When Should Electric Utilities Go Underground

Weighing the expense of burying power lines against the risks of aboveground weather-related damage

Power lines are safer from weather and physical damage underground, but when does it make sense to spend the time and money to bury them?

Many utilities across the U.S. are asking this question as above-ground power lines are increasingly threatened by extreme weather events, wildfires, and physical damage. Utilities often determine how to prioritize undergrounding efforts before they spend time and money on extensive - and expensive - projects. With enough data and climate-informed grid modeling down to individual towers, poles, and components, utilities can locate the most at-risk sections of their above-ground power lines and decide which ones warrant the expense and time of undergrounding projects.

Above ground or underground?

In the rush to electrify the world, millions of miles of power lines were strung above ground. In many existing cities there was no other choice - pre-existing buildings necessitated above-ground power lines. Above-ground lines are also relatively inexpensive and fast to install. Consequently, most of the U.S. power grid - amounting to 5.5 million miles of distribution lines and over 180 million power poles - is above ground.

In colder climates ice can accumulate on power lines, causing them to snap under load or from "galloping" caused by high winds. This can leave people without power and possibly heat in potentially dangerous freezing conditions. In dry climates or mountainous regions, wildfires pose a significant risk to overhead power lines, and fallen lines can ignite wildfires, causing significant environmental and property damage.

Many U.S. suburbs built after 1950 have underground power lines, which are nearly immune to ice and extreme weather events and are more resistant to fires. Underground power lines are also protected from falling tree limbs, automotive accidents, and many other physical incidents.

However, underground power lines are expensive to install, especially in developed cities and areas where digging is a challenge due to poor soil condition, bedrock, frequent flooding, etc. According to the California Public Utilities Commission, the costs for undergrounding existing overhead distribution infrastructure in the state can range anywhere from $350 per foot to $1,150 per foot, or $1.85 million to $6.072 million per mile. In 2012 the Edison Electric Institute conducted a nationwide survey of undergrounding projects. Its report, "Out of Sight, Out of Mind," states that conversion costs ranged from around $150,000 to $3.5 million per mile between 2009 and 2012. Conversion costs have only risen since, and the cost to underground significant portions of power grids could now cost billions.

Additionally, underground lines can be susceptible to moisture damage from flooding and can be more difficult to repair. Given the costs and other difficulties of undergrounding, utility providers typically only convert overhead power lines to underground lines when the costs of power outages or other damages over time outweigh the cost of undergrounding projects.

Risks versus benefits of undergrounding

Risk models support informed decision-making processes, such as how to allocate limited resources to undertake mitigation activities. Risk models rely on an assortment of detailed data used to characterize asset conditions, operation, and maintenance; threats to the assets; and the potential outcomes and consequences of asset failure. By integrating these factors into an analytical framework, the current risks to the assets can be systematically evaluated. However, to be effective in today's world, the data must also account for changing climate factors and extreme weather threats, often on the micro-climate scale. In addition to natural forces (e.g., wind, snow, lightning), utilities can also benefit from considering contact from objects (e.g., vegetation, animals, vehicles), and equipment failures, as additional risk inputs.

Risk models can help utilities decide when undergrounding is warranted for parts of their infrastructure (see Figure 1). For instance, the risks and costs of potential outages due to freezing rain or wildfires over the next 15 years may be great enough to justify undergrounding the most vulnerable sections of power infrastructure as soon as possible.

Wind & Climate Impacts on Sub-Transmission Structures - An Example

Fig. 1: Detailed risk models account for weather patterns, climate change-informed data, and more to deliver the information utilities need to determine whether burying power lines is worth the expense.

Results from these risk models can provide insight into the development of mitigation activities to address the current risks. The results highlight which threats, system vulnerabilities, and outcomes are the greatest contributors. Next, "what-if" scenario analyses can estimate the residual risks following mitigation efforts and repairs. The effectiveness of an activity can then be measured by the amount of risk reduction it achieved, and, similarly, the cost efficiency of a mitigation/repair activity can be measured as the risk reduction achieved per dollar spent. Together, these measures can be used to identify effective and efficient combinations of activities to reduce risk in the utility grid. The risks associated with certain sections of power infrastructure may justify undergrounding, whereas others may justify above-ground modifications or repairs.

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