Abstract: The relay obtains three-phase current signals from two power line sources thereof, typically two independent feeder lines. The two sets of signals are processed by a single processing means, which independently controls the circuit breakers associated with each feeder line, depending on the values of current obtained.

Abstract: The system includes a plurality of voltage transformers in a protective relay for A, B and C-phase power system voltages, responsive to the secondary windings of corresponding voltage transformers for the power system, the secondary windings of the voltage system power transformers being connected in a broken delta arrangement. The primary windings of the relay voltage transformers are arranged in a broken delta configuration and connected to the secondary windings of the power system voltage transformers so that A-phase, B-phase and C-phase voltage are provided at the secondary windings of the relay voltage transformers. The three-phase voltages may then be summed to produce a residual voltage value.

Abstract: Current transformer (CT) secondary currents for the windings of a power transformer are used to produce a differential current. An operating current value is obtained from the differential current value. A restraining current value is obtained from processed winding current values. Second and fourth harmonic values of the differential current are obtained and are summed with a restraining current quantity which is a result of the restraining current multiplied by a slope characteristic factor. If the operating current value is greater than the sum of the restraining current quantity and the second and fourth harmonic values, an output signal is produced which may be used as a trip signal unless it is blocked by selected blocking signals generated by another portion of the system.

Abstract: The system includes a blocking counter which is set to 255 quarter cycles and the output contacts of the protective relay are blocked for five quarter cycles if the first order difference between two successive quarter-cycle samples of current is above a selected threshold, referred to as the last previous and the first samples. The first order difference between the first and the next successive (second) samples is compared against a second threshold and the instantaneous value of the current sample is compared against the second threshold. If both comparisons are above the second threshold, then the output contacts are unblocked. If not, such that a constant DC A/D converter output or an A/D spike is indicated, then the outputs remain blocked for the full five quarter cycles, which is sufficient time to permit the A/D spike to disappear and to recognize a constant DC output from the A/D converter.