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 it’s almost completely replacing copper for overhead applications. Of all the known non precious 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, particularly 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, 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.
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.
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:
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.
REFERENCES:
1. Douglass, Dale A., Economic Measures of Bare Overhead Conductor Characteristics, IEEE Paper 86 TD 502-9 PWRD.
2. Kennon, Richard E., Douglass, Dale A., EHV Transmission Line Design Opportunities for Cost Reduction, IEEE Paper 89 TD 434-2 PWRD.
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 it’s almost completely replacing copper for overhead applications. Of all the known non precious 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, particularly 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
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.
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.
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
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.
REFERENCES:
1. Douglass, Dale A., Economic Measures of Bare Overhead Conductor Characteristics, IEEE Paper 86 TD 502-9 PWRD.
2. Kennon, Richard E., Douglass, Dale A., EHV Transmission Line Design Opportunities for Cost Reduction, IEEE Paper 89 TD 434-2 PWRD.
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