Improvements in Overhead lightning protection
Lightning is one of the major causes of poor performance by medium-voltage overhead lines in distribution networks. On somemedium-voltage networks, for example, up to 78% of equipment failures are the result of lightning activity. Wood-pole splitting and pole-top fires can cause long, frustrating outages and dangerous low-hanging conductors. On medium-voltage overhead line feeders in South Africa, the insulation-to-earth typically comprises an insulator with a 150-kV basic insulation level (BIL) in series with a wood gap consisting of a 150-kV BIL on the bonded wood poles, which offer some power-frequency arc-quenching properties.
The 300-kV total BIL philosophy currently used by Eskom is based on IEEE Standard 1410-2010 and was implemented to ensure no indirect lightning strikes caused a phase-to-earth flashover. Unfortunately, the higher the BIL, the higher the amplitude of the direct and indirect lightning voltage surges on overhead line feeders, which can increase the number of fuse, surge arrester and transformer failures during lightning storms. A disadvantage of the wood gap’s presence is that direct lightning strikes to the feeder generally cause a flashover across the wood gap, which often results in splintering damage to the wood.
Eskom launched an internal investigation to determine the optimal choice of BIL by instigating on-site measurements on an operational 22-kV overhead line feeder. The pole-mounted supply points were an integral part of the feeder in the field trial to identify the most optimal configuration and protection of the equipment. The utility appreciated the fact direct lightning strikes to the overhead line at a pole-mounted installation caused the largest electrical stresses on the equipment.