Most overhead conductors are bi-component systems made of two distinct materials – typically aluminum strands combined with a steel or composite core. This dual-material construction creates unique thermal-mechanical behaviors that must be understood for proper line design.
The “knee point” represents the temperature at which the aluminum strands reach zero tension, transferring their mechanical load entirely to the core. This occurs because aluminum has a higher thermal expansion coefficient than core materials, causing it to expand more rapidly with temperature.
Below the knee point, conductor sag increases relatively rapidly, driven by aluminum expansion. Above the knee point, sag behavior is controlled by the core material’s thermal expansion characteristics. The knee point temperature varies by conductor type based on:
ACSR exhibits a knee point around 125°C, though this temperature is reached in practice. Below the knee point, sag increases linearly with temperature at a rate determined by aluminum’s thermal expansion coefficient. The theoretical behavior above the knee point would follow steel’s expansion characteristics, but ACSR is typically not operated in this region because the hard-drawn aluminum strands would be permanently damaged.
ACSS, using fully annealed aluminum, displays a lower knee point temperature (around 70-80°C) than ACSR. This earlier transition occurs because the annealed aluminum carries less mechanical tension. Above the knee point, ACSS can operate at much higher temperatures with relatively high sag behavior controlled by the steel core.
TS AECC exhibits minimal thermal sag above its knee point (similar to ACSS at around 70-80°C) due to the extremely low thermal expansion coefficient of its pure carbon fiber core, resulting in an almost straight sag-temperature line at high temperatures.
ACCC incorporates glass fiber alongside carbon fiber in its core. This hybrid composition results in a thermal expansion coefficient slightly higher than pure carbon fiber, though still providing excellent thermal sag performance.
ACCR has a relatively high knee point temperature, resulting from the significant tension carried by its high-temperature aluminum alloy strands. Once past the knee point, its composite core provides good thermal sag performance.
These conductors are unique in that their knee point essentially occurs at installation temperature. The aluminum strands are installed with minimal tension, meaning all subsequent thermal sag behavior is governed by the steel core’s properties. While this results in lower initial sag compared to other designs, the steel core’s high thermal expansion leads to significant sag at elevated temperatures.
In transmission line design, conductor sag must be carefully evaluated to ensure adequate ground clearance under all conditions. There are three fundamental types of sag that must be considered: thermal sag, creep sag, and load-induced sag.
Thermal sag occurs as conductors heat up and expand during operation. The amount of thermal sag depends primarily on the core material’s coefficient of thermal expansion. Traditional steel-core conductors experience significant thermal sag due to steel’s relatively high thermal expansion coefficient. In contrast, composite core conductors use materials with much lower thermal expansion coefficients, resulting in significantly reduced thermal sag.
Creep sag develops over time as conductor materials, particularly aluminum strands, gradually elongate under sustained mechanical loading. This is especially significant in conductors without dedicated strength members, such as AAC (All Aluminum Conductor) or AAAC (All Aluminum Alloy Conductor).
Traditional industry practice has been to project ten-year creep behavior based on relatively short-term laboratory tests (around 1,000 hours). However, this approach often underestimates long-term creep, as aluminum strands can continue to elongate well beyond the ten-year mark in conductors without dedicated strength members.
Load-induced sag occurs when external forces, primarily ice and snow accumulation, add weight to the conductor. The total load includes:
A conductor’s response to these loads depends largely on its core’s elastic modulus. First-generation composite core conductors, which incorporated significant amounts of fiberglass, exhibited lower modulus values that could result in excessive load-induced sag. This contributed to a perception that all advanced conductors have sag problems under heavy loading.
Next generation advanced conductor AECC takes a different approach. By eliminating fiberglass, it achieves a respectable modulus that effectively manages load-induced sag. This enables the conductor to handle heavy ice loads and support longer spans, including major river crossings like those over the Mississippi River.
TS Conductor’s AECC technology effectively addresses all three types of sag through its unique design:
Thermal sag is virtually eliminated above the knee point temperature. This is because once the fully annealed aluminum transfers its load to the carbon fiber core, any further temperature increase causes minimal thermal expansion due to the core’s extremely low coefficient of thermal expansion.
Creep sag is minimal because the carbon fiber composite core maintains its strength and dimensional stability over time. Unlike aluminum conductors that continue to elongate, or traditional designs where aluminum strands may creep, AECC’s core provides long-term mechanical stability.
Load-induced sag is effectively managed through the core’s respectable modulus. AECC’s carbon fiber core provides sufficient mechanical strength and modulus to handle ice and snow loading while maintaining appropriate sag levels for reliable operation. Aluminum alloy options are also available, should it be required for special project situations.
The mechanical behavior of conductors fundamentally affects their performance, installation requirements, and long-term reliability. At the heart of this behavior is the relationship between stress and strain – how a conductor responds when mechanical forces are applied.
Traditional ACSR (Aluminum Conductor Steel Reinforced) conductors show a relatively straightforward stress-strain relationship that can be approximated as linear. This simple behavior has led many utilities worldwide to use linear calculations in their sag-tension analysis, an approach that has worked well enough for traditional conductors.
However, both ACSS (Aluminum Conductor Steel Supported) and composite core advanced conductors exhibit distinctly non-linear stress-strain relationships. This non-linearity comes from the same mechanism in both cases – the fully annealed aluminum strands transferring load to the core (whether steel or composite). This means that traditional linear approximations are no longer adequate – polynomial models must be used for accurate sag-tension calculations.
The mechanical behavior of a conductor is largely determined by its core material and construction. A key property is the elastic modulus – defined as the slope of the stress-strain curve – which indicates how much a material stretches under load. A higher modulus means greater resistance to stretching.
Traditional ACSR uses a steel core, providing high stiffness due to steel’s high elastic modulus. The hard aluminum strands also contribute to the mechanical strength, creating a composite structure. This proven design has served the industry for over a century.
ACSS also uses a steel core, giving it similarly high elastic modulus. However, because its annealed aluminum strands don’t contribute significantly to mechanical strength, its overall behavior differs from ACSR despite having the same core material.
First-generation advanced conductors took two distinct approaches to core design, each with inherent limitations.
ACCR contains ceramic fibers in a metal matrix core. While ceramic fibers provide high stiffness, they can only stretch about 0.7% before failing. This limited tensile strength requires the use of specialized aluminum alloy strands for additional strength, compromising electrical performance.
ACCC used a hybrid core combining glass and carbon fibers. While innovative, the inclusion of glass fiber significantly reduced the core’s modulus. Glass fiber’s modulus is only about one-third that of carbon fiber, resulting in lower overall stiffness. This makes ACCC less suitable for areas with heavy ice loading or long spans.
TS Conductor’s AECC technology takes a different approach. By using a carbon composite core without glass fiber, protected by our patented aluminum encapsulation, we achieve a sufficient modulus to handle extreme weather conditions.
When evaluating conductor performance for line design, what matters is the total sag under all operating conditions. This includes both mechanical sag from ice/wind loading and thermal sag from conductor heating during operation.
While steel-core conductors (both ACSR and ACSS) exhibit lower mechanical sag under ice and wind loading due to their higher modulus, their thermal sag characteristics often become the limiting factor in line design. ACSS in particular, despite its high modulus, experiences significant thermal sag at its high operating temperatures.
TS AECC may experience somewhat higher mechanical sag but maintains significantly lower thermal sag. The net result is that TS AECC can maintain required clearances under all conditions while providing greater capacity.
This illustrates why it’s crucial to consider both mechanical and thermal behavior when selecting conductors for transmission line projects. While mechanical properties like modulus are important, they must be evaluated as part of the complete performance picture.
Unlike conventional wire drawing which requires extensive use of lubricants, our process begins with melting cleaned aluminum rod in a specialized furnace. We then extrude the aluminum rather than drawing it, significantly reducing work hardening. This extrusion-based approach offers remarkable flexibility – we can modify aluminum strand shapes and sizes by simply switching dies, a process that takes days rather than months with traditional manufacturing methods.
The elimination of lubricants from our process results in exceptionally clean conductor surfaces. This cleanliness is not merely cosmetic – it has important implications for high-voltage applications where surface contamination can affect corona performance.
Our manufacturing process introduces only minimal work hardening during the encapsulation process and stranding. Feedback from the field indicates that this limited work hardening is beneficial during installation, making our conductors less prone to “birdcaging” compared to batch-annealed ACSS conductors.
We take a conservative approach to conductivity ratings to ensure our utility customers always receive better connectivity than specified. For the stranded aluminum, we guarantee 63% conductivity while often achieving 63.5%.
The clean surfaces produced by our lubricant-free manufacturing process eliminate concerns about hydrophobic surface effects that can create corona issues in EHV and UHV applications. The slight work hardening provides improved handling characteristics during installation while maintaining excellent electrical performance.
TS Conductor fully complies with ASTM B987-20 where applicable to our technology. The standard’s section on galvanic barriers, however, was written specifically for older conductor configurations and isn’t relevant to our design.
Our patented aluminum encapsulation layer completely blocks moisture and oxygen from reaching the core, making galvanic corrosion physically impossible. This protection mechanism is well-proven in the industry – aluminum clad steel wire has effectively prevented galvanic corrosion in steel core conductors for decades.
The path to modernizing industry standards is slow, complex, and more political than it should be. Despite a seven-year effort to update ASTM B987 to better reflect current technology, progress has been impeded by legacy manufacturers protecting their market positions. We continue to work with ASTM to advance these standards while maintaining our focus on verifiable quality measures like continuous X-ray inspection during manufacturing.
A common misconception about composite core conductors is that they are inherently rigid and unbendable, particularly when encapsulated in aluminum. The reality is quite different. TS Conductor’s innovative design actually enables superior flexibility while maintaining structural integrity through a unique pre-tensioning approach.
To understand conductor bending performance, we need to examine the fundamental physics involved. When any conductor bends, it experiences both tensile (stretching) and compressive (squeezing) forces. The upper layer (convex surface) experiences tension while the lower layer (concave surface) in bending experiences compression, with equal magnitude but opposite directions.
In unidirectional composites like those used in conductor cores, there’s an important asymmetry between tensile and compressive strength. While these materials exhibit exceptional strength under tension (fiber-dominated property), their compression strength is typically only 60-70% of their tensile strength (matrix-dominated property). This means that under bending, compression failure will occur before tensile failure—making compression the limiting factor.
The most critical period for conductor bending occurs during installation, when the conductor may not be under tension. Without tension to offset compressive forces, sharp bends or small radiuses can potentially damage traditional composite conductors. This vulnerability explains why first-generation advanced conductors require perfect adherence to installation procedures.
TS Conductor solves this fundamental challenge through pre-tensioning. By building protective tension into the core during manufacturing, we provide inherent protection against compression failure—even when the conductor isn’t under external tension during installation. This built-in safeguard makes our conductor significantly more forgiving during installation.
Our pre-tensioned design allows TS Conductor to handle more extreme bending conditions than first generation advanced conductors. Rather than experiencing sudden failure at sharp bends, it exhibits graceful bending behavior while preserving core integrity. In fact, the minimum bending radius of our conductor is typically determined by the need to prevent birdcaging of the annealed aluminum strands rather than any limitation of the composite core. This combination of flexibility and durability represents a fundamental shift in composite conductor design—moving from products that require perfect handling to a solution that’s inherently protected against real-world installation conditions.
Transmission line conductors experience complex mechanical stresses throughout their operational life. Understanding how conductors respond to these stresses, particularly the phenomenon of creep, is essential for proper transmission line design and long-term reliability. This understanding helps engineers optimize conductor selection, installation procedures, and maintenance practices.
The mechanical response of a conductor is determined by two primary components: the composite core and the aluminum strands. The composite core exhibits purely elastic behavior, meaning it deforms linearly under stress and returns to its original shape when the stress is removed. This elastic response is limited to approximately 2% elongation, making the core highly stable and predictable.
The aluminum strands demonstrate both elastic and plastic behavior. In their initial loading region, they respond elastically like the core. However, once they exceed their yield point and enter the curved portion of their stress-strain curve, they begin to experience plastic deformation. This plastic deformation can reach 20-30% elongation, if without core constraint, and is permanent, unlike the elastic behavior of the core.
The total conductor stress-strain curve represents the mathematical sum of these two components working in parallel. This combined response determines the conductor’s overall mechanical behavior under varying load conditions.
Creep in conductors refers to the gradual, permanent elongation that occurs over time under constant mechanical stress. This phenomenon primarily occurs in the aluminum strands, as the composite core maintains its elastic properties. In fully annealed aluminum strands, creep allows the strands to gradually elongate and transfer their tension load to the composite core.
This load transfer process is a fundamental aspect of conductor behavior. As the aluminum strands creep, they reach a more relaxed state while the composite core takes on a greater share of the mechanical load. This process continues until the conductor reaches a stable configuration determined by the properties of both components.
The creep behavior of conductors, particularly those using annealed aluminum strands, provides significant performance advantages. As the aluminum strands transfer load to the core through creep, they become more mechanically free to move. This freedom of movement enables natural damping mechanisms within the conductor.
The relaxed state of the aluminum strands creates excellent self-damping characteristics for managing aeolian vibration. When combined with trapezoidal wire design, which maximizes surface contact between adjacent strands, the conductor achieves optimal frictional damping. This natural damping mechanism helps protect the conductor from fatigue damage caused by wind-induced vibration.
Installation procedures must carefully account for the mechanical properties of both the core and aluminum strands. During installation, excessive bending must be avoided to prevent permanent deformation of the aluminum strands, which could lead to “birdcaging” where strands permanently separate from their intended positions. Following minimum bend radius guidelines similar to those used for ACSS conductors helps prevent these issues.
Long-term conductor performance depends on understanding and accounting for creep behavior. Engineers must consider the load transfer between core and strands in their structure designs and recognize that the conductor’s self-damping characteristics may reduce or eliminate the need for external damping devices. This comprehensive understanding of conductor mechanics enables optimal design decisions that ensure reliable, long-term performance.