RTDS Based Testing of Bus Differential Protective Relay Performance in Presence of CT Saturation

A power system busbar can experience very high amounts of currents during short circuit faults, resulting in significant and often irreparable damage. Therefore, bus protection systems need to have fast, reliable, and secure operation. Amongst the various bus protection schemes, the low-impedance bus differential schemes have become increasingly popular due to their less challenging application requirements; however, they can be severely impacted by current transformer (CT) saturation during higher fault currents. The modern numerical relays try to mitigate the CT saturation problem by processing the initial saturation free current measurements and using stabilization techniques. In this project, the low impedance bus differential function of a modern bus protection relay is tested under severe CT saturation, such that the saturation free time is minimized to lower than the advertised minimum saturation free time. The testing is based on hardware-in-the-loop (HIL) setup that utilizes the physical relay device and the Real Time Digital Simulator (RTDS).