Computation of power line structure surge impedances using the electromagnetic field method

The power line structure surge impedance is computed using an electromagnetic field approach for various transmission and distribution line structure configurations (ranging from a single wood pole to lattice steel towers) including their grounding systems. The influence of the soil resistivity and different grounding system configurations on the structure surge impedance has been studied. The structure surge impedances are computed for three impressed surge current waveforms: (a) step current wave; (b) ramp current wave; and (c) typical double-exponential lightning surge current. The computed surge impedances using the electromagnetic field approach are compared with those obtained using formulae based on a geometrical model of the power line structure. It is shown that the computed values based on the electromagnetic field theory are, to various degrees, different from the values computed using the formulae based on the geometrical model of the structure. The study also reveals that: (a) the surge impedance of a lattice steel tower decreases only slightly when the soil resistivity is increased from 0.1 Ω-m to 500 Ω-m; and (b) when the footing resistance is held constant, the surge impedance of a single pole changes by about 5%, when counterpoises instead of rod clusters are used