A Multiconductor Model for Determining the Response of Power Transmission and Distribution Lines to a High Altitude Electromagnetic Pulse (HEMP)

A nuclear detonation at an altitude above 40 km above the earth´s surface would produce an intense transient electromagnetic field, known as a high altitude electromagnetic pulse (HEMP). These electromagnetic fields radiated from the blast vary with the detonation location and weapon characteristics. At detonation altitudes ranging from 100 km to 500 km, large portions of electrical power system interconnections would be illuminated by this intense transient field. The intensity of this HEMP field would be on the order of tens of KV per meter. Most of the energy of the HEMP environment lies within the radio-frequency (RF) portion of the electromagnetic spectrum, and ranges from several kilohertz to more than 100 MHz. This intense transient electromagnetic field will almost simultaneously induce fast electrical surges in transmission and distribution (T&D) systems and in elements of the control and communication systems throughout large portions of the nation´s electric grid. Earlier investigations have indicated that the HEMP-induced surges may well exceed the basic insulation level (BIL) on power distribution lines. This could result in flashovers and/or insulation damage. Insulation damage can occur in the form of a puncture or degradation of the insulation strength. On power transmission lines which are designed with a higher BIL, the possibility of line flashover is smaller, but it must be recognized that these lines may run for many miles and consequently, can deliver a large amount of EMP energy into a substation or other facility.