Research On Control Strategy Of The Converter In MMC-HVDC

MMC-HVDC transmission has attracted more and more attention in the field of HVDC transmission,because of its superiority in voltage level and capacity expansion.It is considered to be the most potential new transmission mode.The main applications of VSC-HVDC system include grid access of new energy generation,isolated load power transmission,island or offshore platform power supply,urban distribution network and asynchronous grid interconnection.Modular multilevel converter,because of its advantages of topology,easy expansion,high reliability and good quality of voltage waveform,has become one of the hottest topics in the field of international power electronics in recent years.Based on the modular multilevel converter(MMC)as the research object,the studies of the control strategies in this paper are voltage-balancing control and circulation suppression in HVDC system.The improvement and optimization of the control strategies on the MMC are aimed to achieve good tracking of control objectives in steady state operation,in order to get high efficiency switching device the trigger pulse basis.First,On the basis of MMC topology and model analysis,we have completed the design of MMC basic unit controller,and proposed using the internal model control principle to achieve the tracking control of inner loop current to reduce control parameters,enhance dynamic performance and robustness.For the sub module voltage capacitor balancing problem of MMCHVDC system with a large number of sub modules,a quick sorting partition algorithm and a keep factor method are proposed to improve the sub module capacitor voltage balancing control strategy.Based on the divide and conquer idea,the quick sorting partition algorithm is introduced for semi quick sorting,which greatly reduces the number of sorting,and the keep factor is introduced to keep the input state of the sub module as much as possible,thus reducing the switching frequency.Second,for the problems of MMC circulating current principle and internal circulating current's components,parallel composite control with PI control and repetitive control is proposed to solve the problem of MMC converter.The parallel repetitive control strategy makes the system stable rapidly in large fluctuation,and play a role in restraining the high frequency double harmonic in the circulating current.To effectively improve the stability of dynamic response speed and control effect.Finally,by building back to back dual ended MMC-HVDC model,the loop control of inner loop controller,sub module voltage balancing control and parallel control are simulated and analyzed respectively,and the correctness and feasibility of the proposed control strategy is verified.