A New Protection Strategy for Grid-Forming Renewable Power Outgoing Lines Using Characteristic Signal Amplitude Ratio

Grid-forming (GFM) renewable energy sources are increasingly integrated into power grids to enhance the stability of high-penetration renewable energy systems, while the fault current characteristics of GFM-based outgoing lines lead to the inapplicability of conventional longitudinal differential protection, which suffers from reduced sensitivity or even refusal to operate under weak grid conditions. To address this issue, this paper proposes a novel active detection-based protection strategy for GFM photovoltaic power station outgoing lines based on the amplitude ratio of characteristic harmonic signals. First, the sequence equivalent circuits of the GFM system during grid faults are established to analyze the fault current characteristics, and the inapplicability mechanism of conventional pilot differential protection is revealed. Considering the filter cutoff frequency, harmonic interference avoidance and power quality constraints, the 8th harmonic is selected as the characteristic signal, and a proportional-resonant (PR) controller is adopted to realize the independent and flexible injection of the characteristic signal and power frequency signal. Based on the distribution difference of characteristic signals under internal and external faults, a protection criterion is constructed using the amplitude ratio of the harmonic component of the differential current to the characteristic signal injected on the station side. The simulation results on the MATLAB/Simulink platform show that the proposed strategy can quickly and accurately distinguish various internal and external faults of the transmission line, and operate reliably under different fault types, fault locations and high transition resistance.