New algorithm for low-impedance restricted earth fault protection

A new algorithm for low-impedance restricted earth fault (REF) protection is presented in this paper. The algorithm uses a unique definition of the restraining current for enhanced sensitivity and security on current transformer (CT) saturation. While ensuring good sensitivity and excellent security, the trip time of the actual relay hardware implementing the presented algorithm is below one power cycle. The described algorithm is a standard function on commercially available transformer and generator numerical relays with more than one thousand unit-years of combined field experience to date.