Neutral Line Protection System for HVDC Transmission

This paper described the principle of fault detection and its simulator study, principal circuitry, field test results and field performance records on the neutral line protection system for an HVDC transmission system. In the protection system, 125 Hz AC current injected to the neutral line is utilized as a pilot current to detect the neutral line faults. The pilot current injection is made through the neutral line surge capacitor without detracting its capability. Combined with a metallic return protecting breaker, this protection system has demonstrated a satisfactory performance on the grounding fault clearance without interrupting the power transmission.