There are many different types of transmission line structures (towers, poles) used throughout ATC's service area. Many of our existing transmission lines were built during the 1950s and 1960s, when the common structure design was self-supporting steel lattice towers. The lattice structure is very strong, relatively light and could be erected without the need for heavy equipment and major access roads. However, they take a lot of time to design and build. The base foundations require a large area of land on which to stand, which often interferes with agriculture.
Lattice-style transmission structures, although still in operation today, are generally not used in new construction.
Today, we use a variety of tower and structure designs that are considered more aesthetically acceptable and leave a smaller footprint. Learn about the wires and circuits. We use both wood and steel structures depending on the conditions of the land and surrounding areas, the height needed for the poles, the price of steel and other factors, such as:
- construction
- operation
- maintenance
- cost
- electrical capacity and voltage of the transmission line
- physical electrical clearances
The land the transmission lines will cross is considered during the planning phase along with legal restrictions (such as zoning and land-use regulations) and the appearance of the structures. The width of a right-of-way depends on the voltage of the line and the height of the structures, but can be 75-150 feet or more depending on the type of facilities. Special consideration is given to the impact the transmission line and construction will have on native vegetation and the environment.
Click on the thumbnail images to enlarge the photos.
| Double-circuit, 138-kilovolt transmission line built on wood structures. Local electric distribution lines, cable
and telephone lines are sometimes carried
on the same structures to make efficient use of space. Taller
poles are needed to do this.
|
|
| Double-circuit, 138-kilovolt line build on
galvanized steel poles. This type of design
is often used when the line will carry heavy
electric loads. Higher voltage lines require
taller poles-sometime 100 feet or more.
|
|
| 138-kilovolt single-circuit line on weathering steel. This type of maintenance-free structure will weather (rust)
over time to give the appearance of wood and is generally
used in wooded areas.
|
|
| H-frame wood structure. This type of design allows
for shorter spans (the distance between structures.) In
general the height of the structures is less than single
pole structures.
|
|
| Double-circuit 138-kilovolt steel lattice tower. Transmission structure of this design are no longer used
for new construction. However, ATC operates many lines
on lattice towers.
|
|
| 138-kilovolt steel H-frame. |
|
| 345-kilovolt, double-circuit on single poles. Higher voltage lines
require taller poles and wider rights-of-way.
|
|
About the wires
The wires (conductors) used in large transmission lines are mainly aluminum. Aluminum has replaced copper lines, because of the lower cost and lighter weight. A single-circuit transmission line will have three wires (conductors); a double-circuit line will have six wires. An additional wire (or two), called a shield wire is connected directly to the transmission line towers at the top to protect the main conductors (wires) from a direct lightning strike. If lightning strikes it will hit the shield wires rather than the conductors. If lightning were to strike the main conductors, a short circuit to the ground might occur, which could result in a wide-spread power outage.
Transmission lines are connected to the towers by insulators that are usually made of porcelain. They must be strong enough to support the weight of the transmission lines while preventing a contact between the wires and the tower. Contact between the two would cause a short circuit and disrupt supply.
|
