Improving Lightning Performance of EHV Transmission Lines

Utility Practice & Experience

Lightning has long been known to be a major cause of disruptions giving rise to line outages, especially on transmission systems. This is due to the very nature of how a transmission line operates in an open area, making it particularly susceptible to lightning. For example, every year Malaysia reports that more than 36% of transmission line outages are due to lightning. In China, studies have shown that 29 to 46% of all line outages are caused by lightning and for 500 kV lines in the south, power grid trip outs from lightning have been known to account for up to 87% of outages. In Indonesia, about 66% of line outages on 150 kV lines are caused by lightning while statistics on transmission line outages in Japan have shown that as much as 72% are caused by lightning strike. Similarly, countries such as Brazil and Great Britain also report a high proportion of outages due to lightning events.

Typically, lightning flashover takes place due to a lightning strike to a structure or line conductor. For a transmission line, the conductor is protected from lightning by a shield wire on the top of the tower. Backflashover occurs when lightning hits the shield wire or tower. A high lightning current will be produced by a direct lightning strike to the tower structure and will flow along the tower into the earth. As a result, the impulse potential will be highly detrimental and will subsequently affect the tower and the earthing device efficiency. System failure due to backflashover is measured as backflashover rate (BFR) and this will reflect its performance. Commonly, a high BFR is associated with high tower footing impedance coupled with high soil resistivity.
Tower footing impedance associated with soil resistivity is known to have a significant impact on transmission system failure. Usually, the value of the impedance parameter must be less than the limit of the tower impedance set by the requirement of the power utility. In Peninsular Malaysia, for example, the national grid – Tenaga Nasional Berhad – has fixed the tower footing resistance (TFR) to be less than or equal to 5 ohm for 500 kV lines, although in many countries, a TFR of around 10 Ω to 15 Ω is considered as effective. Thus, an effective earthing design is essential to improve the efficiency of a transmission line and is one of the most effective solutions to this problem. TFR is one of the important parameters to consider. The outcome of improving TFR depends on many factors, including the earth structure and soil resistivity, among others. The arrangement of the earthing design typically has a relationship with tower footing and Table 1 shows some of the practices presently used in several Asian countries.

Table 1: Current Practices of Earthing Design & Arrangement in Several Asian Countries

Another method to reduce TFR is the counterpoise approach, perceived as practical and efficient for high voltage transmission earthing systems. Two types of counterpoises have been widely used for towers located in areas of high soil resistivity, such as rocky and sandy soils: the continuous and the radial type. Other aspects important in earthing design include soil resistivity, number of soil layers and soil thickness in each layer. Thus, a convenient way to reduce the value of soil resistivity and earthing resistance is to use earthing enhancement compounds. Such a solution has recently become popular in electrical power systems, particularly for mitigating the issue of earthing systems by replacing inappropriate soil content to minimize TFR from high cost and space constraints.

Attend the upcoming 2022 INMR WORLD CONGRESS in Berlin this October to attend a lecture by lightning expert, Mohd. Zainal Abidin Ab Kadir, a Professor at the Centre for Electromagnetic & Lightning Protection Research at the Universiti Putra in Malaysia. Dr. Zainal will report on a study analyzing the outcomes of default and new earthing arrangements for enhancing tower footing resistance and impedance, as well as an interpretation of soil profile. He will recommend improving lightning performance of EHV transmission lines with different earthing designs (before and after improvement) and applications of earthing enhancement compounds to achieve the required limit for TFR of towers.