Postfault DC Transient Overvoltage Control for Multiterminal MMC-HVDC System

In a modular multilevel converter-based multiterminal high-voltage direct current (MMC-HVdc) system, unbalanced active power (UAP) between converters is easily induced after ac/dc faults, resulting in severe dc transient overvoltage. To solve this problem, a postfault dc transient overvoltage control is proposed. The generation mechanism of UAP among multiterminal MMCs is analyzed, and the coupling model linking UAP to dc overvoltage is established, revealing a positive correlation between the peak voltage and UAP amplitude. Within the MMC region connected to the wind farm (WF), the proposed method achieves optimal allocation of UAP to suppress the overvoltage peak by dynamically coordinating multiple power units in accordance with the time sequence. Specifically, the submodule capacitor voltage boosting capability in the MMC serves as temporary energy storage, and UAP is redistributed between the WF and the DBR based on the energy equal area criterion. The real-time digital tests show that the proposed method reduces the peak transient overvoltage by over 9.1%, stabilizing at 1.0 p.u. and exhibits superior voltage control capability compared to other methods. Additionally, it minimizes reliance on the DBR, reducing DBR capacity and head dissipation by more than 25% and 25.58%, respectively, thereby enhancing the safety of the MMC-HVdc system.