A novel pilot protection scheme for HVDC lines based on waveform matching

•A novel pilot protection scheme for HVDC lines based on waveform matching is proposed.•It is proved in this paper that after the electrical quantities are low-pass filtered, the HVDC line can be simplified to two PI models from the midpoint. It can eliminate the effect of distributed capacitive current and has simple computation as well as satisfying precision.•The midpoint currents, which can be calculated based on the simplified DC line model, will be matched under external faults while have opposite trend under internal faults.•The improved Pearson correlation coefficient (IPCC) is proposed to measure the similarity between the waveforms of the calculated midpoint currents and establish the protection criterion.•The newly proposed protection in this paper has absolute selectivity, great reliability and strong fault resistance withstand ability. Sensitivity analysis and comparisons with other pilot protections further demonstrate that the proposed protection stands out for high accuracy and strong robustness to many non-ideal factors. The traditional current differential protections for HVDC transmission lines are easily affected by distributed capacitive current. Although some improved differential principles have been presented in literatures, they still risk maloperation caused by synchronization error, noise or parameter errors. To overcome the problems above, this paper proposes a novel pilot protection scheme based on waveform matching. After the electrical quantities on both sides are low-pass filtered, the HVDC line is simplified to two PI models from the midpoint, which can eliminate the effect of distributed capacitive current and has simple computation as well as satisfying precision. The midpoint currents, which can be calculated based on the simplified DC line model with the information of either side, will be matched under external faults while have opposite trend under internal faults. The improved Pearson correlation coefficient (IPCC) is proposed to measure the similarity between the waveforms of the calculated midpoint currents and establish the protection criterion. A ± 800 kV HVDC test system is modeled in PSCAD/EMTDC. Extensive simulations of various fault situations indicate that the new protection has absolute selectivity, great reliability and strong fault resistance withstand ability. Sensitivity analysis and comparisons with other pilot protections further demonstrate that the proposed protection stands out for high accuracy an