A novel restart control strategy for the MMC-based HVDC transmission system

The overhead dc line has higher fault probability, especially the nonpermanent fault probability, than the dc cables. The conventional restart strategy of flexible high voltage dc (HVDC) transmission system deblocks the converter directly, causing a severe secondary damage to the system in the case of permanent faults. For improving the power supply reliability and security, the paper proposed a novel restart control strategy which can identify the fault property (permanent or nonpermanent) correctly for the modular multilevel converter (MMC) based HVDC overhead transmission system. By utilizing the uncontrolled rectifier operation mode and existing starting resistor, the proposed restart strategy is effective to reduce the secondary damage to the system significantly, including the overcurrent damage and overvoltage damage. Additionally, the delay time is introduced to eliminate the influence of the line distributed capacitor charging current on the proposed method, with the setting principle of this delay time also being discussed. The proposed strategy is very simple to be implemented, thus being more valuable for engineering application. Simulation cases based on the PSCAD/EMTDC were carried out finally to verify the feasibility and superiority of the proposed strategy.