Fault Location and Protection for Metallic Return HVdc Grid Based on Natural Frequency Extraction of Modal Derivative Current

In bipolar high-voltage direct current (HVdc) transmission power grids with metal return lines (MRLs), the identification of high impedance faults and the sensitivity of fault location and protection are still challenges. In response to these concerns, a fault location and protection strategy is proposed in an HVdc grid with MRL, which extracts the natural frequency of modal derivative current through the variational mode decomposition-Hilbert transform (VMD-HT) algorithm. First, this study establishes equivalent models of dc grids under different fault types considering the line capacitance. On this basis, the expressions of natural frequency are derived, and the fault location can be accurately inferred from the measured natural frequency. Subsequently, a fault protection method based on a joint criterion of natural frequency and modal derivative current is proposed. Theoretically, this method is not affected by the fault transition resistance and exhibits good sensitivity. Besides, a modal phase plane is introduced for faulty pole selection. Finally, this article establishes PSCAD/EMTDC and experimental platforms to validate the precision of the proposed fault location and protection scheme across different situations.