Abstract: A protective relay having an operational boundary matched substantially to the output performance limit characteristics of a generator to enhance the protection. The protective relay generates a signal representative of a discriminating impedance based on measurements of the generator's terminal voltage and current. The relay utilizes two pairs of predetermined values which characterize in part the generator's output performance limit curves. Two corresponding pairs of vector signals are generated by the relay based on their respectively corresponding pairs of predetermined values and the generated discriminating impedance signal. The angles between the vector pairs are measured by the relay and are compared with predetermined angle values which also represent in part the generator's output performance limitations. Leading and lagging conditions of at least one of the vector pairs affects the selection of the predetermined angle value used in the comparison with the measured angle associated therewith.

Abstract: A differential protective relay for protecting devices on an ac electrical power system. The protected devices can include a transformer, a generator, or a station bus. The differential protective relay evaluates the current flowing into and out of the protected device only during the period when the current transformers are not saturated. The differential protective relay also detects internal faults accompanying external faults by evaluating the ratio between the current flowing into and out of the protective device. When this ratio exceeds a predetermined limit the protective relay evaluates the difference between the current flowing into and out of the device to determine if this difference is sufficiently large to trip the relay. When a current transformer associated with the differential protective relay is saturated, the differential protective relay produces a restraint signal to restrain tripping.