Overvoltages and critical backflashover current of a 150 kV overhead line: influence of lightning current waveform and tower grounding system modeling

This study investigates lightning current waveform effects on the fast-front overvoltages and critical currents that cause backflashover of the insulation of a 150 kV overhead line by means of ATP-EMTP simulations, considering frequency- and current-dependent effects of tower grounding systems. The CIGRE waveform is used to simulate negative first return-strokes. Several cases are investigated by employing front time, maximum steepness, and time to half value statistical distributions. Various approaches to model concentrated and extended ground electrode configurations are applied. The first one uses a resistor with a fixed resistance. The second approach considers the frequency-dependent (FD) response of the grounding system with fixed electrical properties of soil. The third approach the FD response with soil properties varying with frequency. The fourth is a soil ionization model. A hybrid method is used to yield the tower grounding systems FD responses. This method combines electromagnetic field analysis with circuit theory. The modeling approach of the grounding system influences more the critical backflashover current for shorter and steeper wavefronts, as well as longer wavetails. The impact of lightning current waveform parameters is quantified and discussed for the 150 kV line under investigation.