A New Algorithm for Digital Impedance Relays

This paper presents a new algorithm suitable for calculating impedances from digitized voltages and currents sampled at a relay location. Each input is assumed to be composed of a decaying d.c. component and components of the fundamental and harmonic frequencies. Parameters of a digital filter determined by using the least error squares approach are then used to compute the real and imaginary components of the voltage and current phasors. Impedances as seen from a relay location are then calculated. The proposed algorithm was tested using the fault data recorded at the Regina South switching station of the Saskatchewan Power Corporation; some of the test results are included.