Study Of Modular Multilevel Converter In Flexible HVDC System

In renewable energy grid-connected and isolated power supply and other areas, voltage source converter based high voltage direct current(VSC-HVDC) has a huge advantage that has caused the extensive research and application. Voltage source converter is the most important components of VSC-HVDC, of which modular multilevel converter(MMC) with its modular and scalable becomes the most potential converter topology in high-power long-distance HVDC systems. In this paper, the modular multilevel converter based on VSC-HVDC System as the research object, mainly study the topologies of MMC, sub-module capacitor voltage balancing, circulating current suppression, control strategy and other issues.Firstly, we introduce the working principle, mathematical model and operating characteristics of MMC, and then analysis the principle and applicable scope of MMC's common modulation strategies(i.e. the nearest level modulation and carrier phase shifted modulation). Sub-module capacitor voltage balancing method are drawn out for the two modulation strategies, respectively. In view of the circulation currents of converter arms in MMC, the generation mechanism and suppression technology are analyzed.Secondly, In this paper, the topology structure, working principle and the DC fault ride-through capability are detailed analyzed for these three traditional MMC—half-bridge sub-module(HBSM), full-bridge sub-module(FBSM) and clamping double sub-module(CDSM), and the advantages and disadvantages of these different sub- modules are also summarized. On this basis, we propose a hybrid MMC, which is composed by FBSM and HBSM together to form the upper and lower arm of MMC. Its selecting principle of basic parameters is described, and the sub-module capacitor voltage balancing method for hybrid MMC topology is also designed, besides, analyze the system good operation performance under the condition of DC voltage drop.The simulation results validate that the proposed hybrid MMC and improved capacitor voltage balancing strategy are feasible and effective,and point out the hybrid MMC not only has the ability of passing through fault in DC side,but also has higher capacity utilization, lower power consumption and a good application prospect.Finally, we elaborate the basic principles of MMC-HVDC control, establish mathematical model under dq rotating coordinate and design direct current controller. In the actual application of MMC-HVDC, for direct current control exist control complex and calculation intensive, we propose a model predictive control(MPC) strategy. The policy develop the predictive model based on discrete-time mathematical model for MMC-HVDC,the best switching states of each MMC unit can be obtained by solving an optimization problem, which can eliminate the circulating currents and achieve the voltage balance of capacitor by redundant switching states. The simulation results show that the MMC-HVDC system based on MPC strategy operates better, implementation process easy and simple.