Switching Impulse Withstand of HVDC Overhead Lines in Italy: Gap Factor and Clearance Evaluation

The renewable energy transition is forcing European Transmission System Operators to face new challenges; a huge growth in solar and offshore renewable energy sources is foreseen in the next decades in Italy. New <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\pm 500$</tex> kV HVDC backbones will replace existing high and extra high voltage ac overhead lines (OHLs), both in rural and urbanized areas, prompting the need for a sustainable design in terms of environmental impact (e.g., mainly, visual impact) and human interaction (e.g., dc electric field, audible noise and radio noise). As known, site pollution severity, lightning and switching overvoltages represent the main parameters to be considered during the insulation coordination study. Due to the presence of dc circuit breakers, higher switching overvoltages could arise if compared to standard HVDC overhead lines. The paper focuses on the practical calculation of the gap factor, a key parameter that characterizes the non-uniform electric field distribution in the air gap between electrodes. Various methods for determining the gap factor are examined, including empirical and standard approaches.