Evaluation of the Mitigation Effect of the Shield Wires on Lightning Induced Overvoltages in MV Distribution Systems Using Statistical Analysis

Lightning overvoltages are one of the most dangerous factors affecting the power quality of a distribution grid. In medium voltage (MV) overhead distribution systems, lightning transients can be caused by either direct or indirect strikes. Indirect strikes are much more frequent than direct strikes and can cause flashovers, especially when the line insulation level is low. The most widely employed protection measures against lightning overvoltages are the use of the shield wires, and/or surge arresters, and increasing the line insulation level. In particular, many studies have shown a significant reduction in the induced voltage magnitudes, thanks to the use of the shield wires in both transmission and distribution systems. This paper deals with the possibility of adopting the shield wires to mitigate lightning induced overvoltages in MV networks, and proposes an analysis on the main parameters that influence their performances. This aim is pursued by means of a probabilistic and statistical analysis based on the response surface method (RSM), which permits to obtain the equation of the surface that best approximates a given model. The application and efficiency of the proposed RSM-based approach have been illustrated by applying it to a realistic situation of a rural distribution line.