Zone I Distance Relaying Scheme of Lines Connected to MMC-HVDC Stations During Asymmetrical Faults: Problems, Challenges, and Solutions

The exclusive fault behaviors of MMC-HVDC stations bring great challenges to the correct operations of distance relays. This paper presents a new zone I distance relaying scheme that can accurately calculate fault distance in the case of integration of MMC-HVDC system. The two calculation formulas of fault distance are derived from mathematical perspective and geometrical relationship of impedances, respectively. Phase angle of fault current is the key to calculate fault distance. In power system without MMC-HVDC stations, the existing method for solving this phase angle is based on two premises, that is, the positive- and negative-sequence current distribution factors are equal and real numbers. However, owing to the integration of MMC-HVDC system, aforementioned premises are both not true. Consequently, the existing method cannot obtain accurate phase angle of fault current. To solve this problem, the three solutions corresponding to three different types of asymmetrical faults are provided. The proposed scheme has the advantages of satisfactory immunity against fault resistance, applicable to various asymmetrical faults, and performing well under rectifier as well as inverter modes. The performance of the proposed scheme is evaluated by simulation results in PSCAD/EMTDC and compared with prior-art schemes.