电网电压不平衡下基于模块化多电平的高压直流输电控制策略

Control Strategy of HVDC Transmission Based on MMC Under Unbalanced Grid Voltage

研究了基于多电平变流器(MMC)的两端柔性高压直流(HVDC)输电系统。首先建立了MMC数学模型,分析了MMC内部环流产生的原因以及环流危害,接着研究了HVDC输电系统的工作原理和控制策略。系统的控制逻辑可分为3级,其中最低层控制策略为换流站内的阀级控制,包含MMC的调制方法和环流抑制策略。重点研究了换流站级的双环PI控制和阀级的环流抑制。通过搭建输电系统模型实现了正常工况下理论分析和软件仿真论证,同时设计了非正常工况下经过正负序分离后的双环PI控制方法,实现了电网电压不平衡时系统的稳定控制。

This article focuses on the research of the modular multilevel converter(MMC)two-terminal flexible high voltage direct current(HVDC)transmission system.First,the mathematical model of MMC is established,the cause of circulation in MMC and its influence are described;Then,the working principle of HVDC transmission system is analyzed.The control logic of the system can be divided into 3 levels,among which the lowest level control strategy is the valve level control in the converter station,including the MMC modulation method and the circulation suppression strategy.This paper focuses on the study of converter station-level dual-loop PI control and valve-level circulation suppression.The theoretical analysis and software simulation demonstration under normal operating conditions are realized by building a transmission system model.At the same time,the dual-loop design is designed after positive and negative sequence separation under abnormal operating conditions.The PI control method realizes the stable control of the system when the grid voltage is unbalanced.