| Modular Multilevel Converter is a new kind of converter topology,the AC and DC output quality is excellent,transmission capacity is large and it can achieve active and reactive decoupling control,etc.MMC is the most suitable structures for HVDC systems.The main challenges of MMC-HVDC is the output current is unbalance,circulating current contains high-order harmonics and high ripple of direct line voltage under unbalanced fault.In order to solve these problems,this thesis analysis the the work principle of MMC and research the outside/inside optimial control methods of MMC.Mix Sequence Injection Low Voltage Ride Through method is proposed to reduce the unbalance current under unbalanced fault.The MMC circuit structure and working principle are in-depth analysised,MMC average-arm model is built by arm-average method,output current and circulating current controller are peoposed,the overall control system is designed,include circulating current harmonics suppressor,reactive/reactive power controller.According to the grid code,a method which called MSI-LVRT could reduce the d-axis double grid freduency conponent is designed.The effectiveness of MSI-LVRT control strategy is verified by simulation.The direct line voltage ripple is high when the tranditional zero-sequence circulating current controller work under unbalanced condition.A new zero-sequence circulating current contoller which could reduce the direct line voltage ripple under balanced contion is proposed.According to the MMC working principle,the direct voltage controller is designed.The equivalent circuit model is used to analyze the mechanism of DC voltage ripple under balanced condition.According to the analysis results,this paper presents a new type of zero-sequence circulating current controller to suppress the DC bus voltage ripple,effectively suppress the DC bus capacitor voltage double grid frequency ripple.The validity of the zero sequence loop controller is verified by simulation. |
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