Lightning Electromagnetic Fields and Their Induced Currents on Buried Cables. Part I: The Effect of an Ocean-Land Mixed Propagation Path
We use a full-wave finite-element-based solution of Maxwell's equations for the evaluation of lightning electromagnetic fields inside a vertically stratified, two-layer ground (ocean-land mixed propagation path) and their induced currents on the shield of buried cables. For "normal" incidence (with respect to the ocean-land interface), it is shown that the vertical electric field is the component most affected by the ocean-land mixed path when the observation point is close to the ocean-land interface (i.e., 5 m or so). For "oblique" incidence, however, depending on the angle of incidence and the distance between the observation point and the ocean, all the field components are reduced by the ocean-land interface. For the calculation of induced currents, and for the case of a parallel layout (cable laying in parallel to the ocean-land interface); 1) for a strike to the land, when the cable is buried in the soil and the distance to the ocean is greater than about 100 m, the effect of the ocean is negligible. 2) For a strike to the ocean, the induced current magnitudes are appreciable only when the cable is entirely within the land. For the case of a perpendicular layout (cable perpendicular to the ocean-land interface); 1) for a strike to the ocean, when the cable is totally buried in the ocean, the effect of ocean-land mixed propagation is negligible. However, when the cable extends into the land through one end, the induced currents increase at both ends with increasing length of underland portion. 2) For a strike to the land, when the cable is located entirely inside the land, the effect of ocean-land mixed path on the induced currents at both ends is negligible. However, as the cable extends into the ocean, a remarkable enhancement in the induced currents is observed for the termination located inside the land. This enhancement can be as high as a factor of 2 with respect to the case of a cable in homogeneous soil characterized by the properties of the land.