Evaluation of the Impact of Including the Frequency-Dependent Behavior of Grounding Systems on the Lightning Performance of Transmission Lines and on Grounding Systems Design

This paper evaluates the impact of including the frequency-dependent behavior of grounding systems on the lightning performance of transmission lines and on their grounding systems design. The developed overvoltages through the insulator strings and the backflashover rates of a typical 138 kV transmission line are estimated, while the tower-grounding system is represented by two different models, namely: (i) a static model, consisting of a lumped resistance with value equal to the low-frequency grounding resistance, (ii) an impulse impedance model (based on the accurate HEM-Hybrid Electromagnetic Model). A method is proposed to define the length of the counterpoise wires of the transmission line as a function of the soil resistivity. The criterion used was to limit the value of the impulse impedance to 20 ohm which is similar to that adopted by Brazilian power utilities, although they usually adopt this value for the low-frequency resistance of grounding, thus disregarding its frequency-dependent characteristics. From the results obtained, it can be concluded that disregarding the frequency-dependent effect of transmission line grounding leads to errors in estimating their lightning performance. It was also found that by using a more sophisticated modeling for grounding, it would be possible to optimize the design of transmission line grounding systems, reducing the length of the counterpoise wires, therefore resulting in supply and labor gains.