Online Fault Location in Monopolar LCC-HVDC links With Metallic Return Using Modal Transient Data

This paper proposes an online fault localization technique for line commutated converter high voltage direct current (LCC-HVDC) transmission links, which use single core monopolar cables with metallic return. The conducting layers of the cables have mutual off-diagonal elements in the longitudinal impedance matrix. Thereupon, in case of faults in DC transmission section due to the coupling between conducting layers, using the direct measurements for localizing the fault results in a noticeable amount of error. In this paper, based on the modal theory, the longitudinal matrix of HVDC cable bundle is diagonalized into the decoupled modal matrix. Each of the resulted modes has different attenuation, velocities, and characteristic impedances. According to this velocity difference, an online one-sided modal fault location algorithm is presented. The proposed method gives more accurate results than common methods that use direct traveling wave data for fault localization. Discrete Wavelet Transform (DWT) is used for detecting modal initial peaks. CIGRE LCC-HVDC link is modeled in PSCAD/EMTDC and further fault location algorithm is designed in MATLAB. The comparison between obtained results and techniques based on direct measurement proves the accuracy and robustness of the proposed modal fault location algorithm.