| With the continuous development of power electronics technology,it is accompanied by the research and exploitation of various new-type multi-level converter topology and switching devices,which virtually solves relative problems faced by highvoltage and high-power applications.Since Modular Multilevel Converter(MMC)came out in 2002,it has been widely applied in the UHVDC power transmission field due to its good expandability,redundancy and output characteristics,and several flexible multi-terminal transmission projects have been put into operation officially.With the increasing demand for power and voltage level in the field of electrical transmission,MMC converter has gradually become a motor drive control converter with great research value.Therefore,this paper studies the key control strategy of MMC converter.To begin with,this paper simply introduces the common Sub-Module structures and working conditions.Through mathematical modeling of MMC converter,the basic operation mechanism is analyzed.On the basis of working principle,three modulation strategies of multilevel converter are compared.This paper emphatically analyzes the application situation in MMC converter when carrier horizontal Phase-Shift modulation strategy outputs N+1 and 2N+1 level respectively.Then,a control method that can synchronously complete the pre-charging of the three-phase bridge-arm Sub-Module by using the dc-side common dc-bus is simply studied,and the dual-closed-loop control method of the Sub-Module capacitor voltage equalization control of the MMC converter’s basic frequency operation is compared with the basic sorting algorithm.In order to solve the problem of high switching frequency in sorting,an optimal sorting algorithm is researched and its application is completed by combining the modulation strategy of Nearest Level approximation.Next,the generation mechanism and characteristics of the internal circulation current of MMC converter are analyzed,the two suppression strategies of circulating current are compared,and the suppression effect of the circulating current suppression method based on the Quasi-Proportional Resonance controller on the negative second harmonic components is studied emphasically.In addition,aiming at the control of variable frequency operation for MMC converter,this paper expounds the causes of low frequency pulsation in Sub-Module capacitor voltage through the energy analysis of converter bridge arm,and obtains the idea of low-frequency pulsation suppression,which aims to eliminate the lowfrequency components in the bridge arm energy deviation by introducing the freedom degree of control.In order to reduce the additional bridge-arm current switching stress and system loss caused by the basic dual-sine-wave injection,a low-frequency pulsation suppression method based on hybrid harmonic injection is researched.Meanwhile,in order to simplify the complexity of internal circulating current control brought by hybrid harmonic injection,a variable frequency operation control strategy based on the energy equalization control of the auxiliary voltage bridge arm is researched.At the same time,the hierarchical decision control is used to optimize the variable frequency operation control,so as to realize the smooth transition between different output frequency stages of MMC converter and improve the stability of the system.Finally,by establishing the simulation model in Matlab/Simulink and building the three-phase inverter-type MMC converter’s experimental platform,the relevant modulation strategy and control strategy are verified and compared in detail.The results well show the correctness of the theoretical research and the effectiveness of the proposed control strategy. |
PDF Full Text Request