According to specific signal input and the desired frequency extraction

generalized S-transform can freely adapt Gaussian window function to obtain the optimal time-frequency resolution and the characteristics of different frequency components at different time slots by one calculation and extract 12/24/36 th characteristic harmonic at different time. We studied a voltage characteristic harmonic based boundary protection principle for HVDC transmission lines. The operating criterion was proposed

the setting principle was given

and the operating scheme was designed. Through analyzing whether the average voltage characteristic harmonic on the HVDC transmission lines was higher than the setting value during the fault

it could be determined whether the fault was on the line. The fault pole was selected by comparing the voltage characteristic harmonic of two poles. The results from theoretical analysis and simulation demonstrate that the proposed protection scheme can correctly distinguish fault from inside or outside and select the fault pole

which is suitable for the backup protection of HVDC transmission lines.