A fault property identification scheme for the restarting of hybrid HVDC system

The hybrid HVDC system will be widely used in the future. Since transient faults account for the majority of all overhead line faults, the system restart is essential to restoring the power supply of the hybrid HVDC system. However, the traditional restart strategy blindly restores the DC voltage after a fixed time delay, which will result in secondary damage to the system in the case of permanent faults. In order to enhance the safety and reliability of power supply, this paper proposes a fault identification scheme using active signal injection from the MMC to determine the fault properties (transient or permanent) before restarting. This technique uses a steady current signal to detect faults and reduce the risk of large short-circuit currents during permanent faults. The change of line voltage after injecting the signal under different fault types is analyzed, which also serves as the theoretical basis for identifying the fault properties. The proposed scheme is easy to implement and can avoid the secondary damage to the system. A typical hybrid HVDC system is modeled for simulation in PSCAD/EMTDC, and the correctness and effectiveness of the scheme are verified.