Strategic Underground: Resources and How to Best Achieve Expected Reliability Improvements

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According to a Associated Press analysis government data, the number of power outages in the United States caused by severe weather has doubled in the past 20 years as a warming climate brings more destructive storms that cripple large segments of the country’s aging power grid.

The need to improve grid reliability and resiliency has led many of the nation’s largest utilities, such as FPL, P&E, Dominion Energy and WEC Energy Group to initiate the burying of overhead lines at risk.

Similarly, many municipalities, power cooperatives and utilities have developed strategic burying programs, while others are studying the pros and cons of burying at-risk overhead lines.

Whether these projects are aimed at grid reliability, resiliency, security, or residential aesthetics, more and more utilities are choosing to strategically install power lines underground.

Why underground?

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

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

Each utility has unique needs and constraints to consider when deciding to implement strategic underground segments of its distribution system. More and more people are concluding that the benefits outweigh the costs associated with these projects, especially in urban areas prone to extreme weather conditions.

Resources and elements to evaluate

A variety of energy industry groups and government entities provide valuable resources to help assess important aspects of burial.

Non-profit PDi2 is one such industry group. With a mission to increase awareness of options for underground electrical systems, they seek to help utilities make data-informed infrastructure decisions that produce the best long-term outcomes for energy consumers.

The Department of Energy’s Office of Electricity provides further valuable landfill information through its recent online seminar, Resilient power grids: strategically burying power linesand accompanying Documentation.

A thorough examination of the costs and benefits associated with burying power lines is essential for an informed burying decision. The profitability of strategic landfilling is driven by several main factors, including:

  • age of existing aerial infrastructure
  • whether economies of scale can be achieved
  • the vulnerability of areas to increasingly violent and frequent storms
  • the number of customers per line mile

And the main cost/benefit elements that need to be estimated are:

  • life cycle costs
  • benefits from less frequent breakdowns
  • aesthetic costs avoided
  • catering expenses

Underground Reliability Considerations

Although the overall frequency of outages on underground distribution networks is significantly lower than on overhead lines, it is important to be aware of two reliability issues specific to burial.

An underground reliability problem is represented by a bathtub curve. The bathtub curve shown below refers to a relatively higher failure rate for newly underground systems and older underground lines, with a consistently low failure rate throughout the mid-life.

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

With the frequency of outages being about one-ninth that of overhead lines, reliability expectations are understandably high for new landfill projects. But a doubling of outage times can be a nasty surprise for customers when a fault occurs.

The good news is that there is a quick payback technology solution to help utilities dramatically improve the reliability impacts associated with tub curvature, subsurface fault location, and outage times.

Technology that increases underground reliability

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

To ensure visibility of underground distribution arteries, line sensors like Sentient Energy’s UM3+can be installed in electrical cabinets or safes. Downstream of the outgoing switches, in single-phase underground residential distribution (URD), line sensors such as the Sensible energy UM1can 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 shown below, outage detection and location information allows for faster response and less patrol time, helping to reduce outage duration.

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

Small incremental cost with big ROI

Remote monitoring adds only 1-2% to the total cost of the landfill project. Savings associated with the resulting reduction in interrupted customer minutes and savings from load data, reduced operating and maintenance costs, and crew efficiency more than offset the cost of smart line sensors.

Get all the benefits of landfill reliability

There is no doubt that utilities can achieve significant improvements in reliability and resiliency through strategic underground initiatives. But without proactive monitoring of fault detection and location and load data from smart line sensors, utilities can be challenged to achieve the full reliability benefits of underground distribution systems.

Learn more about Sentient Energy’s overhead and underground facilities line sensorsClick here.

For more information on landfill best practices, see PDi2 Utility Resilience Handbook.

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