AC transmission: Technology advances
Hotter wires, real-time ratings and new 765 kV backbones
What this tab covers The newest technologies in service or close to adoption, what each improves, and how the main options compare.The main advances get more out of existing corridors (advanced conductors, dynamic line ratings, power flow control) and push voltages higher where new corridors are built (765 kV in the US, 1,000 kV in China).
Latest advances
Advanced (high-temperature, low-sag) conductors being deployed · 2024Composite-core conductors that run hotter without sagging, strung on existing towers (reconductoring)12
Improves: Can roughly double capacity in an existing right of way at under half the unit-length cost of new lines
Dynamic line ratings being deployed · 2024Sensors and weather data set a line's limit in real time instead of a conservative fixed rating11
Improves: Unlocks spare capacity when wind or cool air cools conductors; FERC Order 1920 requires planners to consider it
Advanced power flow control and transmission switching being deployed · 2024Devices and switching practices that push power from congested lines onto lightly loaded ones11
Improves: Can defer new build
765 kV backbones in Texas and the Midwest being deployed · 2025New extra-high-voltage AC networks instead of piecemeal 345 kV expansion6
Improves: Texas: about 5% lower losses for about 7% more capital than the 345 kV plan (calculated from ERCOT totals)
SF6-free high-voltage switchgear being deployed · 2028Breakers insulated with alternatives to sulphur hexafluoride, a very potent greenhouse gas19
Improves: EU bans new SF6 equipment at 52 to 145 kV from 2028 and above 145 kV from 2032
Comparing the options
| Option | Efficiency | Cost | Capacity or distance | Maturity |
|---|---|---|---|---|
| 345 kV AC (double circuit)56 | About 10% lost per 1,000 km at heavy load (calculated) | About USD 4.2 million per mile in Texas (2025) | About 3,600 MVA thermal | mature |
| 765 kV AC (single circuit)515 | About 5.7% lost per 1,000 km at heavy load (calculated) | USD 5.2 to 6.3 million per mile (MISO 2024 guide) | About 6,600 MVA thermal, about 2,400 MW natural loading | mature |
| Reconductoring with advanced conductors12 | Lower resistance than conventional conductors | Less than half the unit-length cost of new lines | Up to about 2 times the existing line, for lines up to about 50 miles | being deployed |
| HVDC overhead (see HVDC tile)520 | Lower losses on long routes | Converter stations make it dearer on short routes; Brattle break-even about 700 km for new build | 3,000 or 6,000 MW per ±640 kV bipole | mature |
For new corridors, 765 kV AC is usually cheapest up to a few hundred kilometres: MISO puts the break-even with HVDC at roughly 250 to 400 miles, Brattle at about 700 km. In existing corridors, reconductoring and dynamic ratings are faster and cheaper than new lines.
Sources
- Discussion of EHV and HVDC (M. Tackett, MISO, presentation to ERCOT) MISO / ERCOT, 2023
- 2024 Regional Transmission Plan: 345 kV Plan and Texas 765 kV Strategic Transmission Expansion Plan ERCOT, 2025
- FERC addresses long-term planning, regional transmission cost allocation American Public Power Association, 2024
- Accelerating Transmission Expansion by Using Advanced Conductors in Existing Right-of-Way (Chojkiewicz et al., Energy Institute at Haas WP-343R) UC Berkeley / GridLab, 2024
- AEP Companies' comments about extra high voltage transmission lines to the ERCOT region (Project 55249, citing MISO 2024 Transmission Cost Estimation Guide) Public Utility Commission of Texas, 2024
- Transition to SF6-free electrical switchgear will begin in 2026 Finnish Environment Institute (Syke), 2025
- HVDC Transmission: Multi-Value Planning Considerations and Experience The Brattle Group (for MISO), 2023