MMC型级联变换器阻抗建模和稳定性分析

工程中常通过不同类型的变换器灵活级联以实现不同的功能，然而变换器级联系统中前、后级变换器之间的相互作用可能会导致系统稳定裕度不足甚至失稳，不同类型的模块化多电平变换器(modular multilevel converter,MMC)级联也将面临稳定性问题。建立了MMC型级联变换器的稳定性分析模型，并提出了一种基于串联阻尼电阻的稳定性提升策略。首先，建立了MMC型AC/DC变换器和DC/DC变换器的阻抗模型，在稳态工作点附近推导出两种MMC型变换器的小信号模型，从而获得前级变换器的输出阻抗和后级变换器的输入阻抗。其次，分析了MMC型级联变换器的稳定性，结果表明，当前级变换器输出阻抗与后级变换器输入阻抗不匹配时，级联系统稳定性裕度可能不足导致系统失稳。进而，分析了MMC型级联变换器的平波电感、桥臂电感和子模块电容等一次参数对级联系统稳定性的影响。最后，提出了一种基于串联阻尼电阻的稳定性提升方法，并给出了串联阻尼电阻的选择建议。仿真结果表明提出的稳定性控制方法可有效提升MMC型变换器级联系统的稳定性。

MMC-based cascaded converter impedance model and stability analysis

In engineering, different types of converters are often flexibly cascaded to achieve different functions. However, the interaction between the front and rear converters in the converter cascade system may lead to insufficient stability margin or even instability of the system. Different types of modular multilevel converter (MMC) based cascaded converters will face different stability problems. In this paper, a stability analysis model of an MMC cascaded converter is established, and a series resistance-based stability improvement strategy is proposed. First, the impedance models of an MMC-based AC/DC converter and a DC/DC converter are established. The small signal models of the two MMC-based converters are deduced near the steady-state operating point. From these the output impedance of the source converter and the input impedance of the load converter are obtained. Second, the stability of the MMC cascaded converter is analyzed. The results show that when the output impedance of the source converter does not match the input impedance of the load converter the stability margin of the cascade system may be insufficient, resulting in system instability. Then, is analyzes the influence of primary parameters such as smoothing and arm inductances and sub-module capacitance of the MMC cascaded converter on the stability of the cascaded system. Finally, a stability improvement method based on series resistance is proposed, and selection suggestions for series resistance are given. Simulation results show that the proposed stability control method can effectively improve the stability of an MMC converter cascade system.

This work is supported by the Natural Science Foundation of Hubei Province (No. 2018CFB580).