Lightning & Grounding Considerations for Overhead Line Rebuilding and Refurbishing Projects, AC and DC

Introduction, Objectives of this Technical Brochure

Overhead lines have supported the safe and economic development of three-phase AC electric power systems for more than 135 years. Evolution now includes a legacy of dedicated rights-of-way (easements or servitudes) with structures spaced at 60 to 600 m. With a total global length of transmission circuits of about 5 Gm (5 million km) and 100 Gm of distribution circuits in built-up areas, there is increasing pressure for utilities to repurpose existing overhead lines rather than to obtain land for new construction.

The lightning performance of these overhead lines can significantly affect overall reliability of supply. During these and other faults, choices for grounding affect safe operation. Technical studies and literature have addressed individual subjects, such as lightning and soil parameters, remedial measures, and line lightning performance for many decades. The goal of this Technical Brochure (TB) is to consolidate the valuable advice distributed among dozens of existing CIGRE resources on lightning parameters, calculation of lightning performance and the degree to which this performance can be improved by remedial measures.

This TB adds value to overhead line rebuilding and refurbishing projects by promoting a coordinated approach to grounding system design, considering fault currents and lightning surges. Readers will appreciate that minor design modifications – such as providing electrical bond wires across the grout of a micropile to the metal casing, from structure to OHGW and from guy wires to their anchors – can reduce or eliminate the need for supplemental grounding systems. The TB goals are further advanced by text that provides convenient hyperlinks to most of its references, as well as spreadsheet-based calculations to quantify several important aspects.

Scope and Methodology

Existing transmission and distribution network assets are aging naturally. In some cases, lines reach early end-of-life from aggressive operation, neglected maintenance or unexpected climate stresses including wind and wildfire. Five motivations for rebuilding and refurbishing projects include inadequate reliability, inadequate thermal rating, component end-of-life, reduced visual impact and right-of-way (ROW) encroachment.

A survey of typical lines from 1955 confirmed that rebuilding an overhead line to improve its lightning performance demonstrated the value of:

• Overhead groundwires above the phases for shielding
• Externally gapped line surge arresters
• Insulating crossarms
• Under-built groundwires below the phases

In 1955, the arrester tubes and insulating crossarms made use of wood, and the insulators themselves were strings of ceramic disc insulators. Today’s technology has advanced to apply metal oxide surge arresters as well as composite insulators and crossarms. The lines from 1955 are the typical candidates for today’s rebuilding and refurbishing projects, using modern components. They had median 216 m span length, providing lower impulse and power frequency ground impedance than newer lines with longer span length. The 1955 lines also provided at least 500 kV impulse CFO using combinations of wood and porcelain discs, well above the 300 kV now recommended for resisting induced overvoltage for all system voltage levels