The effectiveness of shield wires in reducing induced voltages from lightning electromagnetic fields

The mechanisms from which lightning overvoltages can be produced on a power line depend on the system voltage. In medium voltage (MV) overhead distribution systems, lightning transients can be originated by either direct or indirect strokes. The main methods that can be adopted to improve the line lightning performance concern the increase of the critical flashover voltage (CFO) of the line structures, the installation of surge arresters, and the use of one or more periodically grounded shield wires. This paper deals with the evaluation of the effectiveness of shield wires in reducing the magnitudes of the surges induced by nearby strokes on MV distribution lines. Such effectiveness depends on the combination of several parameters such as the relative position of the shield wire with respect to the phase conductors, the grounding interval, the ground resistance, the stroke current steepness, and the relative position of the stroke channel with respect to the grounding points. Realistic situations corresponding to typical configurations of a rural distribution line with either an overhead ground wire or a neutral conductor are considered. The analysis is carried out based on both computer simulations and test results obtained from scale model experiments performed under controlled conditions.