Modern Relaying for A-C Secondary Network Systems

This paper outlines the application of a new three-phase, single-element induction cylinder network master relay which uses line-to-line connected potential coils. Previous network master relays were three-phase, three-element and employed line-to-neutral connected potential coils. The new relay with line-to-line connected potential coils produces far greater tripping torque during unbalanced primary feeder faults than the older relays with line-to-neutral connected potential coils. Examples are given to show the advantage of the new relay. Modern relays have materially improved tripping characteristics which extend the range of highly leading and lagging currents over which the relays will operate selectively at normal balanced three-phase voltages. The mechanical improvements of the network relays are discussed.