Editing Electric power transmission (section)

== Capacity ==
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The amount of power that can be sent over a transmission line varies with the length of the line. The heating of short line conductors due to line losses sets a thermal limit. If too much current is drawn, conductors may sag too close to the ground, or conductors and equipment may overheat. For intermediate-length lines on the order of {{convert|100|km|mi|abbr=off}}, the limit is set by the [[voltage drop]] in the line. For longer AC lines, [[Power system stability|system stability]] becomes the limiting factor. Approximately, the power flowing over an AC line is proportional to the cosine of the phase angle of the voltage and current at the ends.

This angle varies depending on system loading. It is undesirable for the angle to approach 90 degrees, as the power flowing decreases while resistive losses remain. The product of line length and maximum load is approximately proportional to the square of the system voltage. Series capacitors or phase-shifting transformers are used on long lines to improve stability. HVDC lines are restricted only by thermal and voltage drop limits, since the phase angle is not material.

Understanding the temperature distribution along the cable route became possible with the introduction of [[distributed temperature sensing]] (DTS) systems that measure temperatures all along the cable. Without them maximum current was typically set as a compromise between understanding of operation conditions and risk minimization. This monitoring solution uses passive [[optical fibers]] as temperature sensors, either inside a high-voltage cable or externally mounted on the cable insulation.

For overhead cables the fiber is integrated into the core of a phase wire. The integrated Dynamic Cable Rating (DCR)/Real Time Thermal Rating (RTTR) solution makes it possible to run the network to its maximum. It allows the operator to predict the behavior of the transmission system to reflect major changes to its initial operating conditions.

=== Reconductoring ===
Some utilities have embraced reconductoring to handle the increase in electricity production. Reconductoring is the replacement-in-place of existing transmission lines with higher-capacity lines. Adding transmission lines is difficult due to cost, permit intervals, and local opposition. Reconductoring has the potential to double the amount of electricity that can travel across a transmission line.<ref name="Pontecorvo">{{Cite web |last=Pontecorvo |first=Emily |date=February 20, 2024 |title=There Is a Stupidly Easy Way To Expand the Grid - Heatmap News |url=https://heatmap.news/climate/clean-energy-grid-reconductoring |access-date=2024-03-06 |website=heatmap.news |language=en}}</ref> 
A 2024 report found the United States behind countries like Belgium and the Netherlands in adoption of this technique to accommodate electrification and renewable energy. <ref name="NYT-AR">{{cite news |url=https://www.nytimes.com/2024/04/09/climate/electric-grid-more-power.html |title=The U.S. Urgently Needs a Bigger Grid. Here's a Fast Solution. |newspaper=[[The New York Times]] |author=Brad Plumer |date=April 14, 2024}}</ref> In April 2022, the [[Biden Administration]] streamlined environmental reviews for such projects, and in May 2022 announced competitive grants for them funded by the 2021 [[Bipartisan Infrastructure Law]] and 2022 [[Inflation Reduction Act]].<ref>{{cite web |url=https://bidenwhitehouse.archives.gov/briefing-room/statements-releases/2024/05/28/fact-sheet-biden-harris-administration-launches-federal-state-initiative-to-bolster-americas-power-grid/ |date=May 28, 2024 |title=FACT SHEET: Biden-⁠Harris Administration Launches Federal-State Initiative to Bolster America's Power Grid |publisher=The [[White House]]}}</ref>

The rate of transmission expansion needs to double to support ongoing electrification and reach emission reduction targets. As of 2022, more than 10,000 power plant and energy storage projects were awaiting permission to connect to the US grid — 95% were zero-carbon resources. New power lines can take 10 years to plan, permit, and build.<ref name="Pontecorvo" />

Traditional power lines use a steel core surrounded by aluminum strands ([[Aluminium-conductor steel-reinforced cable]]). Replacing the steel with a lighter, stronger composite material such as [[carbon fiber]] ([[ACCC conductor]]) allows lines to operate at higher temperatures, with less sag, and doubled transmission capacity. Lowering line sag at high temperatures can prevent wildfires from starting when power lines touch dry vegetation.<ref name="NYT-AR" /> Although advanced lines can cost 2-4x more than steel, total reconductoring costs are less than half of a new line, given savings in time, land acquisition, permitting, and construction.<ref name="Pontecorvo" />

A reconductoring project in southeastern Texas upgraded 240 miles of transmission lines at a cost of $900,000 per mile, versus a 3,600-mile greenfield project that averaged $1.9 million per mile.<ref name="Pontecorvo" />