| Modular multilevel converter(MMC)is different from the traditional two-level or three-level converter,which is in high modularity and flexibility.By cascading submodules,the MMC could be applied in any voltage-level applications.As it is used in high voltage and large power applications,MMC could avoid the direct connections of power switching devices,which significantly reduce the difficulties of manufacture.Due to the large number of the voltage levels at the ac side of MMC,the MMC could be connected to the grid without filters,which reduces the cost of the MMC based high voltage direct current(HVDC)transmission systems.Moreover,the submodules in MMC could be redundant to each other,which improves the reliability of the MMC.Owing to its advantages,MMC has been considered as the most promising converter in the HVDC systems.In order to operate the MMC safely,it is significant to investigate the operation control,fault diagnosis,and new topology of the MMC.In this paper,the circulating current suppression,fault diagnosis,no-capacitor-sensor operation and novel submodule topology of MMC are investigated,and the simulation and experimental results verify the effectiveness of the proposed strategies.The main contents of the thesis include the following aspects:1.The circulating current suppression control of MMC is studied.A novel circulating current suppression control strategy is proposed,where the predictive control is used to select the most appropriate redundant state of the MMC to suppress the circulating current.The selection of the redundant state for predictive control is narrowed theoretically,which reduces the calculation burden of the proposed strategy.The simulation and experimental results verify the effectiveness of the proposed strategy.2.The effect of faulty power switching devices on the full-arm voltage and capacitor voltage is analyzed.A fault diagnosis strategy based on extended state observer is proposed,where the change rate of the full-arm voltage and the capacitor voltage are adopted to detect and locate the faulty power switching device.The effectiveness of the proposed strategy is verified by the simulation and experimental results.3.Voltage sensor-based fault diagnosis strategy for power switching device fault is studied.Based on the analysis of the current paths under normal and fault condition,a new configuration of voltage sensor in submodules for fault diagnosis is proposed,where the relationships between the output of the voltage sensor and operating states of the submodule are adopted as the fault indicator.On the basis of the configuration of the voltage sensor,a capacitor voltage observer is established to guarantee the operation of the MMC.In addition,during the period of the positive arm current,the rate of observed and measured increment of capacitor voltage is adopted to estimate the capacitor capacitance in the submodule.The effectiveness of the proposed strategy is verified by the experimental results.4.The operation of MMC without capacitor voltage sensors is studied.A new configuration of voltages sensor is proposed to remove the capacitor voltage sensors.Based on the new configuration of the voltage sensor and Kamlan Filter,a capacitor voltage observer is build up.The simulation and experimental results verify the effectiveness of the proposed strategy.5.Novel submodules for MMC are investigated.Two novel submodules with symmetrical dc fault blocking capability are proposed,which improves the reliability of the MMC based HVDC system.The effectiveness of the proposed submodules is verified by the simulation and experimental results.6.Based on the real-time Micro Lab Box controller,an experimental platform for modular multilevel converter is built,which lay a foundation for verifying the effectiveness of the proposed circulating current suppression control strategy,fault diagnosis strategy and novel submodules. |
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