A non-unit protection principle based on travelling wave for HVDC transmission lines

Travelling wave protection is one of the most attractive line protection techniques in high voltage direct current (HVDC) transmission system. However, it has a low sensitivity in high impedance fault conditions as the initial wave is insignificant. This paper presents a non-unit fault protection principle based on the initial wave as well as the reflected waves to improve the protection sensitivity in high impedance faults. The reflection and refraction characteristics of the line boundaries and the fault point are analyzed to extract the features of the reflected waves, which form the principle of the protection. Furthermore, dyadic wavelet transform is utilized to detect the arrival of travelling wave surge. A simulation model based on CIGRE benchmark is built using PSCAD/EMTDC to validate the protection principle. Simulation results indicate that the proposed protection has a good performance in various fault situations especially in high impedance faults.