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NEWSLETTER

Wooden Pole Fire

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Electrical utilities worldwide use wood poles for distribution systems and lower KV transmission applications claiming that wood is a cost-effective material in terms of both initial line costs and overall life cycle costs. A study by the utility industry concluded: "treated wood offers the most energy-efficient, functional, cost-effective and practical material for use by electric utilities in providing electrical service to the public." (WWPI Wood Pole Newsletter). Wood poles must however be properly inspected and treated because they age, deteriorate and get ignited. Although wooden pole fires are attributed to a number of concurrent conditions, evidently, a combination of insulators' contamination and moist are the prerequisite to the ignition that starts with electrical discharges. Corona cameras, fortunately, can see partial electrical discharges that are the beginning of the determination, and assist utilities identify problems and prevent the imminent hazardous outcome.

Wood poles are in fact tree stems which have the proper natural characteristics to meet the engineering and design standards to support a utility line; and have been harvested, shaped, treated and certified to meet the needs. The strength and resilience of wood, along with protective preservative treatment, enable wood poles to withstand considerable abuse when being stored, hauled, handled and installed. Wood poles are considered safer by linemen due to the materials' non-conductive nature and they require no special climbing gear when bucket trucks are not available. Maintenance is simple and can be performed by trained personnel with the poles in place. Installation is easy, as are modifications to meet field conditions or later changes.

While wood poles are subject to stringent standards for treatment, inspection and replacement they still become old, their anti-rot treatment dissipates and they deteriorate. If they are not properly inspected and replaced within the proper guidelines, they will become a hazard. Visual inspection can reveal split tops, ragged tops, woodpecker holes, abnormal cracks, plants growing, green algae, signs of rotten wood, broken guy wires, broken insulators. But, wood pole fire is triggered by processes that are not seen with bare eyes namely electrical discharges due to leakage currents.

Wooden pole fires cannot be attributed to any single factor but to a number of concurrent conditions with insulators' contamination as a prerequisite followed by atmospheric conditions of wind, fog or heavy mist. When contaminated insulators get wet their surface become conductive and small amounts of leakage currents flow across the insulators attempting to reach ground. The leakage currents can generate enough localized heat to activate the wooden pole preservative treatment and start chemical reactions that lead to pole top fire, especially when pole cross arms and insulators are near or at the end of their service life.

Reducing the leakage current on insulators is what utilities aim at when they try to overcome top pole fire occurrences. Live line washing is one solution that could be an option for lines that are located in mild contaminated areas, though the main drawback of a washing approach is establishing the correct timing of the operation washing is not effective in environments subjected to "instantaneous" polluting conductive mist conditions, e.g. marine salt fogs. In heavily polluted areas utilities use silicone rubber insulators with high hydrophobic characteristics, or insulators with extended shed diameters. Either way, the purpose is to reduce the surface electric stress, reduce the leakage current, and increase the electric strength of the insulators.

Utilities explore and find solutions to avoid pole fire but theses do not replace nor eliminate the need for ongoing inspection and maintenance. When safety is at stake, there is an ongoing need to check the grounds, add insulation, add grounding lead and ground mounting bolts, minimize leakage currents and ensure that no corona or arcing are discharged. Corona cameras are valuable because they pinpoint and reveal the exact location where the processes begin. Moreover, corona is detected in the early stage of deterioration when processes are still reversible, and is therefore a great cost effective signaling method that saves unplanned outages, allows preemptive corrective actions, and assists creating a safe working environment

 

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