A novel distance protection for MMC-HVDC lines based on the equivalent capacitance voltage

•The equivalent capacitance voltage of MMC remains unchanged within a short period after fault occurrence.•The compensating voltage of the DC transmission line has opposite polarity with the equivalent capacitance voltage of MMC under the internal fault while has the same polarity under the external fault.•The faults can be discriminated based on the polarity of compensating voltage during the short period after the fault.•The proposed distance protection for MMC-HVDC lines has clear setting criterion, powerful anti-noise ability, independency of boundary and accurate response under the zero-voltage fault. The present protection configuration for MMC-HVDC transmission line is not perfect enough due to poor reliability and insufficient protection types. Some works have been done on distance protection methods for DC lines. However, they encounter the problems of discriminating the zero-voltage fault or dependency on simulation. To overcome the problems above, this paper proposes a novel distance protection principle for MMC-HVDC lines based on the equivalent capacitance voltage (ECV). It is proved that the ECV of MMC will keep its polarity unchanged within a short period after the DC fault. And a further fault characteristic analysis indicates that the compensating voltage of the DC line has opposite polarity with ECV under the internal fault while has the same polarity under the external fault. Thus, the faults can be discriminated based on the polarity of compensating voltage during the short period after fault occurrence. Comprehensive simulation results have validated the effectiveness and strong robustness of the new method. The proposed distance protection stands out for its clear setting criterion, powerful anti-noise ability, independency of boundary and accurate response under the zero-voltage fault. It can offer a duplication and supplement to the traveling-wave protection.