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High Voltage Power Transmission Lines (Overhead Lines)
April 10, 2012

Introduction

In order to transmit high voltage power, there is a need to use a cable that has the necessary qualities for the transmission of large quantities of electricity. Overhead power line entails transmission of electricity using towers. Moreover, another way to transmit electricity is through utility poles. Overhead transmission method tends to be the most commonly used way of transmitting high voltage power because most of the insulation are provided by air. This method is less costly, especially when transmitting large quantities of electricity.

In order to accomplish this obligation, the most efficient cable to use is aluminum. This paper explores aluminum as the best cable to use in high voltage transmission lines and the process used in making the cable.

Aluminum Cable

Properties of Aluminum

Physical and chemical properties of aluminum

Some of the physical properties include that aluminum is a silvery white metal. The metal is also reflective to heat. Moreover, aluminum metal can be produced in various different forms with the help of machines. This means that the metal can have various surface finishes. Another physical property of aluminum is that it is easily recyclable. One of the chemical properties of aluminum is that it is resistance to oxidation. The other chemical property is that the metal is created using an electronic method.

Good conductor of electricity

Aluminum cable tends to be the best to use in high voltage transmission lines compared to cables made from other metals. One of the advantages of using aluminum cable is that it is a good conductor of electricity (Warne, 2005). This means that the metal has a low electrical resistivity. The reason why aluminum is a good conductor of electricity is that it has 3+ charges. This means that aluminum has three delocalized electrons that tend to move freely in each atom of the metal. The relationship between of electricity and aluminum is that when there is an electrical field applied to the metal, every loose electron is able to move freely. This translates that all the loose electrons will move towards the positive terminal where there is the presence of an electric field. Aluminum ends up being a good conduct of electricity hence eligible to use in transmission lines (Warne, 2005). The metal is also a very good thermal conductor.

Light in weight

The other significant importance that makes aluminum be the best cable while using in high voltage transmission lines is its lightweight. Comparing aluminum with other metals like copper, nickel and brass, aluminum tends to be less by weight to about one-third of the others. The other aspect in relation to weight is that aluminum has a specific gravity of 2.7. This means that the metal is very light in weight. Moreover, aluminum cables being of lightweight makes them most efficient for overhead transmission of electricity. Another significant importance of aluminum being lighter is that cables made from the metal require little support.

Economical

Another significant feature about aluminum is that it is economical. Aluminum cables pose as the most economical compared to other cables from other metals. Moreover, the production of aluminum is also economical. Most of the production sites of aluminum tend to be near the sources areas and costs related to transportation are less. Another significant issue related to less cost in aluminum is that the metal is recyclable. Moreover, aluminum being of low cost enables cables made from this metal to move for a longer distance (Vargel, 2004).

Corrosion resistant

The other reason for choosing aluminum as the best cable is that it is resistant to corrosion. Aluminum is able to resist corrosion because of the presence of a thin layer on its surface. The thin layer lining is made of aluminum oxide. Moreover, using technology, this particular layer can be made stronger by anodizing the metal. Anodizing refers converting the metal to the anode. This is done by the electrolysis of dilute sulphuric acid. However, in order to accomplish this step, there is a need to first etch the aluminum with sodium hydroxide solution. This is done in order to remove the existing oxide layer. After electrolyzing the aluminum article, a thick film of oxide is build up that is highly resistance to corrosion.

Ductility and Malleability

The other feature associated with aluminum is that it is highly ductile. Apart from being highly ductile, aluminum has a high aspect of malleability. Both ductility and malleability are properties related to how the possibility of deformation can occur on a particular metal. Aluminum holds both of these properties and seems to be the best while using as electrical cables. In all metals, aluminum is the second malleable one. While in the aspect of being ductile, aluminum holds the sixth position in all metals. Both of the two properties are of significant importance when using in electrical cables. This is because; malleability refers to the ability of a metal to be deformed by compression. Moreover, this process ought to occur without cracking without or rupturing. This feature also translates that it is possible to roll aluminum into several sheets. Aluminum holds a high percentage of this specific feature. Ductile means that aluminum has the ability to be deformed plastically. This process ought to occur without fracture under tensile force. This feature also illustrates that aluminum can be easily drawn into wires. Aluminum also tends to have a high percentage of this feature. Both of these properties make aluminum be the perfect metal for drawing large cables that are recommendable for overhead lines.

Others

Aluminum has another significant property of being a high strength (Fraden, 2010). This makes aluminum cables to be the best in making overhead lines because the high strength helps the cables not to creep. Aluminum metal is also non-magnetic. This property makes aluminum be the best in making cables because they cannot attach to each other in case they are swung by the wind.

Characteristics of Aluminum cables

One of the characteristics of aluminum cables is that they tend to lose some of their strength during the high temperatures. However, even in during these periods, ductility of the metal remains the same as in low temperatures. This feature makes aluminum to the best in cabling even in cold regions. The other characteristic of aluminum cables is that they are able to form a layer of oxide. The importance of these layers is that they are corrosion resistant. When repeatedly used, the cables made from aluminum tend to lose their strength. Therefore, they require extra care when handling.

Process and Manufacturing the Power Transmission Lines Using Aluminum Cables

High-voltage electric transmission entails the transfer of energy form an electric source to various substations. Most of the substations are located near residential places. This network of transferring electricity from the source to the final consumer is generally known as a distribution system. The importance of using overhead line transmission is because the method is less costly. Most of the aluminum conductors were being manufactured from pure aluminum (ECAL) in the early period. In order to make the overhead transmission lines, most of the manufacturers used wire rods. The wire rods were made through the process of hot rolling. The wire rods were also made through extrusion methods. However, nowadays, in order to process and manufacture transmission lines using aluminum cables, a set systematic system is used to make sure that the best product is made.

Material and properties

The most important materials include the recommended composite conductor. The conductor ought to have a number of zirconium strands that are made from aluminum. These specific strands ought to be of high temperatures. Another significant property required is reinforced composite wires that have to be of aluminum oxide. The reinforced wires are covered by the zirconium strands. The significant importance of both the composite wires that make the composite core and outer core aluminum-zirconium (Al-Zr) is that they help in making the transmission lines to have the overall conductor strength and conductivity.

Composite core

The composite core or the inner strands are made of aluminum composite wires. Most of the wires depend on the conductor size and the wire diameters. In most of the time, the wire diameters range from 0.073” (1.9mm) to 0.114 (209mm). One significant feature of the core wires is that they have the stiffness and strength of steel. However, the core wires have little weight and higher conductivity compared to materials made from steel. This is an advantage to of the aluminum materials in making strong and reliable transmission wires. Each of the core wire is composed of large numbers of aluminum oxide fibers. The fibers are small in diameters and they are of ultra-high strength. The other aspect of the composite core is that the ceramic fibers are continuous. They are oriented in the direction of the wire. The ceramic wires are also embedded with high-purity aluminum. The composite wires are different from aluminum itself in strength. Moreover, the wires tend to exhibit various mechanical and physical properties that are of more degree compared to that aluminum.

Outer strands

The outer strands compromise of a temperature resistance alloy. The specific alloy is aluminum-zirconium. This particular alloy is of made of hard aluminum. The other significant feature is that this particular alloy is designed to maintain high strength. This in many cases occurs at high temperatures. The following figure shows how the outer strands of the transmission wire ought to be.

Tensile strength

After finding the necessary and recommended materials, the other process is making laboratory tensile tests. The test strength is made in a gauge facility that is 10ft in length. During this process, there is a need to take considerable care when handling the materials. Moreover, there is also a significant advantage in cutting and preparing the materials in order to ensure that the wires did not slacken. The disadvantage of slackening is that the wires might decrease their strength values. The other issue that is determined while checking the tensile strength is the breaking load. This is usually done by pulling a conductor to a 1000-lb load. Then the load is further loaded to failure at 10000 lbs/min. After the testing the tensile strength, the breaking load ought to reach the recommended Rated Breaking Strength (RBS).

Stress-strain behavior

Another significant issue addressed during the manufacturing process of the transmission lines is the stress-strain behavior. The behavior is determined according to the set standards by the Aluminum Association. The stress-strain behavior tests are started at 1000lbs (4.4KN). During this process, the strain measurement is set at zero. The load is then incrementally increased to a percentage of between 30 and 75 of RBS. Moreover, curve fitting is then applied to the transmission lines (Kaufman, Rooy & American Foundry Society, 2004).

Short Circuit Behavior

This test is conducted in order to determine whether the aluminum cable is able to sustain the compression that might occur in the case of short circuits. The following figure demonstrates the consequences that might occur during a short circuit while using cables other than aluminum.

Axial Impact strength

This is a test usually done to investigate whether there is slippage in conductor terminations. The other significant importance of this test is to investigate whether there is a possibility to sustain the high shock loads (>100% RBS). In most cases, the loads ought to be sustained by the 795-kcmil Composite Conductor. Moreover, this is done under high rate axial loading. Through this particular process, the shock load is comparable to various loading rates. Some the loading rates include those experienced in certain situations like during ice jumps and galloping events. The following figure demonstrates an axial impact.

Crush strength

The crush strength test is usually done to test the full strength retention of the aluminum cable. The test is done on a 795-kcmil Composite Conductor. The main reason for conducting the test is to simulate the possible damage that might occur during the process of shipping and installation. An example of this process is crushing a section of the aluminum cable between 6-inch steel plates for a period of one minute. If the aluminum cable shows any detection of damage, then the cable it is not fit for the application.

Lightening Resistance

This is a significant process usually undertaken during the process of manufacturing overhead transmission lines. A lightening arc is struck across the aluminum cable to determine whether it can be able to resist very severe strikes (Smith, 2008). The following figures demonstrate two types of cables where one is able to resist occurrence of lightening while the other one is not.

Conclusion

After the aluminum cable is able to pass through all the above manufacturing processes, then it eligible for application in overhead transmission lines. Aluminum tends to be the best metal for making transmission cables as illustrated above. Transmission of high-voltage electricity requires cables that are able to resist various manufacture and environmental unhealthy conditions. Through various developments in technology, it is possible to make aluminum cables in an easy way compared to the methods that were used in the past. Moreover, through technology, there is a possibility of extra advancement in making the overhead aluminum cable lines. Aluminum cables pose to be the best compared to other metals.