Overhead or Underground Power lines

There are thousands still without power in Kentucky and there are some who may remain that way for a week or more yet. This has been the largest outage in State history, slightly worse than that imposed by Hurricane Ike in numbers, more severe in terms of problems. And, on a smaller scale, a vehicle crashing into a power line knocked out power in Oakland this morning. So I thought it timely to make a suggestion that was beaten down after Hurricane Katrina, but is still worth reconsidering. Especially if there are any Federal dollars floating around for construction/reconstruction.

The suggestion, though expensive, is a simple one – bury the power lines, especially those in the urban area. That way they are out of the way of cars, falling trees, and not subject to the weight of accumulated ice. They are still there, undisturbed, after typical hurricanes or tornadoes, and even if disrupted in an earthquake they are still out of the way, and not likely to leave a trail of burnt ground, as I saw in the Mississipi delta after Katrina. (which, had there been any houses still standing would have left them alight).


I am not, at the moment discussing the major power lines that one sees dangling from pylons as the power grid brings the high voltage lines into the local sub-stations and transforms it down for local use. It is, initially those smaller cables, that run alongside roads, through the trees, and over the bushes, to the pole-mounted transformers near your house (and from there in) that seem to make the most, initial, sense to change. Very often the choice is forced by limited availability of land for overhead cables. But once installed they can easily outlast overhead lines, with lower operating and maintenance costs (outside of the advantages in extreme weather).

It is not, admittedly, something that is always without significant cost, but on the other hand there are neighborhoods that require that it be done. After the Hurricanes of 2005, Florida Power and Light became committed to increasing their undergound faciities. With, at that time, some 37% of its system underground, FLP were looking to move another 25% underground and to encourage developers to follow suit. There are land acquisition cost reductions that offset some of the underground supplement. In Connecticut putting the cables underground eliminated the need to buy 100 acres of land and condemn 29 homes and businesses.

It is a process that is being adopted in countries like Mexico where, in the more expensive neighborhoods, burying the cables makes the environment more attractive. There have been over 3 million km of cables buried at medium voltage alone.

It can be an expensive undertaking, one utility calculated that for a 5-mile 230 kV line it would cost around $40 million for burial, as opposed to $5 million on the surface. A significant part of that comes from the much more expensive cable that must be used. Utility companies strongly prefer installing lines overhead and only a small percentage of lines is located underground, falling in percentage as voltage rises. The percentage also varies by country, with those that are more urban burying cables more frequently.

2007 Percentage of total ac circuit length underground in the 220 – 314 kV range.

The three factors that influence the greater cost are the need for better insulation, the need to dissipate heat that build up in the cable and surroundings from the passage of current, and the installation cost to install the cables. Part of the heat problem comes because the insulation required to isolate the power also acts to contain the heat build-up. Some of this can be overcome by using copper as the cable material. Overhead lines are often of aluminum since this is much lighter, the use of buried copper, particularly in larger diameters, would lower the resistance to the current, and therefore the heat generated. But the cable would be heavier. Not that the idea is new, the first cables were placed underground in Berlin, Germany in 1880.

There is a slow move towards the practice, and FPL is now offering incentives to local utilities.

Under FPL’s previous operating tariff, the company required local governments requesting a conversion project to pay the full cost of moving overhead power lines underground. FPL’s revised tariff now credits the sponsoring local government 25 percent of the total expense. 

An analysis by FPL indicates the 25 percent undergrounding credit represents the average savings in storm restoration costs the utility’s entire customer base would share following a major weather event.

It is not, however, a step that all utilities would like to embrace.

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