25 th International Conference on Electricity Distribution Madrid, 3-6 June 2019 Paper n° 263 CIRED 2019 1/5 LIGHTNING PROTECTION OF UNSHIELDED OVERHEAD MEDIUM VOLTAGE POWER LINES IN SOUTH AFRICA Andreas BEUTEL Bruce MCLAREN Hendri GELDENHUYS Eskom – South Africa Eskom – South Africa Eskom – South Africa BeutelAA@eskom.co.za mLarenB@eskom.co.za GeldenHJ@eskom.co.za Willem Dirkse Van Schalkwyk John VAN COLLER Eskom – South Africa Wits University – South Africa vSchalWJ@eskom.co.za John.VanColler@wits.ac.za ABSTRACT South Africa has an extensive overhead electricity distribution network at medium voltage (MV). It also has a significant amount of lightning in most areas. The purpose of this paper is to report on South African experiences with respect to lightning protection of its overhead MV power network, and how designing a line for acceptable lightning performance impacts other aspects such as bird safety, pollution performance and terminal equipment reliability. Strategies covered include different insulation levels, bonding and earthing arrangements and pole-top configurations. Experiences from the laboratory and the field are included. It is concluded that the presently used configuration of a partially bonded structure with lightning insulation level of 300 kV performs more than adequately, but that initial experience with alternative configurations indicates that the network performance could be optimised. INTRODUCTION South Africa has an extensive overhead electricity distribution network at medium voltage (MV), utilizing mainly wood poles and where the predominant voltage is 22 kV between phases. Some of those lines traverse areas with high lightning ground flash density (greater than 14 flashes/km 2 /annum), while others supply customers in areas with relatively low ground flash density, but where there is sparse vegetation and hence the lines are vulnerable to direct lightning strikes. Approximately half of the country has a ground flash density of 3 flashes/km 2 /annum or more. South Africa also has populations of endangered large birds such as vultures, which need to be protected from electrocution. Additionally, much of the coastal (and some other) areas are prone to pollution leakage currents, which may lead to burning of wood pole structures on power lines. These factors, and others such as construction and maintenance costs, need to be factored in when designing a power line. Previous work has explored the pollution leakage current performance [1, 2] and bird safety implications [3, 4] of MV lines in the South African context. A summarised comparison between structure configurations was presented in [5]. The present paper presents a study into the lightning performance aspect of MV overhead line design, using experience gained in South African conditions mainly through Eskom, the country’s largest electricity utility. The first section presents the current standard MV structure configuration used in Eskom. This is followed by an evaluation of alternative configurations and compares their performance to that of the standard configuration. Finally, various field experiences to support the previous sections are included. The work is then concluded. STANDARD CONFIGURATION This is illustrated in Fig 1. This shows a partially bonded wood pole structure, with wood cross-arm and the earthed-end pole-top metal hardware bonded together and connected to earth via an insulation coordination wood gap. This gap increases the lightning impulse insulation level of the structure from the approximately 150 kV provided by the insulators to about 300 kV [2]. This configuration has the advantage that induced lightning surges do not cause flashover, since the insulation level is greater than the voltage of these surges [6]. Should flashover occur due to a direct lightning strike, the arc- quenching properties of the wood of the insulation coordination gap mean that the protection does not necessarily operate [7]. The wood gap also improves bird safety, since it adds an impedance into the earth path – often in the MΩ range [3, 4]. Since induced lightning surges do not cause flashover, terminal equipment such as transformers or their surge arresters are stressed more than they would be if the line had a lower insulation level [8]. Also, since direct lightning strikes still cause flashover, the structures need to be designed to withstand the high energy associated with direct strikes and the resultant power frequency follow current, which can in some cases damage the pole. One way of achieving this design is by mounting a spark gap across the insulation coordination gap [5]. Laboratory voltage lightning impulse test results show the operation of the spark gap in Fig 2. Pole fires can also in rare cases be started in the insulation coordination gap due to lightning flashover and follow current, an example is shown in Fig 3.