Fundamental digital approach to the distance protection of e.h.v. transmission lines

The basis of a new approach to the measurement of impedance, as used in the distance protection of transmission lines, is given. Digital processing is performed and it is shown that an extremely rapid and accurate indication of the circuit impedance can be effected by an online evaluation of the Fourier transform of a small section of the relaying waveforms. Results of an offline study of the response of the new technique when applied to a typical 400 kV line are presented. In contrast to many previously proposed methods, the new technique is shown to impose no fundamental limitation on the form of the relaying waveforms. The possibility of greatly improved protective performance emerges, one of the more important advantages being the possibility of minimal fault-detection times over the whole of the area of the impedance plane covered by a particular relaying characteristic. The paper concludes with a consideration of the effect on performance of filtering the measurands prior to implementing the algorithms.