Adaptive Single-Pole Auto-Reclosing Scheme for Hybrid MMC-HVDC Systems

After the isolation of a single pole-to-ground fault, the adaptive auto-reclosing concept in high-voltage dc (HVDC) systems is difficult to be applied due to little coupling between the poles. With fault ride through capability, the hybrid modular multi-level converter (MMC) can be advantageous for future HVDC projects. This research proposes an adaptive auto-reclosing operation for hybrid MMC-based bipolar HVDC systems, based on fault identification by characteristic signal injection. The proposed method is based on voltage controllability of healthy pole MMC to inject dc voltage perturbations. Healthy line perturbations result in the induced characteristic signals to the faulty line, which contain information about the fault property. An additional control strategy for hybrid MMC, to inject signals with suitable amplitude and frequency, is also discussed in detail. Then, fault identification is realized by wavelet transform of traveling waves caused by the signal injection. Finally, a dc adaptive single-pole auto-reclosing scheme is proposed to shorten the reclose time and to distinguish between temporary faults and permanent faults. Simulation results verify the effectiveness of the proposed method in PSCAD/EMTDC software.