Though overhead transmission lines are quite reliable historically, power outages still sometimes result from a variety of external issues including inclement weather, vehicles striking poles, trees falling on conductors, and wildfires. Heavy winds may damage a variety of above-grade structures and overhead transmission poles are no exception. Other causes of damage to an overhead transmission line can include equipment failure or human interaction.
Overhead transmission lines generally sustain extreme weather events moderately well, though some utilities have concluded it is prudent to begin reinforcing or hardening their facilities rather than continue battling these events year after year. Overhead lines can be designed for extreme loadings or storm conditions, though this is an expensive endeavor and still may not fully stormproof the transmission line.
Other hardening strategies may involve relocating certain transmission sections or even entire circuits underground.
Underground transmission systems are generally less susceptible to storm-related outages, though it should be noted they, too, are not completely stormproof. Underground transmission lines still have above-grade termination structures for cables at substations, and these are the most likely source of storm-related outages. Other causes of damage on an underground line can include cable, splice or termination failure, or damage to the cable caused by a third-party excavation.
When an unplanned outage occurs on an underground transmission line, it must be acknowledged that it can be more difficult to locate the source of the problem and thus require more time to complete repairs, compared with events affecting overhead lines. However, these incidents typically occur less frequently and should be a factor when comparing long-term O&M life cycle costs. Fewer unplanned outages mean more uptime on the network, lower repair costs and more revenue. Other factors that should be weighed include reduced economic and social impacts to residential and commercial customers and avoidance of public dissatisfaction over service disruptions.