Estimation of the minimum backflashover current and backflashover rate of a 150 kV overhead transmission line: frequency and current-dependent effects of grounding systems

Frequency- and current-dependent effects dominate the response of grounding systems. This work investigates the influence of tower grounding system modeling on estimating the minimum backflashover current and backflashover rate (BFR) of a typical 150 kV overhead transmission line. The minimum backflashover current is determined through ATP-EMTP simulations. Four grounding system modeling approaches are evaluated, namely: (i) a constant ground resistance, (ii) the frequency-dependent (FD) response of the grounding system with constant electrical properties of soil, (iii) the FD response with FD soil properties, and (iv) a soil ionization model. Actual concentrated and extended tower grounding systems used in practice for the investigated 150 kV line are considered. The FD response of these grounding systems is obtained using a hybrid code based on electromagnetic field analysis and circuit theory. Results show that simulating the FD response is important for the extended grounding system. For the concentrated grounding system, the minimum backflashover current and BFR are affected considerably by including soil ionization in simulations; FD effects influence BFR results primarily for low soil resistivity values.