Methods for testing transformer differential relays applied to delta-grounded wye transformers using single-phase test currents

This paper provides an alternate method for testing transformer differential relays applied to delta-grounded wye transformers using single-phase test currents. The alternate method is based on the transformer three-line diagram and transformer nameplate. This alternate method simulates actual current flows in the transformer during single-line-ground faults on the wye side and provides confirmation that the relay´s slope characteristics and phase shift settings are operating correctly. The alternate method is based on the transformer three-line diagram and provides verification that the relay´s settings are correct for the transformer application. Three-phase testing is not possible with some test sets due in part by the limited number of current channels available and, in some applications, test sets can also be limited by the high-current values required for differential slope 2 region tests. Traditional methods of single-phase testing typically require the addition of √3 multipliers and other adjustments to account for zero-sequence current removal, further increasing the magnitude of the test currents. Also, traditional tests (both 3-phase and 1-phase) use currents and phase shifts based on the relay settings. The alternate method improves both issues as only 2 channels are required of the test set and, since the test simulates real world, no multipliers are required. The alternate method uses two test currents that are always 180 degrees out of phase of each other for the differentially balanced condition regardless of the transformer phase shift lead or lag. As a result, the alternate method is especially useful for commission testing as it proves the transformer phase shift relay settings are correct for the transformer connection rather than testing the relay settings.