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).