| DC transmission technology has a huge advantage in low-cost transmission.Modular Multilevel Converter(MMC)has a high degree of modularity,good waveform quality,easy installation and disassembly,and easy to increase voltage and power.These advantages make modular multilevel converters a research hotspot in the field of high voltage flexible DC transmission.The MMC operates in a flexible DC transmission system with its AC side connected to the AC grid or AC load.Natural disasters such as lightning strikes and strong winds may cause asymmetrical faults such as single-phase grounding,two-phase grounding,and two-phase short-circuiting on the MMC AC side.Asymmetric faults on the AC side of the MMC cause double-frequency fluctuations in the DC-side voltage and current,causing asymmetry in the three-phase circulating current and large fluctuations in the capacitance of the sub-modules.In this paper,based on three types of typical AC side asymmetry faults,the above problems are studied.First,there are two typical AC side control strategies for asymmetric operation on the MMC AC side.One is the negative sequence current suppression strategy,and the other is the active power fluctuation suppression strategy.In this paper,the causes of the second-frequency fluctuation of the voltage and current on the DC side of the MMC under two AC-side control strategies are analyzed.Through theoretical analysis,the magnitude of the double-frequency fluctuation of the MMC DC side current when the three types of AC side asymmetric faults are compared.MMC has two operating modes:rectification and inverter.In this paper,the influence of the asymmetric fault on the AC side of the MMC on the DC-side voltage and current in the two operating modes is analyzed.The above analysis was verified by simulation.Secondly,the influence of two AC-side control strategies on the sub-module capacitor voltage fluctuation is analyzed.The wave equations of the capacitor voltages of the six bridge arm submodules are derived from the angle of the bridge arm charging current.The relationship between the single-phase voltage drop of the MMC AC side and the capacitor voltage fluctuation of the fault phase sub-module is analyzed under two AC-side control strategies.The above analysis results were verified by simulation.Again,the MMC traditional control strategy uses the regulation of the alternating current to achieve control of the DC voltage.When an asymmetrical fault occurs on the AC side of the MMC,the AC current changes and directly affects the DC side voltage and current.Aiming at this problem,this paper proposes an asymmetric operation strategy of MMC AC side based on AC and DC side power decoupling.The power fluctuation caused by the change of the alternating current is absorbed in the bridge armcapacitor so that it does not affect the DC side voltage and current.For the two operating modes of MMC,the DC side control strategy is designed.The overall control strategy of asymmetric operation of MMC back-to-back system based on AC and DC side power decoupling is designed.The proposed control strategy is verified by simulation.Finally,a three-phase 600V/10 kW MMC experimental platform was built in this paper.The correctness and effectiveness of the asymmetric operation strategy of the proposed MMC AC side are verified by experiments. |
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