一种低开关频率运行的模块化多电平变换器混合调制策略

近年来,基于模块化多电平变换器(MMC)的电压源型高压直流输电技术得到广泛的应用。为了在不增加开关频率的情况下尽可能减小子模块电容电压波动,同时降低系统输出中的谐波含量,通过结合最近电平逼近调制(NLM)和脉宽调制(PWM)两者的优势,提出一种能够满足高压直流输电要求的混合调制策略。该方法在阶梯调制的基础上加入PWM,对误差信号进行二次调制,采用子模块单元选择法并构造电容电压平均、平衡控制和电流闭环控制,对各桥臂子模块投切状态进行实时反馈修正,使系统在较低的开关频率下仍能对子模块电容电压波动以及输出谐波含量进行有效优化,最后在MATLAB/Simulink平台上验证了该方法是有效可行的。

A Hybrid Modulation Strategy for Modular Multilevel Converter Operating at Low Switching Frequency

In recent years, voltage source HVDC transmission technology based on modular multilevel converter (MMC) has been widely used. In order to reduce the voltage fluctuation of the sub-module capacitor as much as possible without increasing the switching frequency, and reduce the harmonic content in the system output, a hybrid modulation strategy which can meet the requirements of HVDC transmission is proposed by combining the advantages of nearest level approximation modulation (NLM) and pulse width modulation (PWM). In this method, PWM is added on the basis of step modulation, and the error signal is modulated twice. The sub-module unit selection method is used to construct capacitor voltage average, balance control and current closed-loop control, so as to carry out real-time feedback correction for the switching state of each sub-module, so that the system can still control the voltage fluctuation of the sub-module capacitor and the output harmonic content at a lower switching frequency. Finally, the method is proved to be effective and feasible on the MATLAB/Simulink platform.