Evaluation of the Backflashover Performance of a 150 kV Overhead Transmission Line Considering Frequency- and Current-Dependent Effects of Tower Grounding Systems

The influence of the frequency- and current-dependent response of tower grounding systems on evaluating the backflashover performance of a typical 150 kV overhead transmission line is investigated in this study. This is achieved by adopting different tower grounding system modeling approaches in ATP-EMTP simulations: 1) a simple resistor (constant resistance value), 2) the frequency-dependent (FD) grounding system response with constant electrical properties of soil, 3) the FD response with FD soil properties, and 4) a soil ionization model. FD responses for concentrated and extended tower grounding systems are obtained via a hybrid method based on electromagnetic field analysis and circuit theory. Backflashover rate, BFR, is estimated by obtaining the minimum backflashover current of the line through ATP-EMTP simulations. Different first return-stroke currents of negative downward lightning flashes are employed in simulations, namely CIGRE WG 33.01 waveforms considering the statistical distributions of their parameters, recorded waveforms, and approximations of the latter with the CIGRE waveform. The impact of the lightning peak current distribution on BFR results is assessed. The evaluated backflashover performance is affected considerably by the FD tower grounding system response for extended systems, whereas by soil ionization for concentrated systems; for the latter, FD effects influence BFR mainly for low soil resistivity values.