Transition from Near-Field to TEM Propagation in Lightning-Struck Shield Wires and Its Impact on Induced Voltages

Lightning strikes to overhead shield wires of transmission or distribution lines generate impulsive currents whose electromagnetic fields induce voltages on nearby phase conductors. Conventional calculations rely on mutual inductance and capacitance formulas that assume infinitely long parallel lines, an approximation that breaks down near the strike point where the fields are highly non-uniform. This paper presents a full electromagnetic analysis of the fields produced by the currents in the shield wire. The induced voltage on the phase conductor is obtained directly from these fields, without invoking mutual inductance and capacitance. It is shown that at distances much larger than the height of the shield wire, the electric field structure of the lightning injected current reduces to that of TEM mode. Only at these distances one can utilize the standard mutual inductance to obtain the induced voltages in the phase conductors. Thus, the mutualinductance/capacitance-based methods become valid only at distances of several tens of meters from the attachment point; closer to the strike, significant errors could occur. The proposed approach therefore provides more accurate estimates of the lightning-induced voltages on phase conductors of the transmission lines.