Patent ID: 7375941

Claim:
A current differential protective relay without user adjustable operational settings for use in protecting a selected line portion of a power system, comprising: at least one phase current differential element, having a first non-user predetermined, fixed threshold value, responsive to phase currents from the power system at a local location of the protective relay and to phase currents from a remote relay on the selected line portion, to detect three-phase faults on the line portion and for providing a first output signal when said predetermined threshold is exceeded by three-phase current; a negative sequence current differential element or two phase current differential elements, having a second non-user predetermined, fixed threshold value, responsive to said local phase currents and said remote phase currents to detect phase-to-phase faults and phase-to-phase-to ground faults and for providing a second output signal when said second predetermined threshold is exceeded wherein said two phase current differential elements include said one phase current differential element for detecting three-phase faults, or two additional phase current differential elements; and a negative sequence current differential element or a zero sequence differential element, having a third non-user predetermined, fixed threshold value, responsive to said local phase currents and said remote phase currents to detect individual phase-to-ground faults and providing a third output signal when said third predetermined threshold is exceeded, wherein if a negative sequence current differential element is used, it is either the negative sequence current differential element used for detecting phase-to-phase and phase-to-phase-to-ground faults or a second negative sequence current differential element, wherein none of the current differential elements include adjustable operational settings.