Effect of current sensor technology on distance protection

This paper presents the investigation on distance protection performance with conventional current sensor device (e.g., current transformer, CT) and non conventional current sensor device (e.g., fibre optic current sensor, FOCS). Distortion of the current sensor output may cause distance relay under-reaching, over-reaching or delayed operation. Various factors contributing to the current distortion, e.g., CT saturation and FOCS current clipping; are presented. While the dc off-set component in the transient fault currents and CT remanent flux as well as total loop resistance in the CT secondary circuit cause the CT to saturate at a lower current magnitude, the FOCS current clipping will only occur if the fault current magnitude higher than the FOCS maximum detectable setting current. The investigation on the impact of the current distortion to the distance relay performance shows that the CT saturation may cause distance relay both under-reaching and over-reaching, meanwhile the FOCS current clipping will not cause the distance relay over-reaching. All those indicate that the modern FOCS can provide the distance protection better security than the conventional CT.