Research On Operation And Control Method Of MMC-HVDC

In the late 1990 s, with the development of full-controlled power electronic devices, the third generation HVDC technology, mainly combined with Voltage Source Converter(VSC), begins to rise. Modular Multilevel Converter, as a new member of VSC, has been used for DC transmission field for its special modular topology structure. The technology of MMC points out a new direction in HVDC field.In this paper, we focus on the working principle, modulation and control methods of the modular multilevel converter in HVDC field. The main content is as follows:(1) The process of HVDC technology development, different topology structure converters appeared with different HVDC technology and research status of different topology structure converters have been briefly introduced at the beginning. In particular, we pointed out the research conducted on MMC in this paper.(2) It attempts to analyze the internal topology structure and the operating principle of sub-module. At the same time, the pre-charging process and operating process on the balance of sub-module capacitor voltage are mentioned here. Comprehensive explanations on these two processes are also made here.(3) Modulation strategy used in VSC and based on PWM is described in detail. After that, according to the modular feature and the need for multi-level output in the actual operation, Nearest Level Modulation(NLM) algorithm has been presented and applied to the simulation model at the end of the text.(4) The control strategy that suitable for MMC-HVDC system is designed in detail. By getting the basic mathematical models of MMC, we could indicate that MMC-HVDC system can use the conventional VSC-HVDC control strategy. Then we will pay more attention on converter station-level control strategy in MMC-HVDC.(5) One 9-level MMC-HVDC transmission model has been built in PSCAD/EMTDC environment. Based on this 9-level model, we could vertify the correctness of pre-charging procedure, capacitor voltage balance algorithm and converter station-level control strategy through the analysis of experimental data generated in this model. At last, for building a experimental prototype, the reference value of the simulation is presented.