三次谐波注入对MMC运行特性的影响

Influences of Third Harmonic Injection on the Operation Characteristics of MMC System

目前,模块化多电平换流器(modular multilevel converter,MMC)的传输容量主要受功率半导体器件通流容量的限制。一种可行的办法是通过三次谐波注入提高MMC的传输容量。为此,首先研究了三次谐波注入对系统稳态特性的影响,包括三次谐波注入后二倍频环流抑制控制对调制波电压以及基频电压幅值的影响,以及三次谐波注入对子模块电容电压谐波特性的影响。然后,以换流器直流侧故障为例,研究了三次谐波注入对系统暂态特性的影响。最后,在PSCAD中搭建了张北四端MMC直流电网模型,在稳态和暂态运行工况下对三次谐波注入调制策略的影响进行了仿真验证,结果表明三次谐波注入在抑制暂态故障电流以及提升传输容量方面有积极的效果,但二倍频环流抑制的投入会在一定程度上限制三次谐波注入对传输容量的提升作用。

The capacity of Modular Multilevel Converter(MMC) is mainly limited by the current rating of power semiconductor devices. An applicable approach is to enhance the capacity of MMC with the third harmonic injection method. First, we analyzed the influences of third harmonic injection on the steady state of MMC system, including the influences of circulating current suppression on the peak value of fundamental frequency voltage and the harmonic characteristics of sub-module capacitor voltage. Then, we analyzed the influences of third harmonic injection on the transient state of MMC system after taking DC faults as an example. Finally, we established a model of the Zhangbei four-terminal DC grid based on MMC in PSCAD. All the theoretical analyses were validated by the simulation results. The results show that the third harmonic injection positively affects the suppression of fault current and enhancement of capacity. Nevertheless, the improvement of power transmission capacity is limited by the circulating current suppression.