Rolled out CT connections of transformer differential relay caused 230/13.8 kV substation tripping

This paper describes a 230/13.8 kV substation nuisance tripping experience caused by rolled out current transformer (CT) connections to the main transformer differential relay. A metering and relaying one-line diagram is included to describe the substation protection scheme and to identify what was overlooked during the design and factory review process. Three-line diagrams that illustrate the change of current in the CT leads to the differential relay caused by the change of CT installation position are included. The transformer differential relay settings and associated sensitivity to the effect of roll out CT wiring are discussed. Recommendations to correct such a design problem are provided.