Should Overhead Lines be Underground?

Should Overhead Lines be Underground?

After every severe storm, there is a call for utilities to put their distribution lines underground.
Hurricane Sandy, of course, is the poster child for why people are asking for distribution lines to be underground.
It’s interesting to note that most distribution lines in Florida aren’t underground, even though Florida gets pummeled by more hurricanes than any other state.

There are approximately 6,000,000 miles of distribution lines¹ in the United States, which should give pause to any call for putting all of them underground.

Table 1 shows the costs involved.

Costs can go over $1,000,000 per mile in both rural and urban areas, such as when there are rock ledges, or when there are high-water tables. Multiplying 6 million miles by $400,000 per mile means the cost of putting all distribution lines underground would be, at a minimum, $2.4 Trillion. This equals 16% of the U.S. GDP.

It should be noted that putting distribution lines underground doesn’t eliminate the threat of outages, though outages would be reduced. Outages would still occur due to flooding, dig-ins and cable failures. This is especially true along the coastline where flooding is caused by storm surge.

After WWII, many new subdivisions put the distribution lines underground. These were in rural areas: mostly farm acreage with few obstructions, where the cost of installing underground cables was very low, well below the average cited in Table1.

But these communities were still subject to outages when the distribution lines feeding the underground sections failed.

Putting sections of the distribution grid underground doesn’t completely solve the problem of outages, though it can help.

Cable life is another uncertainty. The history of overhead lines is that they last for 80 years, while the history for underground cross-linked polyethylene (XLPE) ³ cables indicates a life of 35 years, though, because of their relative newness, this history is incomplete.

The issue of whether to put power lines underground has been studied on numerous occasions, such as by Houston Texas, Florida, Oklahoma, Virginia and North Carolina.

In general the conclusion has been, “Complete undergrounding of all electrical facilities is not the solution to the outage problems caused by storms.”

The impact of failures should be considered when evaluating the issue. Hospitals and other emergency installations need to have reliable backup power no matter what decision is reached … no system will be perfect. Perhaps, gas stations should also have back-up generators.

While much of the above was covered in an earlier article, it’s worth repeating that there is no perfect solution to how to protect the grid from storms.

In established urban communities, the cost of moving overhead lines underground is very high, and the cost should be compared with the potential for having fewer outages.

Notes:

1. An Updated Study on the Undergrounding of Transmission Lines, Edison Electric Institute, 2009.
2. Ibid
3. Cross-linked polyethylene (XLPE) cable is a more recent development, with many utilities still using high-pressure, fluid-filled pipe (HPFF). The advantages and disadvantages are described in a paper Underground Electric Transmission Lines, published by the Public Service Commission of Wisconsin.

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