Impact of grounded shield wire assumption on impedance-based fault location algorithms

Although shield wires are typically grounded through a finite tower footing resistance, positive- and zero-sequence line impedances are calculated assuming no tower footing resistance. Since impedance-based fault location algorithms require sequence line impedances to compute the distance to fault, this paper evaluates the impact of the grounded shield wire assumption on the accuracy of fault locating algorithms. A simple test case, which consists of a 3.73 long transmission line with two shield wires and a tower footing resistance every 0.19 miles, was setup to evaluate the assumption. The analysis shows that tower footing resistance affects only the zero-sequence line impedance. This leads to a marginal increase in error from one-ended impedance-based methods in locating single line-to-ground or double line-to-ground faults. On the other hand, fault location estimates from two-ended methods are not affected as they do not make use of the zero-sequence line impedance in fault location computation.