Control of circulating current can help modular multilevel converters (MMCs) improve the internal performance, but only the conventional circulating current suppression control has been considered in impedance models to investigate MMC-grid interactions. In this paper, the sequence impedance responses of circulating current injection methods based on instantaneous information are modeled for stability analysis. Compared with circulating current control schemes based on predefined references, the circulating current injection methods based on instantaneous information introduce couplings among circulating current, output current and capacitor voltage control loops, which influence the MMC impedance in the low frequency range. This paper reveals that injection of second harmonics in the circulating current reduces the capacitor voltage ripples, but the system stability is compromised due to the couplings. The MMC stability boundaries for different circulating current controllers are defined with the help of sequence impedance model and Bode-plot-based analysis. The impedance model and stability assessment are validated by frequency scan results and case studies.

中文翻译：

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模块化多电平转换器中循环电流控制方案的阻抗整形效应和稳定性评估

控制循环电流可以帮助模块化多电平转换器 (MMC) 改善内部性能，但在阻抗模型中仅考虑了传统的循环电流抑制控制来研究 MMC 与电网的相互作用。本文对基于瞬时信息的循环电流注入方法的序列阻抗响应进行建模，进行稳定性分析。与基于预定义参考的环流控制方案相比，基于瞬时信息的环流注入方法在环流、输出电流和电容电压控制环路之间引入了耦合，影响了MMC在低频范围内的阻抗。本文表明，在循环电流中注入二次谐波可降低电容器电压纹波，但由于耦合，系统稳定性受到损害。不同循环电流控制器的 MMC 稳定性边界是在序列阻抗模型和基于波特图的分析的帮助下定义的。阻抗模型和稳定性评估通过频率扫描结果和案例研究得到验证。