Zone-Packaged Ground Distance Relay II - Design and Performance

A new concept in ground distance fault detection has been reduced to practice in a relay, designated as Type SDG. The relay is a zone-packaged unit which gives the engineer flexibility when designing the relaying system for multiple-zone primary and backup protection. It is an impedance-type relay which has a modified mho circle characteristic when plotted on an R-X diagram. Special circuitry prevents incorrect operation due to over-frequency line oscillations, loss of voltage, or double-phase-to-ground faults. Thus, high-speed operation¿7 to 11 ms is obtained without sacrificing security.