Newly Developed FM Current-Differential Carrier Relaying System and its Field Experiences

This paper presents the theoretical and experimental studies on the newly developed FM current- differential carrier relaying system ( FM relay). FM relay, with outstanding facility compared with any types of conventional relays, can be applied to multiterminal lines as well as two-terminal lines where conventional types of relays may not be considered satisfactory. The principle of this system lies in the current-differential protection in which the instantaneous current values at each terminal are frequency- modulated and transferred via microwave communication system. The results of a long term field test which was successfully performed on a three-terminal tapped transmission line showed the excellent performances of the FM relay.