| With the rapid development of the power electronics, more and more attention is paid to the modular multilevel converter in the field of high-voltage large-capacity technologies. As the new generation of the key facilities of the high voltage direct current(HVDC) technology, the modular multilevel converter(MMC) shows wide application prospects in the field of clean energy transmission、power system interconnection、offshore drilling platform power supply、urban city power supply and isolated island power supply. Currently, extensive studies have been done on the valve-based control strategies, such as start-up control strategy, the sub-module capacitor voltage balancing control strategy and control strategy of reducing the IGBT average switching frequency, In this dissertation, a number of key issues mentioned above are amply discussed following a literature review.(1) The topology of main circuit and sub-module(SM)、basic modulation method and control strategy of the MMC are introduced. Based on the differences of the HBSM and FBSM, a novel three-stage start-up control strategy is proposed, it was proved theoretically that the capacitor voltages of FBMMC after the uncontrolled charging process with the converter blocked were half of those of HBMMC. An ac current-limiting resistance calculating method is brought up in the aspect of maximum arm current. A novel three-stage start-up control strategy of FBMMC-HVDC was came up with to divide the uncontrolled charging process into two stages. Meanwhile, discuss the feasibility and rationality of the sustained conduction of the power devices in the second charging stage of the uncontrolled process. An electromagnetic transient simulation model is established in PSCAD/EMTDC for simulation. The simulation results verify the effectiveness and correctness of the proposed start-up control strategy.(2)A mixed sorting method adopting prime factorization method and Shell sorting algorithm for MMC is proposed. Time complexity of the Shell algorithm applied to the MMC-HVDC is deduced and the calculation formula is proposed for the sort number of the mixed sorting algorithm. Impacts on the reduction of the frequency by the grouping layers are analyzed, conclusion that the optimizing efficiency of the mixed sorting method is in inversely-proportional relationship with the grouping layers is acquired. Comparing vertification is carried out among the traditional sorting algorithm、prime factorization method and mixed sorting method, the simulation results show that the proposed method is able to keep the capacitor voltage in the dynamic balance, and the simulation speed is largely enhanced.(3)A valve-based active voltage balancing control strategy for MMC is proposed. First of all, a modified capacitor voltage balancing method is designed and the mathematical relationship between the amplitude of the capacitor voltages of sub-modules and the IGBT average switching frequency is deduced, which provides a reference for the design of the amplitude of the capacitor voltages in the active-valve-based control strategy. Then, a module for on-line measuring the IGBT average switching frequency is designed, which realizes the row window-type dynamic measurement of the switching frequency. Besides, a closed-loop PI controller for the quantitative control of the IGBT average frequency is proposed. Finally, a simulation system is built in PSCAD/EMTDC for simulation under the steady state、power reversal operation and the fault operation. The simulation results prove the robustness and engineering applicability of the proposed active-valve-based control strategy. |
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