Effect of a lossy dispersive ground on lightning overvoltages transferred to the low-voltage side of a single-phase distribution transformer

This paper presents a study of lightning overvoltages transferred to the low-voltage side of a 10 kVA 7.967 kV/240-120 V single-phase distribution transformer considering dispersive ground effects in the medium-voltage distribution line terminated at its primary side. A two-port wideband model that is capable to represent the transformer behavior under different load conditions is considered. The medium-voltage distribution line is modeled using Marti's transmission line model modified to include the variation with frequency of the ground resistivity and permittivity. Periodical grounding points are also modeled considering dispersive ground effects. Phase-neutral voltages transferred to the transformer secondary are calculated for direct lightning strikes at the medium-voltage line including insulation breakdown, surge arresters, and different load conditions. It is shown that the frequency variation of the ground resistivity and permittivity can significantly affect voltages transferred to the low-voltage side of the transformer if a high-resistivity soil is considered, depending on load conditions and on flashover occurrence. It is also shown that the importance of considering or neglecting frequency-dependent ground parameters on lightning overvoltages transferred to the secondary side of the transformer is minimized by the installation of a medium-voltage surge arrester at its primary side.