MMC-HVDC换流站电流波动抑制策略研究与仿真分析

Strategy Research and Simulation Analysis of Current Ripple Suppression for MMC-HVDC Converter Station

在交流系统不对称的情况下,模块化多电平换流器(modular multilevel converter,MMC)阀侧交流电流会出现基频负序分量,可能导致过流使系统闭锁或停运,同时MMC内部电流会出现2倍频零序分量,导致直流系统电流的2倍频波动。为了抑制交流三相不对称引发的MMC交、直流侧的电流波动,通过分析交流不对称条件下的MMC数学模型,提出一种基于通用控制器(universal controller,UC)的换流站电流波动抑制策略。通过对所提出的通用控制器与二阶谐振控制器(second-order resonant controller,SORC)控制效果的对比分析,理论证明采用UC作为电流控制器的MMC闭环系统具有控制精度高、稳定性好且结构简单的优势。利用RT-LAB仿真平台,建立双端31电平MMC—HVDC输电系统模型,对所提控制策略进行有效性验证。仿真结果表明,在交流系统不对称条件下,该文所提控制策略能够有效抑制MMC交、直流侧的电流波动,保证阀侧交流电流的对称性与直流系统的稳定性,提高MMC系统的运行能力。

In the case of unbalanced AC power systems, negative-sequence components of AC current in valve side at fundamental frequency appear in modular multilevel converter (MMC),possibly causing overcurrent and leading to system block or outage,while zero-sequence component of double frequency will appear in internal,current of MMC,resulting in double frequency current ripple in DC system.In Order to suppress the current ripples on AC and DC sides of MMC caused by AC three-phase imbalance,a current ripple suppression strategy for converter station based on universal controller(UC)is proposed by analyzing the mathematical model of MMC under unbalanced AC system.By comparing and analyzing the control effects of the proposed UC and the second-order resonant controller(SORC),theoretical analysis shows that the MMC closed-loop system with UC has advantages of high coritrol precision,good stability and simple structure.In order to verify effectiveness of the proposed strategy,a two-terminal 31-level MMC-HVDC transmission system model is established on RT-LAB simulation platform. Simulation results show that in unbalanced AC system,the control strategy can suppress AC-and DC-side current ripples of MMC,ensure balance of the AC current in valve side and stability of DC system,and improve operating capability of the MMC system.