Investigation of nuisance trips in microprocessor based transformer differential relay

A “nuisance trip” of a microprocessor based transformer differential relay was observed, with an unknown cause. The relay settings were modified with a higher pickup and an alarm at a more sensitive setting. The relay was then able to capture an event, which then did not cause tripping. The results were analyzed and found to be caused by energization of a downstream transformer. In addition, the CT ratio on the transformer primary was excessively high, which was corrected by auxiliary CTs. Based on these results, no further changes needed to be made to the relay settings.