Development Of Control System For Modular Multilevel Converter

Without clamping devices and separate DC sources, Modular MultilevelConverter (MMC) has a modular structure, so it’s easy to expand to any level. Withcommon DC-link, it’s convenient to work on four-quadrant operation. According tothe mentioned advantage, it attracts widespread attention.Because the voltage step which synthesize AC output voltage is provided bythe Sub-Module(SM) capacitors divide a DC source, the balance of the SMcapacitors voltage is crucial for the stable operation of the MMC. This article firstclassified summarized the research status of the MMC, MMC topology, the basicoperating principles, analytical derivation of the upper and lower leg MMC normalworking voltage, output AC voltage and current of the upper and lower legcirculation and AC output current the relationship between a detailed analysis ofthe nature of the working mechanism of the carrier-based layered and carrier phaseshift modulation MMC modulation strategy,In the presence of MMC topology, a large number of suspended capacitor mustensure the existence of the capacitor voltage before the MMC’s normal work, to doa detailed analysis of the capacitor charging process both the MMC start the project,describes the start in two ways:"It Lai" and "self-Reed both start a detailedanalysis of the working principle of both start-up mode and comparative analysis ofthe advantages and disadvantages of the two kinds of start-up mode to buildphysical simulation model.At low-frequency, the SM capacitor voltage eripple increases rapidly. Based onthe analysis of the ripple, the low-frequency ripple of the arm power change tohigh-frequency ripple by the arm voltage and current reconstruction, so that theamplitude of the SM capacitor voltage ripple is greatly reduced.A three-phasefive-level model is build and presented low-frequency voltage ripple restrainstrategy is simulated, simulation results verify the correctness of the strategy.Finally, three single-phases Nine-level MMC experimental prototype isdesigned and built. These include: design phase MMC hardware systems andthree-phase MMC software control system, including hardware system consists of amain circuit design of three-phase MMC and three-phase MMC DSP controller andFPGA controller design; using DSP and FPGA main from work mode to completethe hardware control system design, software control system includes a mastercontroller DSP software design and auxiliary controller FPGA software design.Completion the normal MMC Experimental demand, and start the program and thelow-frequency equalizing strategy experimental verification, the experimental results further demonstrate the effectiveness of the strategy used.