Transmission Conductors - A Review of the Design and Selection Criteria

F. Ridley Thrash, Jr.Chief Engineer, Overhead ConductorsWire & Cable Technology GroupSouthwire Company

Introduction:

Remarkable changes have occurred in the utility industry since Thomas Edison began the commercial sale of electricity more than 100 years ago. One area that has undergone extensive change has been in the types of conductors available to transmit and distribute electricity. Copper was the first metal used to transmit electricity during the development of the electrical industry in the early 1880's. A review of the selection criteria for transmission and distribution conductors, prior to the extensive use of aluminum, suggests copper conductor sizes were being determined primarily on the basis of mechanical considerations because of the disproportional high conductivity of copper relative to its strength-to-weight ratio. Conductors were, therefore, generally larger than required from the standpoint of efficient electrical conductivity. Because of the weight, span lengths were short, thus increasing the overall cost of the transmission line.

Shortly before the turn of the century, aluminum began to replace copper as the metal of choice for transmission and distribution conductors. The first transmission line using aluminum conductors was constructed in California in 1895, quickly followed by a second line in 1898. The first transmission line using a stranded (7-strand) aluminum cable was constructed by the Connecticut Electric Light Company in 1899 and remained in daily operation for more than 50 years. Starting with these early installations, the use of aluminum electrical conductors has increased steadily until it is the material of choice by transmission line design engineers today. For more than 90 years aluminum has been used by electric utilities for the transmission and distribution of electrical power. Although its almost completely replacing copper for overhead applications. Of all the known nonprecious metals, aluminum ranks second only to copper in volume conductivity. Aluminum possesses a conductivity-to-weight ratio twice that of copper and its strength-to-weight ratio is 30% greater than copper.

When aluminum conductor came into relatively wide use in the early 1900's, experience indicated the need for a conductor with a greater strength-to-weight ratio. Thus, in 1907 a new aluminum-steel composite cable was introduced. This new conductor combined the light weight and high current carrying capacity of aluminum with the high strength of a galvanized steel core. ACSR, as this aluminum conductor, steel reinforced, cable became known, gained rapid acceptance and was used almost exclusively throughout the world until 1939. The excellent conductivity of ACSR, coupled with its excellent strength-to-weight ratio and ease of handling made it the dominant conductor for rural electrification in the United States that began during the early 1920's.

In 1939 a new all aluminum-magnesium-silicon alloy cable was introduced. The new all-aluminum alloy cable (AAAC) was developed to retain the mechanical and electrical properties of ACSR while improving weight and corrosion resistance characteristics. The introduction of the all-aluminum alloy cable and the subsequent development of the composite aluminum conductor, aluminum-alloy reinforced cable provided new alternatives to ACSR. As with most new products, particularily in applications as critical as electrical transmission and distribution, acceptance of the new alloy conductor was slow. In recent years, however, the recognized electrical improvements of alloy conductors over ACSR has led to an increasing trend of usage in aluminum alloy and composite aluminum-aluminum alloy cables.

More recently, many innovative conductor designs have been developed to address the changing needs of the electrical utility industry. New alloys have been developed to provide thermal stability, increased conductivity, vibration resistance and other specific characteristics. With each change there is a compromise. With each compromise there is a new design opportunity.

Conductor design and/or selection for transmission and distribution lines has become a science. The selection of the optimum conductor type and size for a given transmission or distribution line design requires a complete understanding of the characteristics of all the available conductor types. This understanding must encompass more than just the current carrying capability or thermal performance of a conductor. It must include a systems approach to conductor selection: line stability versus current loading; economic operation versus thermal loading; conductor creep and resultant sag under high temperature and adverse mechanical loading; conductor strength as determined by component metal stress-strain performance and metal fatigue characteristics are just a few of the system design parameters to be evaluated.

Types of Conductors:

There is no unique process by which all transmission and/or distribution lines are designed. It is clear, however, that all major cost components of line design depend upon the conductor electrical and mechanical parameters.

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

The various combinations and modifications of these conductor types provide a wide variety of possible conductor designs.

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.

ACSR STRANDINGS

6201 "AAAC" - A high strength Aluminum-Magnesium-Silicon Alloy Cable was developed to replace the high strength 6/1 ACSR conductors. Originally called AAAC, this alloy conductor offers excellent electrical characteristics with a conductivity of 52.5% IACS, excellent sag-tension characteristics and superior corrosion resistance to that of ACSR. The temper of 6201 is normally T81.

6201 aluminum alloy conductors are typically sold as O.D. equivalents for 6/1 and 26/7 ACSR constructions. The O.D. equivalent 6201 conductors have approximately the same ampacity and strength as their ACSR counterparts with a much improved strength-to-weight ratio. 6201 conductors also exhibit substantially better electrical loss characteristics than their equivalent single layer ACSR constructions. However, the thermal coefficient of expansion is greater than that of ACSR. As with AAC conductors, the maximum short circuit temperature of 6201 must be kept below 340°C to prevent dangerous conductor annealing.

As compared to ACSR, AAAC's ligher weight, comparable strength and current carrying capacity, lower electrical losses and superior corrosion resistance have given this conductor wide acceptance as a distribution conductor. It has found limited use, however, as a transmission conductor.

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.

TYPICAL STRANDINGS FOR CONCENTRIC-LAY-STRANDED ACAR CONDUCTORS

AACSR - (6201 Aluminum Alloy Conductor Steel Reinforced) - Is an ACSR with the 1350 aluminum wires replaced by 6201-T81 aluminum alloy wires. The high tensile strength of the 6201-T81 wires combined with the high strength of steel provides an exceptionally high strength conductor with good conductivity. AACSR conductors have approximately 40% to 60% more strength than comparable standard ACSR conductors of equivalent stranding, with only an 8-10% decrease in conductivity. AACSR is available with all core types specified for use with standard ACSR.SSAC - (Steel Supported Aluminum Cable) - SSAC conductor was designed for use as a replacement conductor in upgrading existing transmission and distribution lines with minimum capital outlay. The premise of design is higher conductor operating temperature without detrimental annealing of the aluminum in standard ACSR causing a loss of strength in the aluminum. SSAC conductor is an aluminum-steel composite conductor resembling standard ACSR in appearance, stranding and overall diameter. This is the extent of their similarities however. SSAC uses 1350-0 (fully annealed) aluminum strands with 63.0% conductivity rather than the traditional 1350-H19 hard drawn aluminum used in standard ACSR which possesses 61.2% IACS conductivity. The steel core may be made of conventional or extra high strength steel core wire. Compared to an equal size ACSR, SSAC has a lower resistance, lower breaking strength, lower creep elongation and lower elastic modulus. SSAC can be operated at temperatures as high as 250°C without loss of strength and can be strung at higher unloaded percentage tensions because of its good self damping characteristics.

SSAC has seen limited use in the United States. Even though SSAC has better conductivity, a higher operating temperature and improved damping characteristics when compared to conventional ACSR, it has a lower breaking strength, typically yielding greater initial and final sags. It is, however, a good conductor to consider for line upgrades if the calculated present worth of electrical losses shows a savings over line conversion cost.

Expanded ACSR - This conductor is designed to be used where large diameter single conductors are required to reduce the electrical stress gradient at the surface of the conductor to provide corona-free operation. Expanded ACSR is used when a single conductor rather than a conductor bundle is used at EHV voltage levels. Expanded ACSR is specially fabricated to have a larger outside diameter than could be achieved using the circular mil area of aluminum required. Expansion is achieved by the use of oversized wires widely spaced in successive wire layers near the core. Expansion has also been achieved by the use of extruded metal shapes and various rope, paper or jute fillers. Expanded conductors can offer improved sag characteristics as well as efficient design. Because of the precise fabrication techniques required to manufacture expanded conductors and a history of installation problems, these conductors have not been widely used.

Smooth Body Conductors:

Some cables are designed to produce a smooth outer surface and reduce overall diameter. This smaller diameter reduces the ice and wind loading encountered during severe weather, thereby reducing the pole/tower loading or allowing longer design spans. Smooth body conductors are of two types - compact conductors or trapezoidal shaped wire compact conductors, i.e., TW conductors.Compact Conductors - Compact overhead conductors are typiclly available in both AAC and ACSR with diameter reductions ranging from 8% to 11%. AAC conductors are available in a size range of #8 AWG through 1000 kcmil with standard stranding as listed in ASTM. Compact ACSR conductors are available only in sizes #6 AWG through 336.4 kcmil in constructions with a single steel core wire.

Compact conductors are manufactured by passing the stranded cable through powerful compacting rolls or a compacting die. The strands are deformed, to the degree they loose their circularity, partially filling the interstrand voids and the outer surface of the conductor becomes a relatively smooth cylinder. The resulting reduction in overall diameter not only reduces the ice and wind loading characteristics of the conductor but also reduces the stress gradient at the conductor surface.

150% / 200% ACSR - The terms 150% and 200% ACSR refer to a family of single layer (6/1) constructions of ACSR that have 150% and 200% of the strength of the equivalent construction standard ACSR while exhibiting approximately the same overall diameter. The 150% and 200% smooth body ACSR was developed to provide a conductor with a substantial increase in ultimate strength as compared to standard 6/1 ACSR constructions. This is accomplished by using a larger steel core wire and drastically flattening the aluminum strands to create a smooth cylindrical conductor surface.

150% and 200% smooth body ACSR is fabricated by passing the composite stranded cable through a die or rolls so designed to flatten the aluminum strands and fill the interstices which exist in conventional stranded ACSR. This brings about a reduction in overall cable diameter which means a lower ice and wind load and greater strength to loaded weight ratio.

These conductors were primarily designed for use on rural distribution lines. The reduced diameter and extra high strength provide substantial design and operational advantages for the longer spans of a rural distribution line serving sparcely populated areas subject to severe cold weather conditions.

Trapezoidal Shaped Wire Conductors - Shaped wire compact conductors made from trapezoidal (TW) shaped wires is a relatively new conductor design. These conductors can be provided in AAC, AAAC and ACSR constructions and are designated as types AAC/TW, AAAC/TW and ACSR/TW.

Conventional conductor designs have traditionally used round wires. The use of technology to design and produce trapezoidal wires (TW) provides conductor designers with an alternative to conventional round strand conductor designs. The use of trapezoidal wire designs yields compact conductors with less void area and a reduced outside diameter.

With conventional ACSR strandings, the number of aluminum and steel strands uniquely define the ratio of steel area to aluminum area. For example, all 26/7 ACSR constructions have the same ratio of steel area to aluminum area of about 16%. However, with TW strands the number of aluminum and steel strands do not necessarily define a unique steel to aluminum ratio. Therefore the designation of "type" has replaced the stranding designation to more accurately identify TW conductors. For example a 795 kcmil-26/7 ACSR "Drake" has a TW counterpart designated 795 kcmil Type 16, ACSR/TW. The aluminum area and steel area of both conductors are identical. The use of TW shaped aluminum strands will cause the ACSR/TW to have a smaller diameter.

The following table relates ACSR/TW Type Number with standard conventional ACSR stranding.

COMPARISON OF ACSR/TW WITH EQUIVALENT STRANDING OF ACSR

ACSR/TW TYPE  CONVENTIONAL ACSR 

Number*  Stranding* 

5  42/7 

7  45/7

8  84/19 

10  22/7 

13  54/7 

13  54/19 

15  26/7 

COMPARISON OF ACSR/TW WITH EQUIVALENT STRANDING OF ACSR ACSR/TW TYPE CONVENTIONAL ACSR Number* Stranding 5 42/7 7 45/7 8 84/19 10 22/7 13 54/7 13 54/19 15 26/7

*ACSR/TW type number is the approximate ratio of the steel area to the aluminum area in %.

An alternate design concept is to specify ACSR/TW conductors with equivalent overall diameters to conventional ACSR constructions. In this case, the diameter is matched to that of the standard ACSR while maintaining the same ratio of steel to aluminum by area. Since the aluminum area is increased, the steel area must be increased to maintain the proper area ratio.

If a reduced diameter TW construction is selected, the diameter is reduced by approximately 10% thereby reducing the design ice and wind loading on the conductor. If an equal diameter TW construction is selected, the aluminum area is increased by approximately 20% - 25% providing a decrease in AC resistance of 15% - 20% and increasing the current carrying capacity 8% to 10%.

The use of trapezoidal wires provides a more compact conductor design with mechanical properties at least equal to that of conventional ACSR. Since ACSR/TW designs have the same steel-to-aluminum ratios as their equivalent ACSR constructions, stress-strain and creep data developed for conventional strandings of ACSR can be used to predict sag and tension design data for ACSR/TW conductor constructions.

Types of Overhead Conductors  

Properties of Overhead Bare Conductors:Current Carrying Capacity

Strength

Weight

Diameter

Corrosion Resistance

Creep Rate

Thermal Coefficient of Expansion

Fatigue Strength

Operating Temperature

Short Circuit Current/Temperature

Thermal Stability

Cost

Categories of Overhead Conductors:Homogeneous Conductors:

Copper

AAC( All Aluminum Conductor)

AAAC (All Aluminum Alloy Conductor)

The core consists of a single strand identical to the outer strands. Since all the strands are the same diameter, one can show

that the innermost layer always consists of 6 strands, the second layer of 12 strands, etc., making conductors having 1, 7, 19,

37, 61, 91, or 128 strands.

Non Homogeneous Conductors: ACAR (Aluminum Conductor Alloy Reinforced)

ACSR (Aluminum Conductor Steel Reinforced)

ACSS (Aluminum Conductor Steel Supported)

AACSR (Aluminum Alloy Conductor Steel Reinforced.

the strands in the core may or may not be of the same diameter. In a 30/7

ACSR conductor the aluminum and steel strands are of the same diameter. In a 30/19

ACSR they are not. Within the core or within the outer layers, however, the number of strands always increases by 6 in each

succeeding layer. Thus, in 26/7 ACSR, the number of layers in the inner layer of aluminum is 10 and in the outer layer 16

Categories of Overhead Conductors VR (Vibration Resistance)

Non-Specular

ACSR / SD• (Self Damping)

Choices of overhead depend upon:Power Delivery Requirements

Current Carrying Capacity

Electrical Losses

Line Design Requirements Distances to be Spanned

Sag and Clearance Requirements

Environmental Considerations Ice and Wind Loading

Ambient Temperatures

(1) AAC (All Aluminum Conductors) AAC is made up of one or more strands of hard drawn 1350 Aluminum Alloy.

AAC has had limited use in transmission lines and rural distribution because of the long spans utilized.

Good Conductivity -61.2% IACS

Good Corrosion Resistance

High Conductivity to Weight Ratio.

Moderate Strength

Typical Application Short spans where maximum current transfer is required.

The excellent corrosion resistance of aluminum has made AAC a conductor of choice in coastal areas.

Because of its relatively poor strength-to-weight ratio, AAC has seen extensive use in urban areas where spans are usually

short but high conductivity is required. These conductors are used in low, medium and high voltage overhead lines.

(2) AAAC (All Aluminum Alloy Conductors) AAAC are made out of high strength Aluminum-Magnesium-Silicon alloy.

AAAC with different variants of electrical grade Alloys type 6101 and 6201.

These conductors are designed to get better strength to weight ratio and offers improved electrical characteristics, excellent sag-

tension characteristics and superior corrosion resistance when compared with ACSR. Equivalent aluminum alloy conductors have approximately the same ampacity and strength as their ACSR counterparts with a

much improved strength-to-weight ratio, and also exhibit substantially better electrical loss characteristics than their equivalent

single layer ACSR constructions. The thermal coefficient of expansion is greater than that of ACSR. As compared to conventional ACSR, lighter weight, comparable strength & current carrying capacity, lower electrical losses and

superior corrosion resistance have given AAAC a wide acceptance in the distribution and transmission lines.

Features High strength to weight ratio

Better sag characteristics

Improved electrical properties

Excellent resistance to corrosion

Specifications

Higher Tensile Strength

Excellent Corrosion Resistance

Good Strength to Weight Ratio

Lower Electrical Losses

Moderate Conductivity –52.5% IACS

Typical Application Transmission and Distribution applications in corrosive environments, ACSR replacement.

(3)  ACAR (Aluminum Conductor Al. Alloy Reinforced) Aluminum Conductor Alloy Reinforced (ACAR) is formed by concentrically stranded Wires of Aluminum 1350 on high strength

Aluminum-Magnesium-Silicon (AlMgSi) Alloy core. The number of wires of Aluminum 1350 & AlMgSi alloy depends on the cable design.

Even though the general design comprises a stranded core of AlMgSi alloy strands, in certain cable constructions the wires of

AlMgSi Alloy strands can be distributed in layers throughout the Aluminum 1350 strands. ACAR has got a better mechanical and electrical properties as compared to an equivalent conductors of ACSR,AAC or AAAC.

A very good balance between the mechanical and electrical properties therefore makes ACAR the best choice where the

ampacity , strength , and light weight are the main consideration of the line design. These conductors are extensively used in overhead transmission and distribution lines.

Features Improved strength to weight ratio

Improved mechanical properties

Improved electrical properties

Excellent resistance to corrosion Specifications

Balance of Mechanical & Electrical

Excellent Corrosion Resistance

Variable Strength to Weight Ratio

Higher Conductivity than AAAC

Custom Designed, diameter equivalent to ACSR most common.

Typical Application Used for both transmission and distribution circuits.

(3) AACSR – Aluminum Alloy Conductor Steel Reinforced AACSR is a concentrically stranded conductor composed of one or more layers of Aluminum-Magnesium-Silicon alloy wire

stranded with a high-strength coated steel core. The core may be single wire or stranded depending on the size. Core wire for AACSR is available with Class A, B or C

galvanizing; or aluminum clad (AW). Additional corrosion protection is available through the application of grease to the core or infusion of the complete cable with

grease.

Features Offers optimal strength for line design

Improved strength to weight ratio

Ideal for extra long spans and heavy load conditions

Excellent resistance to corrosion

(4) ACSS – Aluminum Conductors Steel Supported. ACSS is a composite concentric-lay stranded conductor with one or more layers of hard drawn and annealed 1350-0 aluminum

wires on a central core of steel. In an ACSS ,under normal operating conditions, the mechanical load is mainly derived from the steel core as aluminum in fully

annealed stage does not contribute much towards the mechanical strength. Steel core wires are protected from corrosion by selecting an appropriate coating of the wire like galvanizing, mischmetal alloy

coating or aluminum clad. The type of coating is selected to suit the environment to which the conductor is exposed and

operating temperature of the conductor

ACSS are suitable for operating at high temperature without losing the mechanical properties.

The final sag-tension performance is not affected by the long term creep of aluminum.

Features Improved conductivity

High current carrying capacity

Very low sag at high temperature

High degree of immunity to vibration fatigue

Better self damping property

(6) ACCC – Aluminum Conductor Composite Core Aluminum Conductor Composite Core (ACCC) is a concentrically stranded conductor with one or more layers of trapezoidal

shaped hard drawn and annealed 1350-0 aluminum wires on a central core of high strength Carbon and glass fiber composite. The ACCC Conductor uses a carbon fiber core that is 25% stronger and 60% lighter than a traditional steel core.

This allows with the help of trapezoidal shaped strands the ability to increase the conductor’s aluminum content by over 28%

without increasing the conductor’s overall diameter or weight.

Features Excellent Sag properties

Increased current carrying capacity

High operating temperature

Excellent strength to weight ratio

Highly energy efficient.

(7)  ACSR (Aluminum Conductor Steel Reinforced) Aluminum Conductor Steel Reinforced (ACSR) is concentrically stranded conductor with one or more layers of hard drawn 1350-

H19 aluminum wire on galvanized steel wire core. The core can be single wire or stranded depending on the size.

Steel wire core is available in Class A ,B or Class C galvanization for corrosion protection.

Additional corrosion protection is available through the application of grease to the core or infusion of the complete cable with

grease. The proportion of steel and aluminum in an ACSR conductor can be selected based on the mechanical strength and current

carrying capacity demanded by each application. ACSR conductors are recognized for their record of economy, dependability and favorable strength / weight ratio. ACSR

conductors combine the light weight and good conductivity of aluminum with the high tensile strength and ruggedness of steel. In line design, this can provide higher tensions, less sag, and longer span lengths than obtainable with most other types of

overhead conductors. The steel strands are added as mechanical reinforcements.

ACSR conductors are recognized for their record of economy, dependability and favorable strength / weight ratio.

ACSR conductors combine the light weight and good conductivity of aluminum with the high tensile strength and ruggedness of

steel. In line design, this can provide higher tensions, less sag, and longer span lengths than obtainable with most other types of

overhead conductors. The steel strands are added as mechanical reinforcements.

The cross sections above illustrate some common stranding.

The steel core wires are protected from corrosion by galvanizing.

The standard Class A zinc coating is usually adequate for ordinary environments.

For greater protection, Class B and C galvanized coatings may be specified.

The product is available with conductor corrosion resistant inhibitor treatment applied to the central steel component.

Features High Tensile strength

Better sag properties

Economic design

Suitable for remote applications involving long spans

Good Ampacity

Good Thermal Characteristics

High Strength to Weight Ratio

Low sag

High Tensile Strength

Typical Application Commonly used for both transmission and distribution circuits.

Compact Aluminum Conductors, Steel Reinforced (ACSR) are used for overhead distribution and transmission lines.

(8) Trap Wire Constructions AAC/TW  (Trapezoidal Shaped 1350-H19 Aluminum Strands)

ACSR/TW (Trapezoidal Shaped 1350-H19 Aluminum Conductor -Galvanized –Zinc or AW Coated Steel Core Wires)

ACSS/TW (Trapezoidal Shaped 1350-O Aluminum Conductor-Zinc –5% Mischmetal Aluminum Alloy or AW Coated Steel Core

wires)

Comparison of ACSR/TW Type Number with Equivalent Stranding of ACSRType Number                                        Conventional ACSR Stranding

3                                                          36/1

5                                                          42/7

6                                                          18/1

7                                                          45/7

8                                                          84/19

10                                                         22/7

13                                                         54/7

13                                                         54/49

13                                                         24/7

16                                                         26/7 The equivalent stranding is that stranding of conventional ACSR that has the same area of aluminum and steel as a given

ACSR/TW type. The ACSR/TW type number is the approximate ratio of the area of steel to the area of aluminum in percent.

(8-a) ACSR/AS – Aluminum Conductor, Aluminum Clad Steel Reinforced ACSR/AS or ACSR/AWare concentrically stranded conductors with one or more layers of hard drawn 1350-H19 aluminum wires

on Aluminum Clad steel wire core. The core can be single wire or stranded depending on the size.

The mechanical properties of ACSR/AS conductors are similar to ACSR conductors but offers improved ampacity and

resistance to corrosion because of the presence of aluminum clad steel wires in the core. These conductors are better replacement for ACSR conductors where corrosive conditions are severe.

Features Good mechanical properties

Improved electrical characteristics

Excellent corrosion resistance

Better Sag properties

(8-b) ACSS/AW – Aluminum Conductors –Aluminum Clad Steel Supported ACSS/AW or ACSS/AS is a composite concentric-lay stranded conductor with one or more layers of hard drawn and annealed

1350-0 aluminum wires on a central core of aluminum clad steel core. In an ACSS/AW ,under normal operating conditions, the mechanical load is mainly derived from the steel core as aluminum in

fully annealed stage does not contribute much towards the mechanical strength. Aluminum Clad steel has got an excellent resistance towards corrosion.

ACSS/AW are can be safely operated upto 250oC continuously without losing the mechanical properties.

The final sag-tension performance is not affected by the long term creep of aluminum.

Features Improved conductivity

High current carrying capacity

Suitable for high temperature

Excellent corrosion resistance

Very low sag at high temperature

High degree of immunity to vibration fatigue

Better self damping property

(8-c) ACSR/TW – Trapezoidal Shaped 1350-H19 wire Aluminum Conductor, Steel-Reinforced

Shaped Wire Compact Concentric-Lay-Stranded Aluminum Conductor, Steel-Reinforced (ACSR/TW) is a concentrically

stranded conductor , made with trapezoidal shaped 1350-H19 wires over a high strength steel core. There are two possible design variants. In one case ACSR/TW conductors are designed to have an equal aluminum cross

sectional area as that of a standard ACSR which results in a smaller conductor diameter maintaining the same ampacity level

but reduced wind loading parameters. In the second design, diameter of the conductor is maintained to that of a standard ACSR which results in a significantly lower

conductor resistance and increased current rating with the same conductor diameter. manufactures ACSR/TW with Galvanized steel ( in Class A, Class B & Class C), Zn-5Al mischmetal coated steel or Aluminum

clad steel core.

Features High Tensile strength

Better sag properties

Reduced drag properties

Low wind and ice loading parameters

suitable for remote applications involving long spans

(8-d) ACSS/TW – Shaped Wire Aluminum Conductors Steel Supported Shaped Wire Compact Concentric-Lay-Stranded Aluminum Conductor, Steel-Supported (ACSS/TW) is a concentrically stranded

conductor with one or more layers of trapezoidal shaped hard drawn and annealed 1350-0 aluminum wires on a central core of

steel. ACSS/TW can either be designed to have an equal aluminum cross sectional area as that of a standard ACSS which results in a

smaller conductor diameter maintaining the same ampacity level but reduced wind loading parameters or with diameter equal to

that of a standard ACSS which results in a significantly higher aluminum area, lower conductor resistance and increased current

rating.

ACSS/TW is designed to operate continuously at elevated temperatures, it sags less under emergency electrical loadings than

ACSR/TW, excellent self-damping properties, and its final sags are not affected by long-term creep of aluminum. ACSS/TW also provides many design possibilities in new line construction: i.e., reduced tower cost, decreased sag, increased

self-damping properties, increased operating temperature and improved corrosion resistance. The coating of steel core is selected to suit the environment to which the conductor is exposed and operating temperature of the

conductor.

Features High Operating temperature

Improved current carrying capacity

Better sag properties

Excellent self-damping properties

Reduced drag properties

Low wind and ice loading parameters

Decide Number of Conductor and Layer of Conductor: If N: number of conductors [strands], d: Diameter of strands, ,X: number of layers.

Usually the relation between N&X take as followed.

N= 3X2-3X+1 If N is given we can used the above relation get X, then we can get the total Diameter of cable as

dT= (2X-1)d. If Total Number of Conductor (N)=19 Than 19=3×2-3x+1. So Number of Layer (x)=3

Than Diameter of Cable dT = (2x-1)d =5d

What is the history behind the ACSS/TW Product? In 1974, Reynolds Metals patented the ACSS conductor design. Its original name was Steel Supported Aluminum Conductor

(SSAC). The original patents have expired and the product is now known as ACSS. There are currently three major North

American conductor manufacturers that offer ACSS products both round wire and trapezoidal wire (TW). The TW enhancement to ACSS was transferred from existing technology developed for ACSR (Aluminum Conductor Steel

Reinforced) and AAC (All Aluminum Conductor) TW conductors. ACSS/TW is typically manufactured to meet the aluminum

cross-sectional area of a standard round conductor, but allows the overall diameter to be reduced by approximately 10 percent.

ACSS/TW can also be manufactured to meet the existing diameter of a standard conductor, incorporating 20 percent to 25

percent more aluminum cross-sectional area.

What does ACSS or ACSS/TW look like? From the outside, ACSS and ACSS/TW conductors look like traditional ACSR. All are manufactured with steel cores and

aluminum outer strands. The key difference is that the ACSR aluminum is made from hard drawn aluminum, while ACSS uses

soft aluminum (i.e. annealed, or “O” temper). In the ACSS/TW trapezoidal conductor, the aluminum strands are not round but

trapezoidal shaped.

What is so special about using annealed aluminum strands? Both ACSR and ACSS conductors are made from two different metals-aluminum and steel. Consequently, the composite

conductor behavior is determined by the combined electrical and mechanical properties of the two materials that make up the

conductor. Although ACSR and ACSS are made with 1350 alloy aluminum, their electrical and mechanical properties are very

different. Electrically, the conductivity of hard drawn aluminum in ACSR is 61.2 percent; whereas, soft aluminum has a conductivity of 63

percent relative to copper (100 percent). This means that the soft aluminum in ACSS is more efficient at transporting power.

Mechanically, the tensile strength (resistance to breaking) of hard drawn aluminum in ACSR is approximately three times that of

soft aluminum. This means that the aluminum in ACSS conductor contributes much less to the overall strength, and the

composite conductor behaves more like steel.

What are the consequences of elevated conductor temperature on ACSR? When ACSR conductors are operated at temperatures in excess of approximately 93 C, the aluminum starts to anneal. The

annealing weakens the conductor and can potentially cause the conductor to break under high wind or ice conditions. To

prevent this from happening, utilities generally limit conductor temperatures to 75 C for an ACSR conductor. ACSS/TW and ACSS conductors are manufactured using soft (annealed) aluminum, where operation at higher temperatures

has no further effect on the aluminum’s tensile strength. Compared to regular ACSR, predictable installation parameters can be

calculated for the ACSS/TW conductors to take into consideration the sag and tension performance at the higher temperatures.

What is the temperature rating of ACSS? The original temperature limit of 200 C has been in existence for almost 30 years and has proven itself. This was based on a

245 C temperature limit established by steel core manufacturers for the galvanized coating of the steel. Operation of the ACSS

product at higher temperature (e.g. 250 C) warrants the use of an enhanced type of galvanizing, which provides more durable

high temperature endurance performance (Misch Metal-zinc/aluminum alloy coating). Another option for high temperatures is

aluminum clad steel.

How high can the operating temperature realistically go? Theoretically, the 250 C rating would provide the ability to carry more power through transmission lines. However, the question

must be asked, “Is it wise to operate an electrical system at that high of a temperature?” The amount of electrical current passing through the conductor combined with environmental conditions determines the

operating temperature of the conductor. Electrical current causes the following: A) The higher the current, the hotter the conductor and the greater the power losses. Ideally, lines are designed to minimize

these power losses and keep normal day-to-day power loads well below the 200 C operating temperature limits. B) The hotter the conductor, the more it will sag and to compensate, the use of larger and/or stronger structures would be

required. C) Electrical current also passes through the conductor joints (splices) and end fittings (dead ends), forming “weak links” that

can mechanically and electrically fail because of overheating. Conductor supports and insulators also become more susceptible

to failure. To sum things up, pushing the temperature limit to 250 C remains an unproven condition.

What are the best applications for use of the ACSS and ACSS/TW products? System reliability issues push the need for the use of ACSS. Utilities are being pressured to demonstrate system reliability. The

ACSS/TW conductor could enable a tremendous emergency load carrying capability that the utility could call upon when

needed. Cyclic Loads and Peak Demand can be accommodated using ACSS/TW because it can operate at temperatures higher than

ACSR. ACSS/TW enables utilities to plan for future situations of increased power requirements because ACSS/TW has power

carrying capacity already built into the system. Utilities can also turn to ACSS products in situations where they need additional power capacity along existing right-of-ways, but

are facing the environmental challenges of building new lines. The ACSS/TW reconductoring option may be the only solution

available to upgrade lines with minimal changes along existing routes.

TW conductor installation requires no special tools, equipment or training.

ACSR TRAPEZOIDAL SHAPED WIRE CONDUCTOR

Vibration Resistant Conductor Designs:

VR Conductor - A wind induced motion resistant conductor, VR conductor is designed for use as a bare overhead conductor in areas subject to aeolian vibration and galloping due to wind and ice. Use of this conductor allows it to be strung to the maximum allowable NESC design tensions without the need for additional vibration protection.

VR conductor is composed of two identical conductors twisted together with a nine-foot left-hand lay, giving the conductor a spiraling "figure 8" shape. This spiraled shaped disrupts the forces created by steady cross winds by presenting a continuously changing projected conductor diameter to the wind. By disrupting the forces created by turbulent wind flow, conductor vibration is prevented. This unique spiral shape, together with less torsional stiffness and varying bending stiffness also reduces or eliminates conductor galloping due to combined ice and wind loads.

VR conductor can be made of component conductors of AAC, AAAC, ACAR, ACSR, AAC/TW or ACSR/TW meeting the appropriate requirements. The type component or subconductor selected should be based on strength and thermal requirements. Constructions are available in all conductor sizes and are suitable for both distribution and transmission requirements.

VR conductor is typically manufactured and sold as an alternate to standard round conductor. The total circular mil area of both component conductors equals the circular mil area of the VR construction. Conductor constructions are normally referred to by the registered code name of the component conductors followed by the VR designation, i.e., "Ibis/VR".

VR CONDUCTOR

ACSR/SD - Sometimes called SDC (Self Damping Conductor), ACSR/SD 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 aluminum wires. One or more layers of round aluminum wires may be added as required.

Self damping conductor differs from conventional ACSR in that the aluminum wires in the first two layers are trapezoidal shaped and sized so that each aluminum 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 form the steel core by the two smaller annular gaps that permit movement between the layers. The round aluminum wire layers are in tight contact with each other and the underlying trapezoidal wire layer.

ACSR/SD has been very effective in reducing aeolian vibration on transmission lines. However, most contractors charge a premium for installation because of special hardware requirements and specialized stringing methods.

SELF DAMPING CONDUCTOR

Bundled Condctors - 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.

BUNDLED CONDUCTOR

Surface Finishes:

The surface of a conductor must be relatively clean and smooth to perform satisfactorily as an electrical conductor. However, special surface treatments or finishes may be required to reduce reflectivity or impart other desired special appearance, or in some cases aerodynamic, characteristics to a conductor or conductor assembly. The most common surface treatment and one normally required for conductors used for transmission and distribution lines crossing undeveloped Federal Government park lands is one to reduce the reflectivity of aluminum conductors. This type of surface finish is referred to as non-specular.

NON-SPECULAR CONDUCTOR - The term non-specular is used to infer that the surface of an aluminum conductor, any type aluminum conductor, has been either mechanically or chemically treated to produce reduced reflectivity. The conductor surface must have a smooth matte gray finish which blends naturally and unobtrusively with the environment.

This non-specular finish is typically achieved by passing the finished conductor through a deglaring machine (a type of sandblast machine) in which the conductor surface is blasted with a very fine mild abrasive grit producing a dull matte gray finish. The reflectivity and color of the finished cable is specified by ANSI C7.69 Specifications.

The abrasive action of the blast media is extremely mild and in no way affects the mechanical characteristics of the conductor. The ampacity of current carrying capability of non-specular conductors is slightly increased because the emissivity of the conductor is increased from approximately 0.23, for bright shiny conductors, to approximately 0.42 because of the darker matte gray surface. An increase in current carrying capacity in the range of 5% can be achieved, for the same temperature rise, due to this increase in surface emissivity.

Other surface finishes providing benefits such as improved aerodynamic characteristics have been reported. The merits of such finishes must be evaluated to determine if lasting economic benefits exist.

Conclusion:

The selection of the optimum conductor type and size for a given line consists of finding that conductor which results in the lowest present net worth cost spread over the life of the line. The transmission line design engineer is confronted with choosing a conductor type from among this bewildering assortment. This choice must be based on basic conductor parameters.

It is clear that 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

These basic parameters are not necessarily independent of one another. However, certain parameters can be varied independently over a range of design considerations.

It is the hope of this writer that a better understanding of available conductor types and materials will provide a better base for future conductor selections.