Other and advanced conductors
TL;DR More power can travel along existing lines using different conductors (wires). Most transmission lines are wired with technology that has been around for well over 30 yrs. There are several newer (only 20 yrs old!) options, but none is a perfect solution.
Larger Diameter
Dominion, for example, takes every opportunity to install larger diameter conductors. Dominion in 2015 gained about 66 percent in power capacity when it upgraded the traditional 1960s-era Aluminum Conductor Steel Reinforced (ACSR) conductors on the 500 kV Mt. Storm–Doubs line. Dominion is planning to upgrade the three existing 500 kV lines going into Leesburg as part of 2022-RTEP-W3 along with adding three new lines
Different Aluminum
Greater capacity can also be achieved by changing the materials of the conductor. Different aluminum can carry up to 50% more power. The tower may need to be taller to account for sagging of the wires in higher temperatures. They also need to have substations at least every 50 miles to provide voltage support and keep the wires operating at the correct temperatures, which is why they tend to be used for shorter 230kVand lower voltage lines.
Example: Dominion now routinely uses Aluminum Conductor Steel Supported (ACSS) conductors as its standard for 230 kV lines in place of the traditional ACSR conductors. The aluminum is pre-annealed (heat treated) to allow for higher-temperature operation (up to 250 C vs. 100 C) and thus more current (which heats the wire) and thus up to 50 percent more power.
It took more than 40 years for ACSS to reach that standard status.
Different Core Material:
Even greater gains are possible by replacing the steel core with composite core materials. These materials are stronger and lighter and can tolerate higher temperatures (180–200 C) without causing the lines to sag (as occurs with ACSS).
- These lines can carry up to about twice the power of ACSR lines. They weigh about the same as the ACSR lines but have slightly bigger diameter and thus fewer losses and less corona effects.
- Composite core lines are also used to span long distances (taking advantage of the strength) or to operate at lower temperatures than ACSR lines carrying the same power to reduce sagging and avoid brush fires.
- Composite core conductors add about 20 to 30 percent to the cost of a new transmission line; however, this is less expensive than building two lines (alternatively, reconductoring an old line costs less than building a new line).
- Some express concern about how these lines hold up under icing conditions; however, the manufacturers offer different composite mixes to increase the strength of the composite core when icing is a primary concern.
- Like ACSS, these lines need voltage support at least every 50 miles to keep them operating at their thermal limits in order to retain their performance advantage, so cost-effective application of composite core conductors for 500 kV lines needs to be limited to areas where there are already substations at least every 50 miles along the path
- As with higher-voltage lines or double-circuit towers, planners need to consider contingencies for losing a single line with so much power. The solution, as in the case of 765 kV lines, is to have the technology in multiple places and to have pairs of lines in a single corridor.
- The industry in general is reluctant to risk using technology that has been around for only 20 years and has not been used extensively at voltages above 230 kV. It has been in use in Indonesia for about 20 yrs at 500kV, but the industry tends to discount foreign experience. It will wait decades for others to provide long-term performance results unless regulators require it.
Superconductor Transmission Lines
Advanced (composite-core) conductors improve on ACSS: they are lighter and sag less, which allows for less expensive towers and lowers fire risk.
A newer generation from TS Conductor, called Aluminum Encapsulated Composite Core, resolves most of the earlier objections to advanced conductors:
- Less vulnerable to damage and more resistant to icing, thanks to the aluminum encapsulation.
- Installs with the same tools and procedures as existing conductors.
- Its one remaining drawback is the lack of a 20-plus-year track record, which still gives utilities pause at the highest voltages. Even so, the federally operated Tennessee Valley Authority has adopted it as their standard going forward. (TS Conductor)
Advanced conductors have already been used at 500 kV on a 27-mile line in northeastern Maryland, using the older generation of the technology.
In the future, it may be possible to use superconductors in place of the aluminum. This requires a sheath of liquid nitrogen cooling around the conductors and, more critically, a system of small substations for refreshing the liquid nitrogen supply about every 10 miles. This is unlikely to be available at scale before the early 2030s, and the power industry will want to see somebody else testing the long-term endurance and performance of this technology over decades before embracing it.
