Strategic Undergrounding: Maximizing Reliability Improvements

A recent Associated Press analysis of government data revealed that the number of U.S. power outages caused by severe weather has doubled in the last 20 years. “Forty states are experiencing longer outages — and the problem is most acute in regions seeing more extreme weather, U.S. Department of Energy data shows.”

The need for better grid reliability and resilience has led many of the country’s largest utilities like FPL, PG&E, Dominion Energy and WEC Energy Group to initiate the undergrounding of at-risk overhead lines.

Likewise, many municipalities, electric cooperatives and publicly owned utilities have developed strategic undergrounding programs, while more are studying the pros and cons of burying at-risk overhead lines.

Whether these projects are aimed at grid reliability, resilience, safety, or residential aesthetics, more utilities are deciding to strategically underground power lines.

Why Underground?

The case for strategic undergrounding of overhead lines for improved grid reliability and resilience is strong. Electricity disturbance and outage data submitted by utilities to the U.S. Department of Energy analyzed by the AP showed the following:

The number of outages tied to severe weather rose from about 50 annually nationwide in the early 2000s to more than 100 annually on average over the past five years.
The frequency and length of power failures are at their highest levels since reliability tracking began in 2013 — with U.S. customers on average experiencing more than eight hours of outages in 2020.
Maine, Louisiana, and California each experienced at least a 50% increase in outage duration even as residents endured mounting interruption costs over the past several years.
In California alone, power losses have affected tens of thousands of people who rely on electricity for medical needs.

An aging grid is a vulnerable grid, especially where the occurrence and severity of extreme weather is increasing.

As cited recently by The Advanced Research Projects Agency – Energy, the U.S. has 5.5 million line-miles and over 180 million power poles that can be damaged by weather or tree-related incidents. This at-risk infrastructure accounts for 62% of all power outages in the U.S.

Faults are nine times more likely to occur on overhead lines.

Resources and What to Evaluate

Every utility has unique needs and constraints to consider when deciding whether to strategically underground segments of their distribution system. More and more are concluding that the benefits outweigh the costs associated with these projects, particularly in urban areas prone to extreme weather.

A variety of power industry groups and government entities provide valuable resources to help with evaluating important aspects of undergrounding.

Non-profit PDi² is one such industry group. With a mission to increase awareness about options for underground power systems, they seek to help utilities make infrastructure decisions informed by data which produces the best long-term results for energy consumers.

The Department of Energy’s Office of Electricity provides further valuable undergrounding information with its recent webinar, Resilient Power Grids: Strategically Undergrounding Power Lines, and accompanying documentation.

A thorough review of the costs and benefits associated with undergrounding power lines is essential for an informed undergrounding decision. The cost effectiveness of strategic undergrounding is driven by several primary factors including:

Age of existing overhead infrastructure
Whether economies of scale can be achieved
The vulnerability of areas to increasingly severe and frequent storms
The number of customers per line mile

And key cost/benefit items that need to be estimated are:

Life cycle costs
Benefits from less frequent outages
Avoided aesthetic costs
Restoration costs

Underground Reliability Considerations

Although overall fault frequency on underground distribution systems is significantly lower than with overhead lines, it’s important to be aware of two reliability issues specific to undergrounding.

Like other mechanical and electronic systems, underground circuits are susceptible to reliability issues like those represented by a “bathtub curve.” The bathtub curve projects failure rates for underground systems to be higher in early and later life, and lowest in mid-life. For new undergrounding projects the concern is the relatively higher fault rate in the early failure period.

A second reliability awareness point is that while underground outages are significantly less frequent, outages that do occur are typically longer. These faults are more difficult to locate and often take more time to repair. It is not uncommon for utilities to see outages on underground distribution systems last for an average of 2.2 times longer than on overhead systems.

With fault frequency being approximately one ninth that of overhead lines, reliability expectations are understandably high for new undergrounding projects. But a doubling in outage durations can be an unwelcome surprise to customers when a fault does occur.

The good news is that a fast time-to-value technology solution exists to help utilities significantly improve reliability impacts associated with the bathtub curve, underground fault location, and outage durations.

Technology that Augments Underground Reliability

Overhead and underground intelligent line sensors provide visibility of key points along distribution feeders. When deployed at transition points, overhead line sensors help utilities quickly know whether faults are overhead or underground.

To provide visibility of underground distribution feeders, line sensors like Sentient Energy’s UM3+™ can be installed in switch cabinets or vaults. Downstream of the feeder switches, into single phase underground residential distribution (URD), line sensors like the Sentient Energy UM1™ can be installed in transformer cabinets. Both sensors wirelessly report faults, precise location data, load data, and disturbances to a central sensor management software application.

As illustrated below, fault detection and location information enable quicker response and less patrol time, helping to reduce outage durations.

Load data from underground line sensors also helps utilities reduce outage durations. With near real-time load data, operations teams can make smarter switching decisions during outages to restore more customers quickly and safely, without the risk of overloading a section with the switching operation.

Small Incremental Cost with Big Payback

Remote monitoring adds only an incremental 1-2% to total undergrounding project cost. Savings associated with the resulting reductions in customer minutes interrupted and O&M costs more than offset the cost of the intelligent line sensors. For more detail on underground line sensing costs and payback periods try our on-line calculator.

Achieving the Full Reliability Benefits of Undergrounding

There is no doubt that utilities can achieve meaningful reliability and resilience improvements through strategic underground initiatives. But without proactively monitoring for fault detection and location, and load data from intelligent line sensors, utilities may be challenged to gain the full reliability benefits of underground distribution systems.

Visit the Sentient Energy Products page to learn more about our overhead and underground line sensors.

For information on undergrounding best practices see PDi²’s Utility Resiliency Playbook.