A fundamental digital approach to impedance relays

The author presents a technique which can effectively compute transmission line impedances. The algorithm assumes that the input is composed of a fundamental frequency component, a decaying DC-offset transient and a number of nonharmonic components with unknown frequencies. The decay rate is not assumed in advance because it is affected by the resistance of the arc at the fault point, and the effective resistance of the system. To indicate the degree of improvement in impedance measurements obtained by using the technique as compared to earlier techniques, the Fourier algorithm was selected as a reference of comparison. The technique was tested using simulated data generated from a software program, and data recorded at a 220 kV switching station. Some of the test results are included