A dependable and secure adaptive distance protection scheme against resistive faults

Distance relays with set trip boundaries are prone to failure against resistive faults as the apparent impedance may exhibit a significant deviation from the impedance of the faulted line segment. Adaptive distance relaying is a viable solution for this problem. This may include (i) changing the trip characteristics of the relay [1], (ii)  modifying the apparent impedance calculation to compensate for the deviation [2]. While the former approach may compromise relay selectivity, the latter typically assumes structural characteristics such as system homogeneity, which may not always be applicable. 

In this paper, we propose a scheme for adapting the zone-1 operation of a distance relay for transmission lines. The scheme combines merits of both the approaches discussed earlier. This means, to improve dependability, it utilizes the adapted relay characteristics corresponding to the prevalent system operating condition. In addition, to maintain relay security, an internal adaptive check is deployed on the measured apparent impedance to ensure that it is not an out-of-zone fault. This step produces an estimate of a “compensated” apparent impedance which determines fault within or out of zone with respect to the base (fixed) zone-1 boundaries. The proposed scheme utilizes an estimate of the two-port equivalent network across the transmission line, especially, approaches which are amenable for a substation level implementation [3] (also see Fig. 1). The scheme may also utilize two-port equivalent parameters made available from  network level analysis.