According to an Associated Press analysis of government data, the number of U.S. power outages caused by severe weather has doubled in the last 20 years "as a warming climate stirs more destructive storms that cripple broad segments of the nation’s aging electrical grid."
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 Advanced Research Projects Agency - Energy recently reported that 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.
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.
Resources and What to Evaluate
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.
One underground reliability issue is represented by a bathtub curve. The bathtub curve shown below refers to a relatively higher fault rate for newly undergrounded systems and older underground lines, with a consistently low fault rate throughout mid-life.
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 the total undergrounding project cost. Savings associated with the resulting reduction in customer minutes interrupted and savings from load data, reduced O&M costs, and crew efficiency more than offset the cost of the intelligent line sensors.
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.
To learn more about Sentient Energy’s overhead and underground line sensors, click here.
For information on undergrounding best practices see PDi2’s Utility Resiliency Playbook.
