| In recent years,three-level converters have been extensively used in some highvoltage and high-power fields due to their superiority in output waveform harmonics,device voltage stress and loss.However,while the performance is improved,the increase in the number of switches also means that the control complexity of the inverter and the probability of failure are significantly improved,and the security of the system is also threatened.Therefore,in order to avoid the serious consequences that may be caused by the failure,it is very important to study the reliability of multilevel converters.In this paper,the common three-phase three-level flying capacitor(FC)inverter is selected as the research object,and the main research is carried out on its modulation algorithm,fault detection and fault-tolerant control strategy.Firstly,an improved SVPWM algorithm is proposed in view of the shortcomings of the traditional space vector modulation(SVPWM)algorithm which has a large amount of calculation.After a brief introduction to the topology,working principle and the implementation process of the traditional SVPWM algorithm of the three-phase three-level flying capacitor inverter,the space vector is mapped from the rectangular coordinate system to the 120° coordinate system,which greatly simplifies the implementation process.By adding phase-shifted carrier modulation and equivalent duty cycle calculation module in the algorithm process,the automatic balance of flying capacitor voltage is realized,and the inverter obtains better output performance.The inverter model is built in the simulation software,and the advantages of the improved algorithm are verified by comparing and analyzing the SVPWM algorithm in the two coordinate systems respectively on the inverter.Secondly,a fault detection method for a three-phase three-level flying capacitor inverter is proposed,which provides necessary conditions for subsequent fault-tolerant control.The faults of the inverter switching devices are classified,and the load current flow of the inverter under the conditions of no fault,outer tube open-circuit fault,inner tube open-circuit fault,outer tube short-circuit fault and inner tube short-circuit fault is compared,and the variation characteristics of voltage of flying capacitor and load current under different faults are obtained.Accordingly,the flying capacitor voltage and load current are extracted as the characteristic information of fault detection.It is verified by simulation that the method can achieve accurate fault location,and the implementation algorithm is relatively simple and the calculation amount is small.Then,a general fault-tolerant control strategy is proposed.In order to provide redundant current paths under fault conditions,thyristors are required in parallel across each switching device.After locating the faulty switching device of the inverter,the bypass of the faulty device is realized by controlling the conduction of the thyristor,and the single-tube fault is transformed into a short-circuit fault for subsequent processing.According to the space vector distribution of the inverter after the faulty device is bypassed,the fault area is differentiated in the vector diagram.In the non-faulty area,the redundant space vector is used to realize fault-tolerant control,while in the faulty area,the boundary vector is selected to synthesize the reference vector,so as to ensure that the inverter can continue to operate after a fault occurs,which significantly improves the reliability of the inverter.The operability and effectiveness of the method are verified by simulation.Finally,the RT-LAB hardware-in-the-loop simulation platform and its hardware and software design are introduced.Based on this platform,a fault-tolerant control model of a three-phase three-level flying capacitor inverter is built,which verifies the feasibility and effectiveness of the new modulation algorithm,fault detection and faulttolerant control strategy proposed in this paper. |
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