一种适用于少子模块MMC的混合调制策略

A hybrid modulation strategy suitable for MMC with small quantities of sub-modules

由于中低压配电网下电压等级较低,模块化多电平换流器(MMC)子模块数量也较少,因此少子模块MMC的调制方式对MMC系统的性能有重要影响。为改善少子模块MMC的输出质量与输出能力,文中提出一种工作在高调制度的基于电平阶跃点切换的少子模块MMC混合调制策略。在最近电平逼近调制(NLM)的基础上,控制MMC在输出电平阶跃点处进行NLM与载波移相脉宽调制模式之间的实时切换,同时结合环流控制和子模块电压均衡控制,进一步保证少子模块MMC在该混合调制下的正常运行。然后,建立子模块数为4的MMC混合调制的仿真模型,对该少子模块MMC混合调制策略进行仿真验证。结果表明:所提出的混合调制策略兼顾低输出谐波和低开关损耗的特点,提高了其输出质量;且其处于高调制度的工作情况,增大了直流电压利用率,提高了少子模块MMC的输出能力。

As the voltage level is relatively low under medium and low voltage distribution network,the number of modular multilevel converter(MMC)sub-modules is also small.Therefore,the modulation mode of MMC with small quantities of sub-modules has an important influence on the performance of the MMC system.In order to improve the output quality and output capability of the MMC with small quantities of sub-modules,a hybrid modulation strategy suitable for MMC with small quantities of sub-modules based on level step switching at a high modulation degree is proposed.On the basis of the nearest level modulation(NLM),the MMC is controlled to switch between NLM and the carrier phase shift pulse width modulation(CPS-PWM)at the output level step point in real time.At the same time,the circulation control and the sub-module voltage equalization control are combined to further ensure the normal operation of MMC with small quantities of sub-modules under the hybrid modulation.Then,the simulation model of MMC hybrid modulation with four sub-modules is established,and a simulation study is carried out on the MMC hybrid modulation strategy of the MMC with small quantities of sub-modules.The results show that the proposed hybrid modulation strategy not only takes into account the characteristics of low output harmonics and low switching loss,but also improves its output quality.And its operation in high-profile system increases the DC voltage utilization rate and improves the output capacity of the MMC with small quantities of sub-modules.