Introduction
ACSR (Aluminium Conductor Steel-Reinforced) is a high-capacity, high-strength stranded conductor used primarily in overhead power lines. The outer strands are made of high-purity aluminum, chosen for its excellent conductivity, light weight, and resistance to corrosion. The center strand is steel, providing additional strength to support the conductor’s weight and minimize sag.
Design and Components
The aluminium alloy and temper used for the outer strands in the United States and Canada is normally 1350-H19 and elsewhere is 1370-H19, each with 99.5+% aluminum content. The steel core, typically galvanized to prevent corrosion, allows for higher mechanical tension to be applied to the conductor. Steel also has lower elastic and inelastic deformation due to mechanical loading (e.g., wind and ice) as well as a lower coefficient of thermal expansion under current loading. These properties allow ACSR to sag significantly less than all-aluminium conductors.
Steel Core Types
- Galvanized Steel (GA2): Regular strength with class A zinc coating.
- Galvanized Steel (GA5): Ultra-high strength with class A zinc coating.
- Zinc-5% Aluminium Mischmetal (MA2): Regular strength with improved corrosion resistance.
- Aluminium-Clad Steel (AW): Provides increased corrosion protection and conductivity, commonly used in coastal applications.
Properties
ACSR combines the light weight and conductivity of aluminum with the high tensile strength and durability of steel. The aluminum strands carry most of the electrical current, while the steel core provides the mechanical strength necessary to support the conductor. The steel core is typically galvanized to protect against corrosion.
- Conductivity: High, due to the aluminum content.
- Mechanical Strength: Enhanced by the steel core, allowing for higher tension and reduced sag.
- Thermal Expansion: Lower compared to all-aluminium conductors, minimizing sag under load.
- Operating Temperature: Limited to 75°C due to the aluminum annealing. For higher temperatures, Aluminium-Conductor Steel-Supported (ACSS) conductors are used.
Effects
- Skin Effect: Causes current to flow near the surface at higher AC frequencies, increasing resistance. This effect is beneficial as it concentrates current in the low-resistivity aluminum.
- Proximity Effect: Currents in nearby conductors constrain current distribution within each conductor, increasing effective AC resistance.
Sizing and Lay
ACSR conductors are available in various sizes, denoted by the number of aluminium and steel strands (e.g., 72/7). The lay direction of the outer strands is typically right-hand lay in the USA. For example, an ACSR conductor with 72 aluminium strands and 7 steel strands is referred to as a 72/7 ACSR conductor.
Example Sizes
- Grosbeak: 26 aluminium strands, 7 steel strands, total conductor area of 322.3 mm².
- Egret: 30 aluminium strands, 19 steel strands, same total aluminium size but different mechanical characteristics.
Performance Factors
ACSR’s performance in overhead power lines is influenced by several factors, including:
- Ampacity Ratings: Determined by wind speed, sun intensity, ambient temperature, and maximum conductor temperature. Higher emissivity coatings can improve heat dissipation, increasing ampacity.
- Mechanical Loading: The steel core’s high tensile strength allows ACSR to withstand mechanical loading from wind and ice, minimizing sag and maintaining structural integrity.
Applications
ACSR is widely used in high voltage overhead power lines due to its combination of high strength and conductivity. Variations like aluminum-clad steel are preferred in coastal areas for better corrosion resistance. It is commonly used in applications where high mechanical strength and conductivity are required, such as long-distance power transmission.
Special Designs
- Trapezoidal Wire (TW): Uses trapezoidal-shaped aluminum strands to increase the cross-sectional area or reduce the overall diameter.
- Self-Damping (SD): Designed to control wind-induced vibration through internal damping.
- Aluminium-Conductor Steel-Supported (ACSS): Uses fully annealed aluminum for high-temperature operations.
- Twisted Pair (TP): Reduces ice/wind galloping and Aeolian vibration by twisting sub-conductors.
Splicing
Splicing ACSR conductors is essential for maintaining their integrity and performance. Most splices are compression-type (crimps), providing good strength and conductivity. Automatic splices are faster to install and are often used during storm restoration. Special splices are required for SD-type conductors due to their unique construction.
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