Extreme weather incident experienced on 765 kV line in South Africa

Rehabilitation programmes were initiated to strengthen the guy anchors, whereby new anchor foundations are installed and repositioned a short distance from the original foundations, and the existing guy ropes are reused. At least two recent incidents have been investigated where corrosion of guy anchors did not contribute to the failures, as the anchor foundations in both cases had already been replaced. This paper reports on one such case study, where a more detailed investigation was conducted to determine the cause of failure. The investigation focused initially on the design parameters of the line and compared these with the theoretical strength of the guy ropes. Full-scale testing was then conducted to determine the actual strength of the ropes and the magnitude of load required for the guy ropes to pull out of their fittings. A line analysis study, using proprietary software, was subsequently performed to estimate the order of magnitude of wind speed and pressure required to produce the forces identified during full-scale testing, since exact weather data at the time of the storm was unavailable. These results were compared with the original design parameters to identify any anomalies.

In parallel, detailed metallurgical investigations were conducted on the guy rope material, the

U-bolt material, as well as the U-bolt casting (crosshead), to confirm compliance with design and material specifications. Results from the software analysis indicated that wind pressures exceeding the design parameters were responsible for the tower collapses, as no other abnormalities, damage, or failures of tower members or line hardware were identified that could have contributed to the incidents. The conclusion reached was that elevated wind speeds and the resulting wind pressures exceeded the original design limits of the line, providing supporting evidence of increasingly severe climatic conditions contributing to the failure.