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Transmission Q&A

1. How does the transmission grid work?

A transmission grid is a network of power stations, transmission lines (or circuits) and substations that are all interconnected to provide a level of redundancy and, therefore, reliable electric service even if an event were to impact the system.

2. What is the difference between alternating current and direct current?

An alternating current flow direction reverses at frequent intervals. Direct current only flows in one direction. Most transmission lines in the U.S. are alternating current.

3. What is the difference between low voltage and high voltage lines?

Transmission lines considered "high voltage" are classified as 230-kilovolt (kV) and greater (where one kV equals 1,000 volts). High voltage transmission lines are used to move energy farther distances to reduce the energy lost in transmission. Low voltage lines move energy shorter distances.

4. What are electromagnetic fields?

Electromagnetic fields (EMFs) are invisible lines of force that surround any electrical device such as power lines, electric wiring, and electrical equipment. Electric fields are the result of the strength (voltage) of the electric charge. Magnetic fields are the result of the motion (current) of the charge. Wherever electricity is used, EMFs are present.

5. How do electromagnetic fields affect human health?

Since the early 1970s, research has been performed to determine if EMFs pose a health risk. This body of scientific evidence does not show a clear pattern of health hazards.

What factors are taken into consideration for the construction of a transmission line?

Timeline: On average it takes 10-15 years to build a high-voltage transmission line. This timeline includes planning, scoping, mapping, environmental review, public comment, project approval, permitting, land acquisition and construction.

Height: In general, for overhead high voltage transmission lines, the wires are typically at least 30 feet off the ground. For safety reasons, the higher the voltage, the more distance is required between the conductors and anything that touches the ground. There is no single requirement associated with different voltages. The kind of terrain the line will pass over is also a factor, among other considerations.

Location: Clearance requirements are related to a few issues, including: the height of the wires from the ground; the distance that must be between two towers in a single transmission line or the distance between towers from two or more separate transmission lines built within a single transmission corridor; reliability standards; and the proximity of transmission lines to roads and highways. These requirements are mandated by federal, state and sometimes local governments and the specific requirements are a function of exactly where the line and towers will be located.

Underground: The higher the energy transmitted, the more heat is generated. With overhead transmission lines, the air surrounding the lines acts as an insulator and absorbs this waste heat. In underground transmission lines other mediums must be used to dissipate this heat, which to-date has restricted the undergrounding of transmission lines to voltages less than 500-kV except for very short distances.

Another consideration of underground lines is the ground disturbance caused by the tunnels through which the transmission line runs. Instead of impacting the ground only at tower footings, underground transmission construction requires extensive excavation and can disrupt habitats or water resources. Access to the underground transmission line, which is needed for maintenance and repairs, requires construction of "vaults", typically 20' x 30' structures (roughly the size of an average living room) that must be buried in the ground every 750-1,000 feet where the conductors are spliced together. The environmental and land disruption impacts of undergrounding transmission lines can greatly exceed that of above ground transmission, and are factors that are weighed in the planning process.

Co-Location: Co-location is adding a new transmission line to a pre-existing transmission infrastructure. Clearance requirements and reliability standards both come into play when co-location is being considered. Some key considerations in evaluating whether co-location is a viable option include whether an existing tower can support the added weight of an additional set of wires and how reliability will be hampered by two lines relying on the same towers.

Transmission Line Design
The Big Picture
Transmission Safety Tips



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