Assessment of lightning performance of high voltage transmission lines (TLs) is an important parameter to be considered by electric power utilities to ensure the quality of the power delivered to the consumers. This assessment is associated with calculation of the number of lightning outages of such TLs, which considers the mechanisms of backflashover and shielding failure. Present work focuses on the backflashover mechanism, which predominates on transmission lines equipped with shield wires that are installed in areas with moderate to high resistivity soils. Also, it considers the assessment of the TL lightning performance related to negative cloud-to-ground flashes, since this event represents more than 90 % of cloud-to-ground flashes and their median peak current is typically two to three times higher than that of subsequent strokes.

Traditionally, the calculation procedures determines backflashover outage rate (BFOR) per 100 km of the TL per year, according to equation (1), where SF is the span factor that accounts for the effect of those lighting strikes along the span, NL is the expected number of lightning strikes to the line (per 100 km per year), and BF% is the probability of backflashover occurrence that consists of the percentage of currents that exceeds the critical current in a cumulative peak current distribution – (%I > I_C).

BFOR = SF × NL × (%BF) (1)

Critical current, I_C, defined as the minimum value of lightning current able to lead TLs insulators to flashover, has a fundamental role in the lightning performance of transmission lines. It is obtained from the calculation of the overvoltage developed across line insulators due to the simulation of lightning striking tower top and the application of a flashover model to assess the occurrence of backflashover on such insulators.

In this context, lightning return stroke current is the fundamental element that determines insulation failures on electrical systems and may lead to equipment damage and line flashovers. The influence of representations of lightning current, mainly the current waveform and associated parameters such as peak current and current front time is a scientific issue under continuous evolution.

Since lightning current is a random parameter, lightning protection studies adopt cumulative peak current distributions to estimate the probability of a given current value to exceed the critical current of the system.

Several cumulative peak current (PC) distributions are presented in literature to be applied in studies of this nature with distinct characteristics and obtained from different measurement methods. In this work, PC distributions are classified as (i) pure distributions, with data obtained only from direct current measurements on instrumented towers, which resemble grounded transmission line towers, and (ii) enlarged distributions, that mixes data obtained from instrumented towers with those provided by indirect measurements using magnetic links, such as those from IEEE and CIGRE standards. Fig. 1 illustrates traditional cumulative peak current distributions presented in the literature.

Also, the representation of current front time is an important aspect of influence on the calculated overvoltage across TL insulators and, consequently, on the corresponding critical current, affecting the resulting lightning performance of the transmission line. Most of Td30 front time of first return stroke currents varies in the 1.5-to-10 μs range, as indicated in Table I. Considering the current dataset from Mount San Salvatore (MSS) and Morro do Cachimbo (MCS) stations, it is expected that just about 5% of negative first stroke currents may have current front time larger than 10 µs. The dispersion of current front time influences the resulting overvoltage across TL insulators due to direct strikes to the line, affecting the resulting lightning performance.

Plan to attend the upcoming 2025 INMR WORLD CONGRESS, where international expert, Fernando Henrique Silveira from Brazil will deliver a paper and presentation that addresses the constraints on applying cumulative peak current distributions of negative first strokes and different representations of current front time on assessment of lightning performance of transmission lines. He will also discuss the main characteristics and differences of traditional cumulative peak current distributions with focus on their application to evaluating lightning performance of electrical systems. Impact of representing current front time by its median value and as correlated parameter with lightning peak current will be assessed as well.