The Effect of an Ocean-Land Mixed Propagation Path on the Lightning Electromagnetic Fields and Their Induced Voltages on Overhead Lines

We use a full-wave approach based on the finite-element method solution to Maxwell´s equations for the evaluation of the effect of an ocean-land mixed propagation path on the above-ground lightning electromagnetic fields and their induced voltages on overhead lines. Two cases of normal and oblique strikes are studied. For normal strike, it is shown that neither the vertical electric field nor the azimuthal magnetic field is affected by the considered mixed propagation path. For an oblique strike, however, the azimuthal magnetic field is slightly affected when the observation point is close to the ocean-land interface (i.e., 5 m or so), while the vertical electric field remains unchanged. For both normal and oblique strikes, at moderate and far distances from the channel base and when the observation point is located in the vicinity of the ocean-land interface, the radial electric field is markedly affected by this interface. For the calculation of lightning-induced voltages, two cases, namely, the ocean-side and the river-crossing transmission lines are studied. For the ocean-side transmission line, as the overhead line gets closer to the ocean, the induced voltages on the line midpoint decrease while increasing behavior is observed in the voltages induced on the line terminations. For the river-crossing transmission line, the lightning-induced voltages along the line experience a significant change in terms of their peak values and waveshapes. In some cases, the enhancement in induced voltages can be as high as a factor of 2 with respect to that obtained by assuming a homogeneous propagation path characterized by the electrical properties of the land.