Leakage current in wooden structures used for power distribution

In Australia and many other parts of the world, electricity is distributed to industries and households via overhead distribution lines, and most lines are mechanically supported by wooden structures consisting of poles and crossarms. Seventy per cent of the 8.5 million wooden structures in service as part of electricity distribution infrastructure in Australia are over 35 years old. Leakage current flow in these ageing wooden structures imposes continuous stress and has caused numerous pole fires, not only in Australia but in many other countries.

In this thesis, the electrical characteristics of aged wooden structures are examined and used to further our understanding of leakage current performance; this is the first study of this phenomenon to have been conducted worldwide. The study proved conclusively that leakage current performance of wooden structures deteriorates with age. Next, scanning electron microscopy was conducted and allowed inferences about structural differences in ageing wood specimens. It was found that structural changes occur during the service lives of wooden structures and play an important role in predicting the leakage current behaviour. The study’s outcomes will assist the assessment of the electrical performance of wooden structures in greater depth, with far greater understanding of the role of ageing in wooden structures, and in developing cost effective asset maintenance and replacement programs which will help in reducing wooden pole fires. Investigation to study the influence of coastal salt deposition on the surface of ageing wooden structures on leakage current was conducted. It was concluded that in coastal areas seawater distributed by wind and other means allows salt to enter the structure of wood through cracks and fill internal pores. In addition, the accumulation of salts on the surface of wood, particularly when it is wet, significantly reduces electrical resistance and deteriorates the leakage current performance of the wood.

The influence of CCA treatment on the leakage current performance of spotted gum poles was also investigated in detail. It is concluded that CCA-treated spotted gum poles have reduced electrical resistance so are vulnerable to smouldering leading to pole fires. Although CCA treatment of spotted gum poles against fungal and termite attack offers several advantages and is a practice widely used by power utilities, the compromise made with reduced electrical resistance of CCA-treated wood can often lead to excessive leakage current thereby increasing the chances of pole fires. This research points to the need to make effective and balanced decisions on CCA treatment of poles taking into consideration the site weather conditions and location, hence minimising life-cycle costs of inspection and refurbishment of CCA treated wood poles. Finally, a novel mathematical model based on dimensional analysis was developed to establish relationships between key physical variables and leakage current in wooden structures. Validation of the complete mathematical model was undertaken by comparing the experimental results and model results and they were found to be in good agreement.