模块化多电平变流器调速系统变频控制

Variable-Frequency Control Strategy of Modular Multilevel Converter

模块化多电平变流器(MMC)凭借着诸多优势成为高电压大功率工况下的核心拓扑。但MMC变频调速系统运行于低频状态时存在桥臂能量分配不均衡、子模块电容电压波动严重等问题,不仅影响变频器全速域运行能力,甚至威胁系统安全。为解决上述问题,提出一种基于共模电压与偏置电压控制的MMC变频调速系统全速域运行方法,旨在通过控制系统各桥臂瞬时功率以快速抑制子模块电压波动。首先,构建系统数学模型,分析悬浮电容电压波动影响因素;其次,设计变频调速系统的低频控制器与在线模式切换环节;最后,为验证所提控制策略的可行性和有效性,对其进行仿真和实验的对比分析。实验结果表明,所提控制策略能有效抑制MMC变流器子模块电压波动,完成不同频段平滑切换,降低系统损耗,改善系统输出品质,提升MMC系统安全运行能力。

With the advantages of modularization, flexible structure and convenient expansion, modular multi-level converters （MMC） become the core topology under the condition of high voltage and high power. In low frequency status, the problems, such as unbalanced energy distribution in arms, severe pulsation of sub-module capacitor voltage in MMC variable-speed drives system, will exist. These problems will not only affect the running ability of full speed area, but threaten the system safety Therefore, a full speed area running method based on common voltage and bias voltage control for MMC variable-speed drives system is proposed. The proposed method can suppress the pulsation of sub-module capacitor voltage by controlling instantaneous power of each arm. Firstly, the mathematical model of MMC system was built to analyze the influencing factors for pulsation of sub-module in low-frequency. Then, the low frequency controller and on-line link of mode switch were designed for variable-speed drives system. Last, the comparisons of the simulations and experiments verified the proposed control strategy. The results show that the control strategy proposed can effectively suppress the pulsation of MMC sub-module capacitor voltage, accomplish smooth switch during different frequencies, reduce system loss, and hence improve the system output quality and the safety performance of MMC system.