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.