Patent ID: 7425778

Claim:
An apparatus for selecting and providing to a differential relay an operational vector-group compensation setting pair that automatically provides correction for a phase shift occurring between currents of at least two windings of a power transformer of a three-phase power system, the apparatus comprising: a means for deriving a first and a second plurality of digitized current sample streams from corresponding secondary currents provided by a first current transformer group connecting the differential relay to a first winding of the at least two windings and a second transformer group connecting the differential relay to a second winding of the at least two windings respectively; and a microcontroller operatively coupled to the means for deriving the first and the second plurality of digitized current sample streams, the microcontroller having a microprocessor and a memory operatively coupled to the microprocessor, the microprocessor being configured to: using the first and the second plurality of digitized current sample streams, calculate a corresponding first and a corresponding second plurality of phasors, provide a plurality of 3-by-3 compensation matrices, the plurality of 3-by-3 compensation matrices representing a corresponding plurality of vector-group compensation settings, after establishing a baseline compensation matrix pair configuration, iteratively calculating a number of test sets of three-phase operate current values and three-phase restraint current values using a corresponding plurality of different pair combinations of the plurality of 3-by-3 compensation matrices applied to the first and the second plurality of phasors, and based on a comparison of each of the three-phase operate current values of each of the number of test sets to a per-unit threshold value, select one of the plurality of different pair combinations of the plurality of 3-by-3 compensation matrices as the operational vector-group compensation setting pair, wherein application of a first vector-group compensation setting of the operational vector-group compensation setting pair to the first plurality of phasors and application of a second vector-group compensation setting of the operational vector-group compensation setting pair to the second plurality of phasors automatically provides correction for the phase shift occurring between currents of the first winding and the second winding.