Study On Control Strategy Of MMC-HVDC System Under Unbalanced Grid Voltage

Recently,when compared with traditional AC transmission,Voltage Source Converter based High Voltage DC(VSC-HVDC) has advantages such as decomposition of active and reactive power, power supplication to passive network and improvement of system stability and transmission capacity. That is why efforts have been made in the development of VSC-HVDC and a lot of scholars have studied on converter and control strategies of DC transmission system. In recent years,Modular Multilevel Converter(MMC) has replaced traditional 2-level converter for its advantages such as high quality output voltage waveform,easy modularization manufacturing and strong troubleshooting ability in HVDC areas. In this research content,this paper study control strategies of MMC-HVDC system under unbalanced grid voltage based on Matlab/Simulink simulation system. This paper can be summarized as follows:1. Study topological structure and working mechanism of MMC and establish mathematical model at AC side of MMC under balanced and unbalanced grid voltage conditions and discuss operating characteristics of MMC-HVDC system under unbalanced grid voltage. This paper proposes a control strategy to suppress negative sequence at AC side of MMC under unbalanced grid voltage, which can realize three-phase symmetric AC current.2. Establish two order non homogeneous linear differential equations with constant coefficients of circulating current in MMC under unbalanced grid voltage conditions and discuss circulation components of MMC based on these differential equations. And this paper proposed a control strategy to suppress circulating current of MMC under balanced and unbalanced grid voltage conditions.3. Control strategies mentioned above need to be achieved in positive sequence synchronous rotating coordinate and that is why this paper designs Phase-Locked Loop(PLL) to lock phase of positive sequence voltage accurately under unbalanced grid voltage conditions. At the end of this paper it designs PLL to ensure the realization of proposed control strategies.