Discriminative performance of distance protection under fault operating conditions

The paper is devoted to an evaluation in detail of the protection coverage afforded by earth- and phase-fault distance relays in terms of the range of apparent impedances presented to them for which the protection correctly operates and initiates circuit-breaker tripping. This requires a knowledge of both the effective polar characteristics of relays for specified system-fault conditions and also, for the same conditions, the apparent impedance to which the relays respond. Owing to the wide variation encountered in both of these in practice, the ordinary static characteristics of relays provide only a limited indication of the discriminative properties of distance relays in different applications, but it is shown how a comprehensive assessment may be made. In particular, investigations are described of the performance of mho relays when applied to 132kV and 400kV circuits, and the results of the investigations are summarised. The studies include the response to faults within the protected zone, the discrimination against external reverse faults, against healthy-phase relay operation and, on double-circuit-line constructions, the discrimination against healthy-circuit relay operation. The time response of comparators in impedance measurement is introduced, and it is explained how, evolving from this, the coverage afforded by the protection may be directly related to its operating time. In this way a basis is provided on which the operating time of distance relays may be systematically linked to its discriminative properties.