Control Strategy Research For HVDC Transmission System Based On MMC

The traditional two level converters(VSC)in the field of high voltage direct current(HVDC),the performance and voltage level are more and more limited,and the modular multilevel converter(MMC)benefits from its unique topological structure,it has the advantage of many other low level converters that are not comparable,so that MMC-HVDC technology has been developed rapidly in the field of power transmission.In this paper,we study the MMC topology,the operation mechanism,the modulation technology,the bridge arm voltage equalization and the circulation suppression,and propose an improved algorithm for the control of the voltage and the current loop suppression algorithm based on memory,compared to the traditional control algorithm has the beneficial effect.Firstly,the main circuit topology and operating mechanism of MMC are analyzed.Based on this,the working principle of three-phase MMC and the main modulation strategy are described.The mathematical model of MMC is established,and the relationship between the AC side and DC side is analyzed,and the energy exchange mechanism between the bridge sub module capacitance and the bridge arm inductance element is analyzed,then,the operating characteristics of MMC are analyzed,and then the control principle is revealed,this laid the foundation for the later chapters of the coordinate transformation and control strategy.Secondly,on the basis of analyzing the principle of sub module capacitor voltage fluctuation and the mechanism of the bridge arm circulation,the sub module voltage equalization and the bridge arm circulation suppression strategy are proposed.In the research of voltage balance control,this thesis mainly introduces the conventional capacitance voltage sequencing control strategy based on the nearest level modulation,on the basis of this,an improved voltage equalization algorithm is proposed,and the control effects of the two methods are compared in the Matlab/Simulink platform;In circulating current control,the principle of memory control is introduced in detail,and it is applied to the circulating current suppressor,its performance is compared with that of the traditional PI controller,and the simulation is verified.Finally,the control strategy of MMC-HVDC system is analyzed;the effect of the control strategy based on the two-phase static coordinate system and the two-phase rotating coordinate system is compared under the condition of steady state control and AC system transient control.This paper presents the design of controller with proportional resonant control in coordinate system,and the simulation results are given.In terms of steady-state control strategy,the mathematical model based on dq coordinate system and ?? coordinate system is analyzed and compared,and the conclusion that there is no cross coupling in the ?? coordinate system is obtained.Based on the PR controller,the current inner loop controller is designed,and then matches the power outer loop controller based on PI control,instead of the traditional dual PI controller as the steady state controller of MMC-HVDC system.In the case of asymmetric fault in AC system,the mathematical model of positive and negative sequence components in the dq rotating coordinate system and the ?? static coordinate system is established by using the symmetrical component method,In this paper,the PR control is selected as the negative sequence current controller in the ??coordinate system,which simplifies the structure of the controller.