A current story - When primary met secondary

Protective relays require accurate voltage and current measurements from potential transformers (PTs) and current transformers (CTs) to reliably protect the power system. In this paper, we describe a unique event in which the secondary wiring of two CTs, one going to a bus differential relay and the other to a transformer differential relay, made contact with the primary system and caused a fault. This subjected both relays to primary-level currents, causing them to measure false differential current and operate. The paper discusses the outage that occurred and the investigation the utility performed, followed by how relay event reports were used to substantiate the utility's findings and gain additional insight into the fault. The paper also discusses the damage that can occur to a relay when CT secondary wiring comes in contact with the primary system.