Directional Correction of Over-Current Relay Using Voltage Slope in Multi Terminal DC System with SFCL

Abstract

In the DC system, a larger fault current flows due to capacitor discharging than in an AC system. In the event of a fault in the MTDC (multi-terminal DC), although the magnitude of the current flowing through each DCCB (DC circuit breaker) varies depending on the fault location, all capacitors in MTDC may discharge and all DCCB may interrupt. In this paper, the trigger type SFCL (superconducting fault current limiter) were constructed to limit large fault current, and DC OCR (overcurrent relay) was designed to prevent interrupting malfunction according to the impact of SFCL. When the fault current was small by operation of SFCL, the DCCB did not interrupt or trip time is delayed. These problems cause malfunction even when DC OCR is applied. Therefore, DC OCR requires an additional correction method. In this paper, a new correction method was proposed in consideration of the current direction and voltage slope. The simulation was conducted according to four cases, and the proposed OCR correction was verified to eliminate malfunctions through case studies.