Apparatus and method for compensating secondary currents used in differential protection to correct for a phase shift introduced between high voltage and low voltage transformer windings

A method for selecting and providing to a microcontroller of 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 method comprising:

• receiving a first and a second plurality of digitized current sample streams derived 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 power transformer differential relay to a second winding at least two windings respectively;
• using the first and the second plurality of digitized current sample streams to calculate a corresponding first and a corresponding second plurality of phasors;
• providing 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.

A method for providing to a microcontroller of a differential relay an operational vector-group compensation setting pair that automatically provides correction for a phase shift occurring between currents of a first winding and a second winding of a power transformer of a three-phase power system, the method comprising:

• based on a first and a second plurality of digitized current sample streams derived from a plurality of secondary currents corresponding to currents of the first winding and the second winding respectively, calculating a corresponding first and a corresponding second plurality of phasors;
• selecting a pair of vector-group compensation settings based on three-phase operate current values calculated using the first and second plurality of phasors, each of the pair of vector-group compensation settings corresponding to a 3-by-3 compensation matrix;
• applying a first vector-group compensation setting of the selected vector-group compensation setting pair to the first plurality of phasors to form a first set of three test phasors and applying a second vector-group compensation setting of the selected vector-group compensation setting pair to the second plurality of phasors to form a second set of three test phasors;
• and
• after adjusting each of the three test phasors of the first set by one-hundred and eighty degrees to form an adjusted first set of three test phasors and if phase angles of each of the adjusted first set of three test phasors is within a pre-determined angle range of each of the like-phase phasors of the second set of three test phasors, enabling application of the vector-group compensation setting pair as the operational vector-group compensation pair.