Research On The Control Strategy Of Flexible HVDC System Based On Modular Multilevel Converter

As a new type of converter topology,modular multilevel converter(MMC)is widely used in HVDC transmission system because of its advantages of convenient voltage sharing,highly modular structure and high operation reliability,In this paper,the high voltage direct current based on modular multilevel converter(MMC-HVDC)is studied.The topology,basic working principle,control strategy of mmc-hvdc system under normal condition and unbalanced grid voltage condition are studied,The details are as follows:Firstly,the topology of modular multilevel converter is introduced,and the working principle and operating characteristics of MMC sub module are described in detail.Two common modulation strategies of MMC and the generation mechanism of internal circulating current are deeply analyzed.At the same time,the basic operating principle of MMC-HVDC system is introduced.Secondly,the control principle of mmc-hvdc system is introduced,and the conventional PI current control principle based on the mathematical model of dq coordinate system is described.Aiming at the problems of the conventional PI current control principle,such as complex control system design and large amount of calculation,an improved MMC hierarchical model predictive control strategy is proposed,The predictive model of mmc-hvdc system is established.Through the hierarchical optimization control of MMC AC side current,sub module capacitance and internal circulating current,the coordinated control of internal and external different control objectives of mmc-hvdc system is realized.The corresponding simulation model is established on matlab/Simulink platform,and the effectiveness of the proposed control strategy is verified.Finally,the AC side operation characteristics of mmc-hvdc system under unbalanced grid voltage are analyzed,the power compensation principle based on direct power control is described,and a new direct power control method without power compensation is introduced.In view of the complexity of the traditional power outer loop current inner loop double-layer control structure,based on the direct power control principle and combined with the MMC hierarchical model predictive control strategy proposed in Chapter 3,a MMC direct power predictive control strategy under unbalanced grid voltage condition is proposed,which realizes the synchronous control of the AC side of the system,the internal circulating current of MMC and the capacitor voltage of sub module under unbalanced grid voltage,and is implemented in Matlab/Simulink A 51 level two terminal mmc-hdc simulation model is built to verify the effectiveness of the proposed control strategy.