Distance protection relay and method

摘要

The invention relates to a protective relay for an electrical power distribution system. The protective relay comprises first and second current transformers arranged to sense a respective current flowing into a power line, and a voltage sensing device arranged to sense the voltage of the power line. The protective relay further comprises a first and a second device for determining the direction of a fault within the system based on a current signal from the first and second current transformers and a voltage signal from the voltage sensing means. The protective relay further comprises a third device for determining the fault direction based on a sum of the current signals from the first and second current transformers and a voltage signal from said voltage sensing means. Controller means determines whether a fault is internal or external based on the fault directions determined by the first, second and third devices. The invention also relates to a corresponding method.