Preventing substation tripping

This article describes an actual case history of substation nuisance tripping caused by a transformer differential relay due to rolled-out current-transformer (CT) connections. One set of differential relay CTs was installed averse order, and it´s wiring was installed as if the CTs were stalled in the correct order. It was after several weeks of substation initial energization, when all plant auxiliary loads, such as lighting and small 480-V equipment, were in operation, the substation main transformer tripped on a line-to-ground arcing fault on the 480 V-system. Transformer differential relay settings, which caused tripping of the transformer for this through fault, were suspected to be in error. It was thought that perhaps arcing ground fault on the 480-V system contained a relatively higher magnitude of harmonic currents. Relay-setting calculations, especially the harmonic restraint, percent slope, tap ratio adjustment, and percent mismatch were reviewed and found to be within acceptable ranges. After thorough checking of relay settings and visual inspection of the substation transformer and switchgear, the field crew reenergized the substation while the technical investigation by the author was still in progress.