis introduces the basic working principle of the T-type three-level converter system,including the system topology display and the converter’s output vector analysis.The discrete mathematical model of the LCL filter,the system’s neutral-point voltage,and the reference value’s mathematical expression are constructed.The opencircuit faults of horizontal and vertical devices in a T-type three-level converter are analyzed theoretically,and the conclusion that fault-tolerant control should be designed differently for horizontal and vertical faults is obtained.The judgment criteria of healthy and unhealthy states in fault operation are given,providing a sufficient theoretical basis for subsequent research content.Secondly,aiming at the healthy state and unhealthy state of a T-type three-level inverter,the weightless FCS-MPC and the finite-control-set-optimized sequential model predictive control(SMPC)are designed,respectively.The weightless FCS-MPC simplifies the control of the neutral-point voltage by deriving the neutral-point voltage output vector table and finding the sector corresponding to the reference output vector.The finite-control-set-optimized SMPC simplifies and reconstructs the finite control set to achieve fault tolerance for open circuit faults of horizontal devices.Compared with the traditional FCS-MPC,the above two proposed methods can reduce the weighting numbers’ trial time and the algorithm’s computational complexity.The effectiveness of the fault-tolerant control is demonstrated in simulation and experimental analysis.Thirdly,aiming at the fault-free state of the parallel T-type three-level inverters,the zero-sequence circulation current(ZSCC)model of the system is analyzed,and a pseudothree-layer SMPC is proposed based on the principle of finite-control-set-optimized SMPC.The grid-side current quality and ZSCC suppression under fault-free conditions are guaranteed.With the help of the grid-side and converter-side simulation waveforms under fault,the inaccuracy of mathematical modeling under the open-circuit fault of vertical components is analyzed.Based on the simulation analysis,a phase-deficient SMPC is designed,which solves modeling imprecision under fault conditions and realizes the approximate fault-tolerant control of the parallel system.The effectiveness of the fault-tolerant control is verified in the experimental analysis.Finally,for T-type three-level APF,based on its topology analysis and harmonic extraction method,and according to its operation characteristics under fault-free,horizontal,and vertical fault conditions,an adaptive DC bus voltage control method is proposed,which can ensure sufficient modulation range under any condition.According to the principle of finite-control-set-optimized SMPC,two fault-tolerant controls,threelayer SMPC,and weighting-number-optimized SMPC are designed,which can realize the open-circuit fault tolerant control of any switching device of the active power filter.In addition,the weighting-number-optimized SMPC can output the weighting number adaptively,which shows the non-integer characteristics of the weight number at the macro level and makes the control effect more flexible.The effectiveness of the fault-tolerant control is verified in the experimental analysis.