An improved distance protection method based on multi-frequency characteristic harmonic injection for DEWFAS

As large renewable-energy bases are increasingly integrated into flexible DC transmission projects, double-ended weak-feed AC systems (DEWFASs), which consist of renewable generation stations and sending-end voltage-source converter stations, have become an important research topic. In such systems, both terminals of the AC transmission line are interfaced with power-electronic weak sources. Consequently, the fault response of the line is no longer determined mainly by the network impedance, but is closely related to the control behavior of the converters. When transition resistance is present during a fault, conventional distance protection using fundamental-frequency measurements may suffer from reduced reliability or even fail to operate. To address this problem, this paper develops a distance protection method employing multi-frequency characteristic harmonic injection. Leveraging the flexible controllability of the MMC, a selected harmonic component is superimposed on the d-axis current reference and mapped through the dq–abc transformation into two characteristic frequency components on the AC side, thereby enabling simultaneous dual-frequency harmonic injection into the DEWFAS. Then, based on the voltage equations of the fault loop, a set of equations using electrical quantities at the two different frequencies is established to determine the fault location. Simulation results show that, at a sampling rate of 4 kHz, the proposed method can accurately distinguish internal faults from external faults within 60 ms after fault occurrence, tolerate transition resistance up to 300 Ω, and maintain reliable operation of distance protection under 30 dB noise interference.