Tower grounding improvement versus line surge arresters: comparison of remedial measures for high-BFOR subtransmission lines

This paper presents a technical/economic comparison between remedial measures aimed at improving the lightning performance of an existing Italian three-phase 150-kV overhead line. The line is characterized by a very high back-flashover rate (BFOR), due to large grounding resistance values. Two countermeasures are proposed: grounding system improvement with additional vertical rods and line metal oxide surge arrester (MOSA) installation on one or all phases. A Monte Carlo ATP-EMTP procedure developed by the authors, which takes into account both the tower grounding nonlinear transient response due to soil ionization and MOSA nonlinear response, has been applied to evaluate and compare the effectiveness of the proposed countermeasures. The installation of MOSA on all phases is technically the best option, but it is relatively expensive. Tower grounding improvement and MOSA installation on the lower phase yield very similar BFORs: the economic comparison strongly depends on tower's accessibility and soil nature.