Overhead Transmission Lines

These are an amazing part of the landscape: 163,000 miles of high-voltage lines, typically on towers rising 130-200 feet.¹ As an amateur photographer, I've spent hours editing their cables out of pictures, so it's not like I was instinctively drawn to them.  But these towers are potentially fascinating.

WHAT ARE THEY?

Transmission lines are part of the electricity distribution system in most of the world.  They connect power stations to substations, which are those big structures/complexes you see outside your neighborhood with lots of transformers.  Because transmission and distribution (T🙵D) are a network, there are several paths for electricity through the lines carried by the towers.

When electricity is generated, it has to be converted into a current suitable for transmission.  Most electricity is transmitted in AC format, meaning that the voltage varies as a sine curve at 60 cycles per second (60 hertz, or 60 Hz); because of Ohm's Law (V = R × I ²), this means that one can increase the efficiency of transmission by reducing the current and increasing the voltage.  If you live in North America, those lines connecting the grid to your circuit box are 120V-1000A (the circuits inside your house are far lower—same voltage, but amperage is 15-50 amps).  But the river of juice flowing to your city flows at 230KV-600KV, while the amperage may vary depending on engineering and load.³


The single-circuit towers tend to be wide, like the one shown in the photograph; those towers are probably around 130 feet tall.  Each tower carries three cables, corresponding to each of the phases of a circuit.⁴ In some cases, it makes more engineering sense to create a much taller tower with two-circuits (hence, six lines with one 3-phase circuit represented on each side).

WHY AREN'T THEY UNDERGROUND?

In the USA, only about 0.5-0.6% of high-voltage (>230 KV) electric transmission is underground. Indeed, underground power lines are virtually confined to applications where they're absolutely imperative, like near airports.  One reason is obviously cost; another is the difficulty of repairs. To repair a breach in a cable, you need to first know where it is.  If a storm knocks over an EHV tower, then it takes hours or days to fix.  If it's underground, then it may take weeks (XCel Energy 2014, p.1).

Nevertheless, running powerlines is an attractive concept.  Underground lines are less vulnerable to weather disruption, and design improvements could conceivably make it much easier to locate and repair breaches.

Still, underground lines cannot dissipate heat generated by resistance as fast as those underground, so more material is required (see figure).  Construction of underground powerlines requires a lot of construction.  Overhead lines of around 230KV require towers every 800-1,000 feet.  With underground placement, a trench is required continuously (including under roads) that is at least 7 feet deep.  Access vaults of concrete are needed as well. The lines last half as long underground (40 years versus up to 80 years for overhead) and cost 15 times as much.

Despite this, there is a lot of demand for more "undergrounding" of EHV/HV lines, and more utilities are doing it.  One compelling reason is that overhead lines, in addition to being more vulnerable to storms and wildfires, are unpopular and residents hate looking at them.

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NOTES

1. NAC (2009), "Electricity Transmission and Distribution", p.564


2. Ohm's Law: V is voltage; R = resistance; and I = current in amperes (i.e., coulombs per second).  See "Ohm’s Law - How Voltage, Current, and Resistance Relate," All about Circuits.  This can also be expressed as I = V/R.  The amount of power, or wattage, is just I × V, so if you could just get electrical resistance super extremely low, it would revolutionize the whole electricity business.


3. "United States Electricity Industry Primer" (PDF) Office of Electricity Delivery and Energy Reliability U.S. Department of Energy (2015), p.18.


4. H. Lee Willis, Power Distribution Planning Reference Book, Second Edition, CRC Press (2004), p.396 explains why grid designers prefer 3-phase lines terminating as close as possible to the point of use, and compares US to European grids. 

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SOURCES 🙵 ADDITIONAL READING

C. van Amerongen, "Overhead Transmission Lines," The Way Things Work: an Illustrated Encyclopedia of Technology, New York: Simon 🙵 Schuster (1967)

Dan T. Babor & Loredana Judele, "Environmental Impact of Concrete" (PDF), Bulletin of the Polytechnic Institute of Jassy, CONSTRUCTIONS. ARCHITECTURE Section LV (LIX)(4) · January 2009 

"Electricity Transmission and Distribution,"America's Energy Future: Technology and Transformation, National Academy of Sciences (2009)

"Transmission Lines," Occupational Safety 🙵 Health Administration (accessed 14 Jan 2014)

"Overhead vs. Underground: Information about burying high voltage transmission lines" (PDF), XCel Energy (2014)