Aluminum conductor steel-reinforced cable (ACSR) is a type of high-capacity, high-strength stranded conductor typically used in overhead power lines. The outer strands are high-purity aluminium, chosen for its good conductivity, low weight, low cost, resistance to corrosion and decent mechanical stress resistance. The centre strand is steel for additional strength to help support the weight of the conductor. Steel is of higher strength than aluminium which allows for increased mechanical tension to be applied on the conductor. Steel also has lower elastic and inelastic deformation (permanent elongation) 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. As per the International Electrotechnical Commission (IEC) and The CSA Group (formerly the Canadian Standards Association or CSA) naming convention, ACSR is designated A1/S1A.
Design
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+% aluminium content. The temper of the aluminium is defined by the aluminium version's suffix, which in the case of H19 is extra hard. To extend the service life of the steel strands used for the conductor core they are normally galvanized, or coated with zinc to prevent corrosion. The diameters of the strands used for both the aluminum and steel strands vary for different ACSR conductors.
ACSR cable still depends on the tensile strength of the aluminium; it is only reinforced by the steel. Because of this, its continuous operating temperature is limited to , the temperature at which aluminium begins to anneal and soften over time. For situations where higher operating temperatures are required, aluminium-conductor steel-supported (ACSS) may be used.
Steel core
The standard steel core used for ACSR is galvanized steel, but zinc, 5% or 10% aluminium alloy and trace mischmetal coated steel (sometimes called by the trade-names Bezinal or Galfan) and aluminium-clad steel (sometimes called by the trade-name Alumoweld) are also available. Higher strength steel may also be used.
In the United States the most commonly used steel is designated GA2 for galvanized steel (G) with class A zinc coating thickness (A) and regular strength (2). Class C zinc coatings are thicker than class A and provide increased corrosion protection at the expense of reduced tensile strength. A regular strength galvanized steel core with Class C coating thickness would be designated GC2. Higher strength grades of steel are designated high-strength (3), extra-high-strength (4), and ultra-high-strength (5). An ultra-high-strength galvanized steel core with class A coating thickness would be designated GA5. The use of higher strength steel cores increases the tensile strength of the conductor allowing for higher tensions which results in lower sag.
Zinc-5% aluminium mischmetal coatings are designated with an "M". These coatings provide increased corrosion protection and heat resistance compared to zinc alone. Regular strength Class "A" mischmetal thickness weight coated regular strength steel would be designated MA2.
Aluminium-clad steel is designated as "AW". Aluminium-clad steel offers increased corrosion protection and conductivity at the expense of reduced tensile strength. Aluminium-clad steel is commonly specified for coastal applications.
IEC and CSA use a different naming convention. The most commonly used steel is S1A for S1 regular strength steel with a class A coating. S1 steel has slightly lower tensile strength than the regular strength steel used in the United States. Per the Canadian CSA standards the S2A strength grade is classified as High Strength steel. The equivalent material per the ASTM standards is the GA2 strength grade and called Regular Strength steel. The CSA S3A strength grade is classified as Extra High Strength steel. The equivalent material per the ASTM standards is the GA3 strength grade called High Strength. The present day CSA standards for overhead electrical conductor do not yet officially recognize the ASTM equivalent GA4 or GA5 grades. The present day CSA standards do not yet officially recognize the ASTM "M" family of zinc alloy coating material. Canadian utilities are using conductors built with the higher strength steels with the "M" zinc alloy coating.
Lay Lay of a conductor is determined by four extended fingers; "right" or "left" direction of the lay is determined depending if it matches finger direction from right hand or left hand respectively. Overhead aluminium (AAC, AAAC, ACAR) and ACSR conductors in the USA are always manufactured with the outer conductor layer with a right-hand lay. Going toward the center, each layer has alternating lays. Some conductor types (e.g. copper overhead conductor, OPGW, steel EHS) are different and have left-hand lay on the outer conductor. Some South American countries specify left-hand lay for the outer conductor layer on their ACSR, so those are wound differently than those used in the USA.
Sizing ACSR conductors are available in numerous specific sizes, with single or multiple center steel wires and generally larger quantities of aluminium strands. Although rarely used, there are some conductors that have more steel strands than aluminum strands. An ACSR conductor can in part be denoted by its stranding, for example, an ACSR conductor with 72 aluminium strands with a core of 7 steel strands will be called 72/7 ACSR conductor. Cables generally range from #6 AWG ("6/1" – six outer aluminum conductors and one steel reinforcing conductor) to 2167 kcmil ("72/7" – seventy two outer aluminum conductors and seven steel reinforcing conductors).
Naming convention
To help avoid confusion due to the numerous to combinations of stranding of the steel and aluminium strands, code words are used to specify a specific conductor version. In North America bird names are used for the code words while animal names are used elsewhere. For instance in North America, Grosbeak is a (636 kcmil) ACSR conductor with 26/7 Aluminium/Steel stranding whereas Egret is the same total aluminium size (, 636 kcmil conductor) but with 30/19 Aluminium/Steel stranding. Although the number of aluminium strands is different between Grosbeak and Egret, differing sizes of the aluminium strands are used to offset the change in the number of strands such that the total amount of aluminium remains the same. Differences in the number of steel strands result in varying weights of the steel portion and also result in different overall conductor diameters. Most utilities standardize on a specific conductor version when various versions of the same amount of aluminum to avoid issues related to different size hardware (such as splices). Due to the numerous different sizes available, utilities often skip over some of the sizes to reduce their inventory. The various stranding versions result in different electrical and mechanical characteristics.
Ampacity ratings
Manufacturers of ACSR typically provide ampacity tables for a defined set of assumptions. Individual utilities normally apply different ratings due to using varying assumptions (which may be a result in higher or lower amperage ratings than those the manufacturers provide). Significant variables include wind speed and direction relative to the conductor, sun intensity, emissivity, ambient temperature, and maximum conductor temperature.
Conducting properties
In three phase electrical power distribution, conductors must be designed to have low electrical impedance in order to assure that the power lost in the distribution of power is minimal. Impedance is a combination of two quantities: resistance and reactance. The resistances of ASCR conductors are tabulated for different conductor designs by the manufacturer at DC and AC frequency assuming specific operating temperatures. The reasons that resistance changes with frequency are largely due to the skin effect, the proximity effect, and hysteresis loss. Depending on the geometry of the conductor as differentiated by the conductor name, these phenomena have varying degrees of affecting the overall resistance in the conductor at DC vs AC frequency.
Often not tabulated with ACSR conductors is the electrical reactance of the conductor, which is due largely to the spacing between the other current carrying conductors and the conductor radius. The reactance of the conductor contributes significantly to the overall current that needs to travel through the line, and thus contributes to resistive losses in the line. For more information on transmission line inductance and capacitance, see electric power transmission and overhead power line.
Skin effect
The skin effect decreases the cross sectional area in which the current travels through the conductor as AC frequency increases. For alternating current, most (63%) of the electric current flows between the surface and the skin depth, δ, which depends on the frequency of the current and the electrical (conductivity) and magnetic properties of the conductor. This decreased area causes the resistance to rise due to the inverse relationship between resistance and conductor cross sectional area. The skin effect benefits the design, as it causes the current to be concentrated towards the low-resistivity aluminum on the outside of the conductor. To illustrate the impact of the skin effect, the American Society for Testing and Materials (ASTM) standard includes the conductivity of the steel core when calculating the DC and AC resistance of the conductor, but the IEC and CSA Group standards do not.
Proximity effect
In a conductor (ACSR and other types) carrying AC current, if currents are flowing through one or more other nearby conductors the distribution of current within each conductor will be constrained to smaller regions. The resulting current crowding is termed as the proximity effect. This crowding gives an increase in the effective AC resistance of the circuit, with the effect at 60 Hertz being greater than at 50 Hertz. Geometry, conductivity, and frequency are factors in determining the amount of proximity effect.
The proximity effect is result of a changing magnetic field which influences the distribution of an electric current flowing within an electrical conductor due to electromagnetic induction. When an alternating current (AC) flows through an isolated conductor, it creates an associated alternating magnetic field around it. The alternating magnetic field induces eddy currents in adjacent conductors, altering the overall distribution of current flowing through them.
The result is that the current is concentrated in the areas of the conductor furthest away from nearby conductors carrying current in the same direction.
Hysteresis loss
Hysteresis in an ACSR conductor is due to the atomic dipoles in the steel core changing direction due to induction from the 60 or 50 Hertz AC current in the conductor. Hysteresis losses in ACSR are undesirable and can be minimized by using an even number of aluminium layers in the conductor. Due to the cancelling effect of the magnetic field from the opposing lay (right-hand and left-hand) conductors for two aluminium layers there is significantly less hysteresis loss in the steel core than there would be for one or three aluminium layers where the magnetic field does not cancel out.