Using Multifunction Bus Differential Relays for CT Saturation Issues

Current transformer (CT) saturation is a common issue in the world of power system protection. Typical switchgear design methodology is to size multi-ratio current transformers based on breaker rating. The ratios of instrument transformers are typically adjusted based on a feeder’s full load amps and cable ratings, primarily to provide more sensitive overcurrent set points and to provide a better metering range. However, lowering taps on the CT also reduces the ANSI class and rating, causing faster saturation and lower tolerance to fault currents. Switchgear space restrictions can limit the ability to install the desired CT class and rating, resulting in smaller CTs that will saturate under fault conditions. Low-impedance multifunction differential relays implemented with overcurrent protection provided a cost effective redundant solution that was used in a liquefied natural gas facility. Testing and modeling was completed and relay performance is presented.