Generator protection and CT-saturation problems and solutions

This paper presents a new generator relay design, stator differential function in particular, coping with extreme cases of current transformer (CT) saturation: both ac saturation under large currents and/or residual flux; and dc saturation under low currents but large and long-lasting dc components. The algorithm is a subcycle algorithm with fastest times reaching half a cycle including overall response of the relay hardware. Analysis of the response of a traditional differential function under CT saturation is included to provide setting guidelines for legacy relays.