The HEMP Response of an Overhead Power Distribution Line

The level of transient electrical surges in power system distribution lines plays an important role in determining the overall response of the electrical power network to a high altitude electromagnetic pulse (HEMP) produced by a nuclear explosion. A previous paper has discussed a multiconductor coupling model which permits the computation of the HEMP-induced voltage surges on above-ground lines, given a specified HEMP environment incident on the line. The use of this model with the Bell Laboratory HEMP waveform for performing a series of calculations of power system responses has resulted in line responses which are unreasonably large, and this has prompted EMP investigators to consider alternate, more physical, HEMP environments for such power system studies. Recently, the results of a parametric study of the HEMP-induced surges on a single conductor located 10 and 20 meters over the earth were reported, using a new HEMP environment. Peak line-to-earth voltages approaching 600 kV were observed for the 10 m line, and peak voltages exceeding 1 MV were noted for the 20 m line. For power transmission lines having a high basic insulation level (BIL), such surges may not be important. However, in power distribution systems, such overvoltages may pose serious problems to the insulators and load equipment, such as transformers, which are attached to the lines. Consequently, a study of the HEMP response of a realistic power distribution system shown in Figure 1 was undertaken, and its results are described in this paper.