Using impedance measurement to improve the reliability of traveling-wave distance protection

Two common problems associated with distance protection schemes based on traveling waves are close-up faults and faults with small inception angles. This paper examines how the reliability of such a relay may be improved by considering the measured impedance at the relay location. The composite algorithm describes how the traveling wave information and line impedance measurement can be used in parallel in a single protection scheme to develop a high-speed, reliable distance relay. Such a scheme will be extremely useful for long EHV transmission lines where high speed fault clearance is essential. The effects of close-up faults, fault inception angle, and external faults on a three-phase 500 kV power system are examined using an electromagnetic transient program. A laboratory prototype of the relay has been developed using a high speed signal processor.