Distance-dependent detection reliability of fixed threshold residual earth-fault indicators in medium-voltage feeders

Abstract

Residual earth-fault indicators installed in medium-voltage overhead feeders operate with fixed pickup thresholds, while the available residual current decreases with feeder distance. As a result, indicator operation becomes progressively less reliable toward remote feeder sections, and deterministic short-circuit analysis alone is insufficient to cha-racterize practical detectability limits. This study investigates distance-dependent detecta-bility of fixed-threshold residual earth-fault indicators using a combined experimental, si-mulation-based, and probabilistic framework. First, the relay–RTU–SCADA event chain is verified through controlled current-injection testing in a 35 kV reserve feeder bay. Second, residual earth-fault current along the 9.4 km Prizreni 1–Zhuri overhead feeder is evaluated using IEC 60909 single-phase-to-earth simulations implemented in DIgSILENT PowerFac-tory. Probabilistic threshold-exceedance analysis is then applied to estimate detection relia-bility as a function of feeder distance. The simulated residual current decreases from 350.577 A near the source substation to 299.793 A at the feeder endpoint. For the deployed 300 A pickup threshold, the detection probability decreases from near-unity values in upst-ream feeder sections to approximately 0.49 at the remote end of the feeder. Additional sen-sitivity analysis shows that fault resistance, uncertainty level, pickup threshold, and feeder electrical length significantly influence practical detectability limits. The results demon-strate that residual earth-fault indication in long feeders should be evaluated as a probabili-stic distance-dependent problem rather than as a purely deterministic threshold-crossing condition. The presented framework provides an engineering basis for evaluating reliability limits and pickup-threshold adequacy in medium-voltage distribution.