A Fault identification Method for MMC based DC Grids with Hybrid DCCBs

A reliable restart strategy is required to guarantee the power supply reliability and power system stability for overhead modular multilevel converter (MMC) based DC grids. The traditional auto-reclosing scheme will cause a second strike to the system when it recloses to a permanent fault. Besides, the second trip of DC circuit breaker (DCCB) will absorb extra energy, resulting in the increase of interruption capacity. Therefore, a fault identification method is proposed to achieve adaptive reclosing for DC grids with hybrid DCCBs. Subsequent to deionization of faulted lines, the active control of DCCBs is conducted, thereby a designed voltage signal is injected into faulted lines. Then, based on the propagation characteristics of voltage traveling-wave, fault characteristics (nonpermanent or permanent) under different faults are analyzed. Using the wavelet transform modulus maximum (WTMM) to detect the differences, permanent faults can be assessed. The proposed method is robust to fault distance and fault resistance, without extra investments and second strike to the system. Finally, the effectiveness of the proposed method in fault identification is verified under the PSCAD/EMTDC platform.