Lessons learned through commissioning and analyzing data from transformer differential installations

Ensuring correct setting and installation of a transformer differential relay is critical. A transformer differential relay must detect internal faults, damaging overloads, and through-fault currents while remaining secure against misoperation. The ability to use wye-connected CTs with microprocessor-based relays has simplified installations, and these relays provide better commissioning tools than those available with traditional relays. However, installations and commissioning remain complicated. Installation and settings errors continue to be widespread, implying a need for more rigorous commissioning tests. Even with greater commissioning effort, occasional wiring problems develop over time, and these can best be resolved through analysis of relay event report data. In the interest of reducing transformer differential misoperations, we share in this technical paper practical lessons we have learned through experience with commissioning and relay event report analysis.