Fault Property Identification and Fault Recovery Scheme for Flexible DC Lines Based on Voltage Injection

In overhead line MMC-HVDC systems, hybrid modular multilevel converters (MMC) can limit fault currents by reducing the voltage during DC faults. For successful system recovery after fault isolation, it is critical to accurately identify the fault characteristics. However, blindly restarting the system risks causing further damage within a short time frame, compromising overall system safety. To address these concerns, this paper proposes a fault identification and system restoration scheme tailored for hybrid MMC-HVDC transmission systems. The proposed approach enhances the control of hybrid MMC injection signals, ensuring fault identification while minimizing the risk of damage to converter valves. A thorough analysis of system responses under various fault conditions is conducted. Additionally, a criterion is established using the Euclidean distance between the voltage reference value and voltage measured value to distinguish fault property. Strategies for voltage restoration and power recovery in the event of temporary faults are also presented, significantly reducing recovery time. The effectiveness of the proposed scheme is validated through simulation results under various fault conditions.