A digital multifunction protective relay

The author describes the development of a digital multifunction relay for the protection of the intertie between a customer-owned generator and a utility system. The relay uses state-of-the-art digital signal processing techniques to measure the relay parameters, thereby eliminating analog hardware. The hardware design uses a dual microprocessor architecture to achieve flexibility and high speed operation. The relay uses the DFT (discrete Fourier transform) to obtain the fundamental frequency phasor measurements, and hence the frequency and power related functions are immune to harmonic distortion in the voltage and current signals. In addition to costing less than the existing package of single function relays, the relay also provides improved performance, user friendly human interface, remote communication, self-checking, and self-calibration functions