A novel pilot protection for flexible dc lines based on difference of fault traveling waves

The pilot directional protection based on amplitude comparison of the traveling wave (TW) has good performance in flexible DC transmission systems. However, this type of protection may fail to detect internal line faults correctly, as the amplitude of the forward voltage TW may be greater than that of the backward voltage TW in some fault cases. To address this issue, a novel pilot protection method based on the difference of fault TWs has been proposed. The proposed method fully considers the polarity of backward voltage TW and current TW. By utilizing the electrical quantity relationship at the line boundary, the difference between the two TWs mentioned above is transformed into the difference between the forward and backward voltage TWs. In this way, a conclusion is drawn that the absolute value of the integral ratio of forward and backward voltage TWs is less than 1 in the case of forward faults, and much greater than 1 when reverse faults occur. Therefore, a reliable directional identification criterion based on the forward and backward voltage TWs has been developed. Hardware-in-the-loop experiments and test results showcase that the proposed protection method has higher sensitivity and reliability for line faults.