Aluminium 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 and low cost. The center strand is steel for additional strength to help support the weight of the conductor. Steel is 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. These properties allow ACSR to sag significantly less than all-aluminium conductors.
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Aluminium 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 and low cost. The center strand is steel for additional strength to help support the weight of the conductor. Steel is 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. These properties allow ACSR to sag significantly less than all-aluminium conductors. The aluminium alloy and temper used for the outer strands in the United States and Canada is normally H19 and elsewhere is H19, each with 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 another material 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. 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.
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. 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. Canadian utilities are using conductors built with the higher strength steels with the "M" zinc alloy coating.
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. Going toward the center each layer has alternating lays. Some conductor types e. 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. To help avoid confusion due to the numerous combinations of stranding of the steel and aluminium strands, code words are used to specify a specific conductor version. In North America bird are used for the code words while animal names are used elsewhere.
For instance in North America, Grosbeak is a 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.
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. 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 AC vs DC 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. The skin effect decreases the cross sectional area in which the current travels through the conductor as AC frequency increases. 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. 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 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.
The hysteresis effect is negligible on ACSR conductors with even numbers of aluminium layers and so it is not considered in these cases.
For ACSR conductors with an odd number of aluminium layers however, a magnetization factor is used to accurately calculate the AC resistance. The correction method for single-layer ACSR is different than that used for three-layer conductors. Due to applying the magnetization factor, a conductor with an odd number of layers has an AC resistance slightly higher than an equivalent conductor with an even number of layers. ACSR is widely used due to its efficient and economical design.
Variations of standard sometimes called traditional or conventional ACSR are used in some cases due to the special properties they offer which provide sufficient advantage to justify their added expense. Special conductors may be more economic, offer increased reliability, or provide a unique solution to an otherwise difficult, of impossible, design problem. The main types of special conductors include "trapezoidal wire conductor" TW - a conductor having aluminium strands with a trapezoidal shape rather than round and "self-damping" SD , sometimes called "self-damping conductor" SDC.
A similar, higher temperature conductor made from annealed aluminium is called "aluminium conductor steel supported" ACSS is also available.
Ontario Hydro's trapezoidal-shaped wire TW designs utilized the same steel core but increased the aluminium content of the conductor to match the overall diameter of the former round-wire designs they could then use the same hardware fittings for both the round and the TW conductors. They do not use designs which have odd number of layers three layers due to that design incurring higher hysteresis losses in the steel core. It is a concentric-lay stranded, self-damping conductor designed to control Aeolian-type vibration in overhead transmission lines by internal damping.
Self-damping conductors consists of a central core of one or more round steel wires surrounded by two layers of trapezoidal shaped aluminium wires.
One or more layers of round aluminium wires may be added as required. SD conductor differs from conventional ACSR in that the aluminium wires in the first two layers are trapezoidal shaped and sized so that each aluminium layer forms a stranded tube which does not collapse onto the layer beneath when under tension, but maintains a small annular gap between layers. The trapezoidal wire layers are separated from each other and from the steel core by the two smaller annular gaps that permit movement between the layers.
The round aluminium wire layers are in tight contact with each other and the underlying trapezoidal wire layer. Under vibration, the steel core and the aluminium layers vibrate with different frequencies and impact damping results. This impact damping is sufficient to keep any Aeolian vibration to a low level.
The use of trapezoidal strands also results in reduced conductor diameter for a given AC resistance per mile. Annealing the aluminium strands reduces the composite conductor strength, but after installation, permanent elongation of the aluminium strands results in a much larger percentage of the conductor tension being carried in the steel core than is true for standard ACSR.
This in turn yields reduced composite thermal elongation and increased self-damping. Twisted pair TP conductor sometimes called by the trade-names T-2 or VR has the two sub-conductors twisted usually with a left-hand lay about one another generally with a lay length of approximately three meters nine feet.
The conductor cross-section of the TP is a rotating "figure-8". The sub-conductors can be any type of standard ACSR conductor but the conductors need to match one another to provide mechanical balance. Many electrical circuits are longer than the length of conductor which can be contained on one reel. As a result, splicing is often necessary to join together conductors to provide the desired length. It is important that the splice not be the weak link.
A splice joint must have high physical strength along with a high electrical current rating. Within the limitations of the equipment used to install the conductor from the reels, as long of a length of conductor is generally purchased that the reel can accommodate to avoid more splices than are absolutely necessary.
Splices are designed to run cooler than the conductor. The temperature of the splice is kept lower by having a larger cross-sectional area and thus less electrical resistance than the conductor. Heat generated at the splice is also dissipated faster due to the larger diameter of the splice. Failures of splices are a significant concern as a failure of just one splice can cause an outage that affects a large amount of electrical load.
Most splices are compression-type splices crimps.
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