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COMMENTARY ARCHVES
   

JANUARY COMMENTARY
by Share Brandt

Spotter’s Guide to
Transmission Poles and Lines

Transmission structures and towers are like interstate highways for electricity, carrying mass volumes of high-voltage current over large distances. These structures stand 55 ft. tall or more and connect power plants, including utility scale renewable projects, to a series of substations and tie one bulk power region of the grid to another. The towering behemoths, surrounded by cleared land (called right-of-way), seem simple and unadorned.

But a closer look reveals interesting details. With a little practice, you can identify devices attached to towers and even guess voltage levels.

Volts Vary

First step? Determine what you’re looking at. Higher voltages on power lines require more space between each other and other objects, allowing people, vehicles, and other equipment to move freely underneath. For this reason, transmission towers usually stand 55 ft. to 150 ft. high. Most are made from steel, but some are concrete, wood, or even ductile iron. Wooden distribution poles, found in neighborhoods (unless your lines run underground), are generally about 40 ft. tall.

Transmission voltages usually run between 23,000 volts and 765,000 volts. Compare that to the voltage of your home’s electrical sockets: 120 volts for most outlets, 240 volts for a clothes dryer or stove range. Voltages in your home are enough to kill you, let alone what power lines carry.

Transmission facilities carrying power for long distances operate between 115,000 volts and 765,000 volts. Substations reduce power to 23,000 volts to 69,000 volts for delivery on smaller transmission lines to your electric co-op’s distribution system. There, local substations step power down further, normally to 12,500 volts, so it can be safely carried by distribution lines to communities. A transformer outside a home or business lowers the voltage to 120 volts or 240 volts.

Estimating volts

The first rule of thumb for estimating volts: the higher the transmission tower, the greater the voltage. Transmission tower conductor, the energized lines made of steele-reinforced aluminum cable, can’t touch the towers that support them—otherwise, the current flows to the ground. They’re separated from towers by bell-shaped insulators (known as “bells”).

The rule of separation works here too—higher voltages require more separation between conductors and towers—hence more bells. This table shows the general correlation between bells and voltage.

Line Voltage

Number of bells

69,000

4

115,000

7

138,000

8

161,000

10

230,000

12

345,000

18

500,000

22

More uses for Transmission Towers

Towering transmission structures often double as weather data collectors. You may notice spinning cups of an anemometer measuring wind speed, or other meteorological equipment.

Early tower designers discovered large birds like to build nests on the girders. Birds can cause an outage if excreted waste, which is very conductive, lands on an insulator and triggers a short circuit. Rather than have birds nest in potentially dangerous spots, designers include platforms for nests.

The Path of Power

This “spotter’s guide” helps you understand what you’re looking at and provides a better understanding of the electric co-op network. But remember:

  • Do not climb utility structures. Stay a safe distance from all described equipment.
  • These measurements and descriptions represent common configurations; in the real world, design varies.

Source: Maurice Martin, Cooperative Research Network.

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z© Wisconsin Electric Cooperative Association