High Temperature Low Sag Conductors (HTLS)

High Temperature Low Sag Conductors (HTLS) can withstand operating temperatures of up to 210 °C, thus carrying higher power compared to conventional conductors. These conductors can be applied when there is a need to use an existing OHL that has clearance problems (ampacity limitations) and restrictions to the use of new and higher towers. HTLS conductors will allow an increase of the ampacity without the need to modify most of the existing towers.

Technology Types

HTLS conductors can be one of four types [11]:

• Type 0: Conventional steel core reinforced aluminium conductors ACSR or ACSR / TW for operating temperatures < 100 °C
• Type 1: Conductors consisting of a strength member made of steel, coated steel or steel alloy, and an envelope for which the high temperature effects are mitigated by means of thermal-resistant aluminium alloys (TACIR, TACSR)
• Type 2: Conductors consisting of a strength member made of steel, coated steel or steel alloy, and an envelope for which the high temperature effects are mitigated by means of annealed aluminium (ACSS)
• Type 3: Conductors consisting of a metal-matrix composite (MMC) strength member, and an envelope for which the high temperature effects are mitigated by means of thermal-resistant aluminium alloys or annealed aluminium. (ACCR, ACMR)
• Type 4: Conductors consisting of a polymer-matrix composite (PMC) strength member, and an envelope for which the high temperature effects are mitigated by means of annealed aluminium or thermal- resistant aluminium alloys for HTLS applications (ACPR)

The aluminium alloy or annealed aluminium will give the conductor the characteristics to withstand high temperatures without losing mechanical properties. The special core will give the low sag characteristics due to low coefficient of thermal expansion (CTE) and high Young modulus.

Components & enablers

N/A

Advantages & field of application

The cost and thermal losses of HTLS conductors are typically higher than conventional conductors. The main advantage is that HTLS conductors can enhance security reserves and transmission capacity without impacting the negotiated right-of-way, ideally with minor modifications of towers (mostly clamps of the conductors and their mountings or light tower’s reinforcement) and sometimes fewer towers. Although existing corridors are used, in some countries such projects have to go through an authorisation or impact assessment procedure again, especially when magnetic field levels are increased, as the expected currents are higher.

Technology Readiness Level

Due to the variety of possible material solutions, not all conductor types can be seen as mature technology (e.g. PMC) but many mature conductor types are already commercially available and ready for full-scale deployment.

• 2020: TRL 4-9
• 2025: TRL 6-9
• 2030: TRL 8-9

Research & Development

Current fields of research: Electrical and mechanical aging, surface treatment, expanded conductors, thermal modelling, system integration, thermal design margins, environmental loading conditions including high wind and / or ice loading and high temperatures, grid planning with HTLS conductors

Innovation priority to increase overall TRL: Conductor material, electrical and mechanical aging, reducing cost of special cores using other materials

Other: For future new materials and compositions of conductors: Full-scale outdoor tests are necessary to conclusively verify the results obtained in the research laboratory

Best practice performance

Maximum thermal capacity per circuit: ca. 3,200 MW (380 kV line)

Current rating: 5.0 kA per quad bundle (ZTAL/HACIN)

Standard definition: IEC 62818 ED1 (expected in 2019)

Best practice application

References

[1] Bmwi. Technologieübersicht. Das deutsche Höchstspannungsnetz: Technologien und Rahmenbedingungen. [Link]

[2] CIGRE. 220-kV field study of different high temperature low sag conductors. [Link]

[3] CIGRE. Experience with the mechanical performance of non-conventional conductors. [Link]

[4] VTT. Maximising power line transmission capability by employing dynamic line ratings – technical survey and applicability in Finland. [Link]

[5] D. Stengel, R. Bardl, C. Kühnel, S. Großmannn and W. Kiewitt.Accelerated electrical and mechanical ageing tests of high temperature low sag (HTLS) conductors. [Link]

[6] 3M. 3M™ Aluminum Conductor Composite Reinforced Customer Installations - High Load Growth. [Link]

[7] J. Lobry and D. Guery. Theoretical Study of Dielectric Breakdown in a New Composite Core HTLS Conductor. [Link]

[8] EPRI. Demonstration of Advanced Conductors for Overhead Transmission Lines. [Link]

[9] Best Paths. Best Paths Project. [Link]

[10] Cigre. Guide for Qualifying High Temperature Conductors for Use on Overhead Transmission Lines. [Link]

[11] Cigre. Conductors for the uprating of existing overhead lines. TB 763. April 2019.