Control Strategy Reserch Of VSC-HVDC Based On Modular Multilevel Converter

VSC-HVDC,with large capacity and low loss,has incomparable advantages over traditional HVDC and AC transmission in solving the long-distance transport of power,new energy grid,the urban distribution network capacity improvement and etc.,which gradually becomes the research focus nowadays.While VSC-HVDC system based on modular multilevel converter,which is more flexible,economical,and environmental,will play an important role in DC power transmission field.MMC converter is the connection between AC and DC,and its voltage equalization strategy,circulation current suppression strategy and the control strategy of MMC-HVDC system are the core of this research:Firstly,the research object is the modular multilevel converter.On the basis of theoretical analysis and mathematical modeling to the operating characteristics of the MMC converter and its sub modules,the principle of selecting main parameters and modulation strategies for MMC are expounded.The voltage sorting can control the fluctuation of the capacitance voltage effectively without controller,which proposed as the voltage balancing strategy of MMC.The simulation is verified preliminarily combining with the NLM modulation strategy as the output mode of modulation wave.Secondly,the mechanism of circulation current and the properties of double frequency and negative sequenced are analyzed through the perspective of energy transfer.On the basis of the traditional circulation current suppression strategy,a new circulation current suppression strategy is designed by the quasi-proportional resonance controller under two-phase stationary ?? coordinate system.The control strategy can realize the accurate and rapid control of the circulation current while avoiding the adverse effect of the current coupling term on the control system,and the simulation of two kinds of circulation current suppression strategies are compared and analyzed.Finally,a control strategy without PLL is designed by studying the operation principle and control strategy of MMC-HVDC system.The control strategy can achieve the same dynamic response compared to the double closed loop control strategy under dq coordinate and its decoupling performance is better.A double-ended MMC-HVDC system is simulated under the conditions of power flip and DC voltage step with PSCAD/EMTDC software.The effective of the proposed control strategies are verified through the analysis of the simulation results,which can ensure the stable operation of MMC-HVDC system under these conditions.