Tapped-resistance character analysis of distance relay for UHV lines

UHV/EHV long transmission lines have large distributed capacitance and shunt reactors, which may have significant effects on the operation of a traditional distance relay. The distance relay without considering any of the above two factors will operate with serious overreaching or underreaching. Several papers have analysed these effects on the measuring impedance of distance relay and proposes different schemes to ensure the reliability and selectivity of distance relay for UHV/EHV long transmission lines. Because of the differences of the definition of the apparent impedance or measuring impedances, and the setting values in these schemes, it is hard to compare these schemes in a uniform impedance plane. The better way is comparing them with their tapped-resistance character. In this paper, three schemes are first introduced briefly, and then their tapped-resistance characters are analysed using a program called PRESC. Some beneficial conclusions are obtained, which are instructive to the selection and the setting of ground distance relay in UHV transmission line projects.