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Year 2026 · Volume 3 · Issue 3
Coupled Frequency-Dependent and Ionization-Dependent Tower Footing Response and the Seasonal Weighting of Back flashover Rate for a 150 kV Double-Circuit Transmission Line in Continental Greece
Published Online: September-December 2026
Pages: 01-10
Cite this article
↗ https://www.doi.org/10.59256/indjeee.20260303001
Design practice for the lightning performance of transmission lines represents the tower footing either by a constant resistance, by a current-dependent resistance accounting for soil ionization, or by a frequency-dependent (FD) soil model, but rarely by the two dispersive mechanisms together. This paper computes the coupled FD and ionization response of the standard footing of the Hellenic 150 kV double-circuit Series-4 tower and propagates it to the backflashover rate (BFR) of a 42 km line in the karst of Sterea Ellada. Three results are reported. Omitting either mechanism is a first-order error: a constant-resistance footing overestimates the impulse impedance by up to 344 % and the BFR by up to 185 %, while an FD-only or an ionization-only representation still overestimates BFR by up to 51 % and 109 % respectively. Within a transmission-line representation of the electrode the two mechanisms are almost exactly multiplicatively separable, because ionization acts on the geometric factor of the electrode while frequency dependence acts on the propagation constant of the soil, so two independent correction factors suffice. Because BFR is a convex function of low-frequency resistivity, weighting the seasonal resistivity of Mediterranean karst by the observed bimodal Greek lightning climatology raises the computed BFR by 43 % to 267 % relative to the conventional annual-average calculation, which is therefore systematically non-conservative. Revised effective counterpoise lengths for the standard 10 mm steel counterpoise are given as a design chart.
