直流配电网中MMC的谐波分析与调制方法设计

Harmonic Analysis and Modulation Design of MMC in DC Distribution Network

将模块化多电平换流器(modular multilevel converter,MMC)应用于中低压直流配电网时,由于子模块数较少,输出电压低次谐波含量大,电流畸变程度较高,因此为直流配电网中的MMC设计合适的调制方法显得尤为重要。在分析MMC已有的最近电平逼近调制(nearest level modulation,NLM)与载波移相脉冲宽度调制(carrier phase shifting pulse width modulation,CPS-PWM)原理及谐波特点的基础上,提出了基于最近电平逼近的脉冲宽度调制(NL-PWM)方法,并推导了其谐波的解析表达式,给出了MMC调制的设计方法。所提出的NL-PWM方法谐波特性更好,并且均压简单,通用性好,易于与NLM方法相互转换,使MMC可根据实际需求方便地选择输出PWM波或阶梯波。对于中低压MMC,增加模块数和引入PWM波调制均可达到同样的谐波改善效果,文章的谐波分析结果为MMC的调制方法设计提供了理论依据,并引入新的谐波指标,即纯电感负载时电流总谐波畸变率,衡量2种方案的谐波大小。最后给出了6 kV MMC的设计算例,并通过仿真验证了所提出NL-PWM方法的可行性,以及其谐波分析结果的正确性。

For the Modular Multilevel Converter（MMC） applied in medium and low voltage DC distribution grids,there will be high content of low frequency harmonics in output voltage and severe current distortion due to significant decrease of the module number. Therefore,it is meaningful to design a suitable modulation method for the MMC in the DC distribution grids. This paper proposes a hybrid modulation method of Nearest Level Modulation（NLM） and Pulse Width Modulation（PWM）. The proposed Nearest Level PWM（NL-PWM） method not only reduces the current distortion significantly,but also avoids the complicated voltage balancing control in each module. The NL-PWM method is easy to convert with the NLM method,which allows the MMC to output PWM wave or ladder wave conveniently according to the actual demand. Increasing the number of modules or introducing PWM can suppress the harmonics to the same degree for medium and low voltage MMC. Results of harmonic analysis in this paper provides theoretical basis for modulation design of MMC and a new harmonic index-the total current harmonic distortion for the pure inductor load is proposed to measure the harmonics of the two modulation methods. Finally,combined with a design example of 6 kV MMC,the harmonic characteristics and feasibility of the proposed modulation method are verified by simulation.