Power Equalization Control Strategy for MMCs in Hybrid-Cascaded UHVDC System

Abstract

Based on the hybrid-cascaded topology of ultra-high-voltage direct current (UHVDC) engineering, this study clarified the mechanism of unbalanced power generation among modular multilevel converters (MMCs) at the inverter side following the fault of the AC system at the rectifying side, and then proposed the power equalization strategy for MMCs. By performing closed-loop control on the active power deviation between constant-voltage and constant-power MMCs, it was possible to achieve automatic power equalization among MMCs after the occurrence of a fault so as to avoid the detrimental effect of a single MMC’s power fluctuation on the connected AC system. Meanwhile, the control enabling logic was designed to ensure the reliable input and stable exit of the control strategy throughout the disturbance period. Finally, a PSCAD/EMTDC platform was used to simulate various types of faults in the AC system at the rectifier side in order to validate the effectiveness of the proposed power equalization strategy.