Phase Delay Compensation and Average Capacitor Voltage Based Currentless Voltage Balancing Methods for Modular Multilevel Converters

Voltage balancing algorithms (VBAs) are vital for the reliable operation of modular multilevel converters (MMCs). The existing VBAs depend on arm currents to adjust the capacitor voltages. However, the repetitive transmissions of arm currents in the hierarchical control system of MMCs increase communication burdens and failure risks. In this article, the performance of an existing currentless VBA is improved and a new currentless VBA is proposed. Firstly, the phase delay compensation is added to improve the currentless VBA with the filter. Then, in the proposed VBA, the polarity of an arm current is reflected by the minimum capacitor voltage. To eliminate the disturbance of measurement noises, the average of the minimum capacitor voltage is further utilized in the proposed VBA method. Compared with the existing currentless VBAs, both the improved and proposed methods can provide more similar performances as the conventional VBA with the sampling arm current. Meanwhile, the improved and proposed methods are both immune to the measurement noises and robust to several types of disturbances in MMC. Finally, the feasibilities of the two methods are verified by the simulation studies with the professional tool Matlab/Simulink.