Improved Approach to High-Frequency Current Injection-Based Protection for Grounding Electrode Line in High-Voltage Direct Current System

High-frequency current injection-based protection is widely used for fault detection of a grounding electrode line in high-voltage direct current (HVdc) system. Since the grounding electrode line is several times longer than the wavelength of the injected current, the relationship between the measured impedance and fault distance is very complex. Due to the complex impedance characteristic, the current injection-based protection has a dead zone under some circumstances. To eliminate the protection dead zone, an improved approach to high-frequency current injection-based protection was proposed. First, the characteristics of the high-frequency impedance of the grounding electrode line under different fault conditions were investigated. Analysis results showed that when the line length was close to the integer multiple of the half-wavelength of the injected current, the measured impedance of the grounding electrode line may be the same as normal operation impedance. Thus, the current injection-based protection will be unable to recognize the fault. Second, a two-layer optimum method was proposed to select the frequency of the injected signal. With the new frequency, the current injection-based protection was able to eliminate the dead zone of the traditional protection. Finally, a ±800-kV HVdc simulation model was constructed, and the simulation results verified the accuracy of our conclusions and the effectiveness of our proposed protection method.