Design and development of a digital multifunction relay for generator protection

This paper presents the design and development of a rotor earth fault protection function as part of a multifunction generator protection relay. The relay design is based on a low frequency square wave injection method in detecting rotor earth faults. The accuracy of rotor earth fault resistance measurement is improved by applying piecewise quadratic approximation to the nonlinear gain characteristic of the measurement circuit. The paper also presents the hardware and software architecture of the relay