| Increasing crisis of energy and environment has brought great distress to the survival of mankind, It is difficult to maintain the use pattern of traditional fossil energy, which is characterized with high consumption and heavy pollution, optimizing the resources allocation and seeking clean alternative energy are the keys to achieving sustainable development. Currently, the energy landscape, which puts electrical energy as the center, is further highlighted. Therefore establishing an efficient, strong and flexible transmission network and promoting large-scale renewable energy integration to the grid have become the fundamental way to alleviating environmental pressure, cracking energy dilemma, boosting energy production and consumption revolution.Because of its significant advantage in long-distance, high-power transmission and renewable energy integration to the grid and other fields, the flexible HVDC transmission technology based on modular multilevel converter has been given strong attention and universal favor in the industry. The MMC-HVDC system is selected as study object for the paper, on the basis of giving detailed description about its baisc principle and relevant characteristic, the control strategy of MMC-HVDC system is treated as focus and given in-depth study, and the application of MMC-HVDC system in offshore wind power integration to the grid is expanded specific analysis.First of all, the topology structure, working principle and operating characteristic of MMC are analyzed in detail, the mathematical models of MMC-HVDC system are constructed in three coordinate systems respectively, and the pre-charging control strategy of MMC is studied and designed.Secondly, the application of carrier phase-shifting pulse width modulation(CPS-PWM) strategy and nearest level modulation(NLM) strategy in MMC-HVDC valve-level control system is introduced respectively. On the basis of reasoning out the capacitor voltage fluctuation mechanism and inter-phase circulating current generating nature of MMC, the conventional capacitor voltage sorting algorithm is improved and optimized in order to reduce the switch frequency, then combining with the method of superimposing balance component, a kind of capacitor voltage balancing control strategy is designed. Additionally, by introducing discrete mathematical model, a kind of inter-phase circulating current suppressor based on discrete mathematical model is designed. Lastly, the effectiveness of the proposed control strategy and controller is confirmed by simulation.Futhermore, the hierarchical control structure of MMC-HVDC system is expounded, the control principle and control mode of MMC station are analyzed, and the double closed-loop control strategy of MMC station are expanded research focusly, then the inner-loop current controller and outer-loop power controller of MMC station are designed specifically. On the basis of integrating system-level control with valve-level control, and aiming at two kinds of different occasions of two-side active system and the system supplying to a passive network, the total control strategy of MMC-HVDC system is designed respectively. Finally, the proposed control strategies are subjected to analyzing and testing by simulation in a variety of steady operating conditions, the simulation result proves their correctness.Finally, the mode and topology for grid integration of offshore wind farm are discussed, and the superiority of MMC-HVDC system in grid integration of offshore wind farm is pointed out, the basic structure and control principle of DFIG are given a preliminary analysis. The control strategy of grid integration of offshore wind farm by MMC-HVDC system is studied in particular, additionally, based on the relational properties of offshore wind farm, the controller of WFMMC station is devised especially. Finally, the proposed control strategy is studied and analyzed by simulation in different transient operating conditions, the simulation result verifies its validity. |
PDF Full Text Request