Circulating Current Suppression Combined with APF Current Control for the Suppression of MMC Voltage Fluctuations

Modular Multilevel Converters (MMCs) are widely used in HV and MVDC transmission. However, their application causes the voltage level to increase, and the number of sub-modules also increases. Problems such as circulating currents and sub-module voltage fluctuations should not be neglected. Considering the coupling relationship between the circulating current and the sub-module capacitor voltage fluctuation, the circulating current suppressing controller (CCSC) and the active power filter (APF) techniques are used to solve the two problems mentioned above simultaneously. Firstly, in order to reduce the influence of clutter on the tracking of the target components of the CCSC, a Second-Order Generalized Integrator (SOGI) is added to accurately lock the main 2nd and 4th harmonic components in the circulating current. Secondly, an APF is added on top of the circulating current suppression, and the two methods can be mutually reinforcing in their roles. The APF applies the strategy of current inner-loop dominant and voltage outer-loop bias control. It is regarded as a whole for absorbing the voltage fluctuations of the sub-module and also eliminates the error caused by the inductive voltage. Finally, the effectiveness of the above method is verified in MATLAB/Simulink, which demonstrates that the proposed method provides better suppression of both circulating current and sub-module voltage fluctuations compared to the conventional MMC that only incorporates APF.