Types of Overhead Conductors

Types of Conductors:

There are four major types of overhead conductors used for electrical transmission and distribution.

·            AAC - All Aluminum Conductor

·            AAAC - All Aluminum Alloy Conductor

·            ACSR - Aluminum Conductor Steel Reinforced

·            ACAR - Aluminum Conductor Aluminum-Alloy Reinforced

All the major cost components of a transmission line depend upon conductor physical, mechanical and electrical parameters. A list of these basic parameters are:

·            conductor diameter

·            weight per unit length

·            conductivity of material(s)

·            crossectional area(s)

·            modulus of elasticity

·            rated breaking strength

·            coefficient(s) of thermal expansion

·            cost of material(s)

·            maximum unloaded design tension

·            resistance to vibration and/or galloping

·            surface shape/drag coefficient

·            fatigue resistance

AAC - All Aluminum Conductor, sometimes referred to as ASC, Aluminum Stranded Conductor, is made up of one or more strands of 1350 Alloy Aluminum in the hard drawn H19 temper. 1350 Aluminum Alloy, previously known as EC grade or electrical conductor grade aluminum, has a minimum conductivity of 61.2% IACS. Because of its relatively poor strength-to-weight ratio, AAC has had limited use in transmission lines and rural distribution because of the long spans utilized. However, AAC has seen extensive use in urban areas where spans are usually short but high conductivity is required. The excellent corrosion resistance of aluminum has made AAC a conductor of choice in coastal areas.

ACSR - Aluminum Conductor Steel Reinforced, a standard of the electrical utility industry since the early 1900's, consists of a solid or stranded steel core surrounded by one or more layers of strands of 1350 aluminum. Historically, the amount of steel used to obtain higher strength soon increased to a substantial portion of the cross-section of the ACSR, but more recently, as conductors have become larger, the trend has been to less steel content. To meet varying requirements, ACSR is available in a wide range of steel content - from 7% by weight for the 36/1 stranding to 40% for the 30/7 stranding. Early designs of ACSR such as 6/1, 30/7, 30/19, 54/19 and 54/7 strandings featured high steel content, 26% to 40%, with emphasis on strength perhaps due to fears of vibration fatigue problems. Today, for larger-than-AWG sizes, the most used strandings are 18/1, 45/7, 72/7, and 84/19, comprising a range of steel content from 11% to 18%. For the moderately higher strength 54/19, 54/7, and 26/7 strandings, the steel content is 26%, 26% and 31%, respectively. The high-strength ACSR 8/1, 12/7 and 16/19 strandings, are used mostly for overhead ground wires, extra long spans, river crossings, etc.

The inner-core wires of ACSR may be of zinc coated (galvanized) steel, available in standard weight Class A coating or heavier coatings of Class B or Class C. Class B coatings are about twice the thickness of Class A, and Class C coatings about three times as thick as Class A. The inner cores may also be of aluminum coated (aluminized) steel or aluminum clad steel. The latter produces a conductor designated as ACSR/AW in which the aluminum cladding comprises 25% of the area of the wire, with a minimum coating thickness of 10% of the overall radius. The reinforcing wires may be in a central core or distributed throughout the cable. Galvanized or aluminized coats are thin, and are applied to reduce corrosion of the steel wires. The conductivity of these thin coated core wires is about 8% IACS. The apparent conductivity of ACSR/AW reinforcement wire is 20.3% IACS.

ACAR - (Aluminum Conductor-Aluminum Alloy Reinforced) - ACAR combines 1350 and 6201 aluminum alloy strands to provide a transmission conductor with an excellent balance of electrical and mechanical properties. This conductor consists of one or more layers of 1350-H19 aluminum strands helically wrapped over one or more 6201-T81 aluminum alloy wires. The core may consist of one or more 6201 strands. The primary advantage of the ACAR conductor lies in the fact that all strands are interchangeable between EC and 6201, thereby permitting the design of a conductor with an optimum balance between mechanical and electrical characteristics. In effect, ACAR is a composite aluminum-aluminum alloy conductor which is designed for each application to optimize properties. Inverse ACAR conductors are also available with the harder 6201 aluminum alloy wires being on the outer surface of the conductor and the 1350 aluminum making up the heart of the conductor.

Bundled Conductors - A bundled conductor arrangement with two or more conductors in parallel, spaced a short distance apart is frequently used for HV and EHV transmission lines. Many electrical reasons can be cited in favor of bundled conductors. From the stand point of current density per unit area, smaller conductors have higher possible current densities, thus greater metal efficiency. The use of multiple conductors per phase having the same total area as a single conductor will operate at lower temperatures yielding lower resistances and losses for equal loads.

Multiple conductors offer significant improvements in reactance over a single conductor of equal area. The inductive reactance of a two conductor bundle is only about 50% of the reactance for a single conductor having the same circular mil area as the bundled pair. Obviously, the greater the spacing between subconductors, the lower the reactance.

Although important, the electrical advantages of bundled conductors may not be the most important factor influencing their use. The concerns of corona and radio noise may dictate the use of bundled conductors since corona loss of a conductor is a function of the voltage gradient at the conductor surface. The subjects of corona and RIV have been well investigated and will not be further discussed here.

The number and size of conductors per phase have not been standardized. It is dependent upon many factors. Today conductor bundles are a standard design practice for transmission lines designed to operate at 345 kV or higher.

Any of the above discussed conductors including VR Cable, can be used as subconductors for bundle conductor designs. This presents the transmission design engineer with limitless design options.