Discriminative response of distance protection in reverse-fault operation

The paper provides a detailed investigation of the principal properties of distance protection response, for the particular conditions of faults immediately behind the intended zone of protection coverage, and against which the protection is required at all times to discriminate. Based, first, on a nontransient evaluation model and stability indices defined for each earth- and phase-fault element of a complete protection terminal, the combinations of conditions for which reverse-fault stability margins are likely to be lowest are identified. The dependence of the margins on the fault level at the protection location, protection forward-reach settings, the crosspolarising level, and the power transfer at the instant of fault inception are examined. From this basis, the extent to which stability margins might be impaired by the presence of transients in comparator input signals is investigated using extensive dynamic simulation facilities. Of principal concern in close-up reverse-fault operation is the transient response of capacitor-voltage transformers, from which protection signals are derived, but the paper shows that, for all asymmetrical fault conditions, dynamic stability margins remain high. Protection discrimination is most severely tested by symmetrical 3-phase-to-earth faults, for then all primary input voltages to capacitor-voltage transformers collapse, and the subsequent protection response is determined mainly by their residual transients. In this extreme fault condition, all earth- and phase-fault comparators are close to an operating threshold, but the paper gives the form of capacitor-voltage-transformer response which guarantees stability.