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Overview
America’s reliance on electricity has continued to grow. Disruptions in electric service from weather events or natural disasters have tended to repeatedly prompt the seemingly logical next question “does it make sense to underground (UG) power lines to help minimize outages?” Additionally, there has been a continuing trend for the removal of poles and overhead (OH) power lines to improve the aesthetics of a neighborhood or area. While undergrounding power lines may seem on the surface to be a good way to go, the reality is that making the decision to put power lines underground is more complicated, and considerably more expensive. Over the years, electric cooperatives and their regulating agencies or bodies have studied the undergrounding of power lines. The large majority of these studies conclude that the cost of undergrounding is far more expensive than OH power systems. New construction of UG facilities or conversion of existing OH power system facilities are both high cost alternatives for undergrounding. These costs can also vary from location to location, but are considerably higher for UG than OH in all instances. Besides the cost and aesthetics, factors regarding reliability need to be considered. Overall, the question becomes “would the benefits achieved outweigh the costs incurred?”

The Edison Electric Institute (EEI) is the association of shareholder-owned electric companies that represent approximately 70 percent of the U.S. electric power industry. They conducted a 2012 poll of electric customers to determine how willing they might be to pay for undergrounding. The results of their poll showed the following:

The results indicated that 60 percent of electric customers were willing to pay at least 1−10 percent more on their power bills for undergrounding and another 11 percent of customers were willing to pay up to 20 percent more. However, fewer than 10 percent of the customers polled were willing to incur a bill increase of 100 percent to pay the more realistic cost for undergrounding. This information confirms the experience of most utilities and state commissions that the cost of undergrounding is a very important consideration and that customers have limited tolerance for higher costs for utility services to pay for undergrounding. (Hall, 2013, p. v)

The cost of undergrounding continues today to remain a challenge for electric cooperatives and their customers who want lines put underground. If UG costs were the same as OH costs, the decision would be an easy one. Despite the higher cost of UG, electric cooperatives do find value in building UG facilities in some instances. For instance, new housing developments in the US are more and more being constructed with UG distribution power lines. But, the construction of new UG transmission lines has been more varied and more rare as UG transmission lines are much more expensive.

Hall (2013) also cites U.S. Energy Information Administration (EIA) data as showing that for all different types of storms or “disturbances”, hurricanes/tropical storms, summer storms, and winter storms (ice/snow) make up “more than 97.8 percent of all the events recorded” (p. 14). All events included earthquake, flooding, heat storm, hurricane/tropical storm, summer storm (lightning/high winds), wildfire, and winter storms (ice/snow). The EIA deducted that storms that produce strong winds are the “major cause of warm weather and grid failures” (p. 15). The data further showed that “hurricanes/tropical storms and summer storms cause 80 percent of all major outages and that snow and ice accumulation are the major cause of system outages during the winter” (pp. 15-16). Given this, one might conclude that UG systems would be less susceptible to damage. But, in reality, most underground systems are generally fed electricity by overhead facilities. So, an event that causes overhead line power disruption will inevitably cause disruption to the underground system as well.

The EEI also studied major storm data for a period of nine years to determine trends and impacts these events had on the electric industry. The data was somewhat inconclusive in that storm patterns were increasing, but average outage time per customer declined in some instances. This may have been due to improvements in restoration response time due to increased use of mutual assistance from other electric utility companies. Additionally, UG facilities seemed to have had a slightly better performance than OH facilities in some instances, while a much better performance in others. UG facilities were particularly susceptible to poor performance in areas where flooding occurs.

In order to get an understanding of how to determine the decision to go underground, one needs to understand the benefits and challenges associated with these decisions. The following lists of benefits and challenges is provided directly from the 2012 EEI poll responses (Hall, 2013), and is a comprehensive listing of all aspects surrounding these. Each of these listed may or may not apply to one specific area or company, but instead represent the poll feedback from their association members across the US. Benefits include improved reliability in some instances, aesthetics, and others as listed. Challenges include costs, operation and maintenance, failure issues, and others as listed.

Benefits of Undergrounding

Reliability

Benefits such as robustness to most weather events and less exposure to wildlife
Increased reliability during high winds and storms
Reduced exposure to lightning
Reduced exposure to outages caused by trees
Better voltage support
Decreased tree trimming costs
Newer UG cable systems, in general, tend to be more reliable and require less maintenance than OH installations
In very dense urban areas, overhead construction becomes impractical, so the utility benefits by having the option of installing underground network systems in these areas where overhead can't be installed

Aesthetics

Customers prefer underground construction
Easier to obtain an easement for underground lines
Helps with public image
The primary benefit to an electric utility for an underground system is customer satisfaction
One of the major benefits is to help create positive community relations by mitigating visual impact

Other

Transmission - less public EMF concerns
Transmission - fewer maintenance repairs
Reduced congestion in high density areas
Ability to maintain facilities at ground level, rather than from poles and bucket trucks
Better public safety
Lower feeder energy losses
The cost of tree maintenance is removed entirely during the life of underground facilities
Reduced route congestion near substations
Increased customer acceptance for new projects
Less resistance from towns for project approvals
Significant reduction in right-of-way (R/W) maintenance costs and vehicular caused outages

Challenges of Undergrounding

Costs

Underground systems are normally more expensive to install than overhead systems
Higher facility replacement costs
Increased project costs associated with UG systems
Increased material costs and longer installation timeframes vs. overhead
Design redundancy/significantly higher capital costs for installation
Higher operations and maintenance (O&M) cost offsets corresponding reduction in R/W maintenance costs
Geographic areas with severe frost and rocky conditions can increase costs significantly
Underground cable mitigation tends to be very expensive compared to other types of equipment repairs/replacements. This is due to the labor intensive nature of locating faults and repairing cable, the need for specialty contractors for replacement or mitigation work, and the need for additional crew resources to restore customers’ power when a failure occurs.

Operation and Maintenance

Older cables are more likely to fail and older tile or fiber duct systems are more likely to collapse when failed cable is pulled
Repair times for UG construction are substantially higher than for OH construction, driving up maintenance costs and duration-based reliability indices
Underground facilities experience many dig-ins by those who do not follow proper procedures to identify the location of underground facilities before excavating
More complex operational needs, such as visual inspection, is impossible, making it more difficult and costly to maintain and repair
Difficult repair due to frozen ground
Installation of underground services requires much more coordination between the utility and customer than similar overhead service installations
Although UG construction eliminates some outage causes, UG systems are still vulnerable to lightning and equipment failure
Difficulty locating space for padmounted gear
Increased stray voltage concerns
Specialized training/equipment for manhole/vault access
Surface-mounted equipment inspections critical to protect public
Difficult access for outage restoration in heavy snow areas
Underground facilities are susceptible to flooding

Failure Issues

Much of the cable installed in the 1970s and 1980s is reaching the end of its useful life, creating a peak in the need for infrastructure investment
Customer satisfaction is at risk due to the connected nature of UG feeds. Multiple failures in a segment on a single tap interrupt power to the same set of customers. Customers often become frustrated since it is not visually apparent as to the cause/location and because failures often occur under warm, dry conditions.
Power outages last longer because damage is more difficult to locate and takes longer to repair
Outages involving the underground system take more time to resolve as faulted cable/equipment takes more time to locate and subsequently replace
Customer perception that undergrounding their service or neighborhood should dramatically improve their reliability, not taking into account exposure of overhead portions of the system upstream

Other

Submersible transformers, in particular, have created a significant safety hazard for crews attempting to locate and repair failed equipment
Conflicts with other subsurface construction and utilities
More specialized skillset and equipment required for installation and repairs (pp. 25-27)

Costs of Undergrounding

The EEI also collected data in it’s 2012 survey on cost per mile of UG vs. OH construction. The following tables represent their findings.

Table Legend:

Urban: 150+ customers per square mile

Suburban: 51 to 149 customers per square mile

Rural: 50 or fewer customers per square mile

Table 6.1 Cost per Mile: New Construction Transmission

	Overhead			Underground
	Urban	Suburban	Rural	Urban	Suburban	Rural
Minimum	$377,000	$232,000	$174,000	$3,500,000	$2,300,000	$1,400,000
Maximum	$11,000,000	$4,500,000	$6,500,000	$30,000,000	$30,000,000	$27,000,000

For rural electric cooperatives, the survey data suggested that new construction transmission costs at the Minimum level could range 8 times the amount of overhead costs to construct underground facilities while at a Maximum level could range 4 times the amount.

Table 6.2 Cost per Mile: New Construction Distribution

	Overhead			Underground
	Urban	Suburban	Rural	Urban	Suburban	Rural
Minimum	$126,900	$110,800	$86,700	$1,141,300	$528,000	$297,200
Maximum	$1,000,000	$908,000	$903,000	$4,500,000	$2,300,000	$1,840,000

For rural electric cooperatives, the survey data suggested that new construction distribution costs at the Minimum level could range 3.5 times the amount of overhead costs to construct underground facilities while at a Maximum level could range 2 times the amount.

Table 6.3 Cost per Mile: Converting Overhead to Underground Transmission

	Urban	Suburban	Rural
Minimum	$536,760	$1,100,000	$1,100,000
Maximum	$12,000,000	$11,000,000	$6,000,000

For rural electric cooperatives, the survey data suggested that the cost of converting overhead to underground transmission facilities could range from a Minimum of $1,100,000 per mile to a Maximum of $6,000,000 per mile.

Table 6.4 Cost per Mile: Converting Overhead to Underground Distribution

	Urban	Suburban	Rural
Minimum	$1,000,000	$313,600	$158,100
Maximum	$5,000,000	$2,420,000	$1,960,000

For rural electric cooperatives, the survey data suggested that the cost of converting overhead to underground distribution facilities could range from a Minimum of $158,100 per mile to a Maximum of $1,960,000 per mile. (Hall, pp. 30-31)

Utility Cooperative Forum: Do the Benefits of Underground Power Lines Outweigh the Costs?