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Library of Congress Cataloging-in-Publication Data Electrical power cable engineering / edited by William A. Thue p. cm.- (Power engineering; 7) includes index. Electrical Power Cable Engineering - CESC | Power Utility. Pages · · Electric Power Generation, Transmission, and Distribution. Edited by The . development of the extensive underground power systems in operation Petersburg by using an electrical pulse sent through a cable insulated with strips.
The requirement for the length of lay is set forth in ASTM specifications, [, to be not less than 8 times nor more than 16 times the overall diameter OD of that layer. In , W. Vast improvements in the materials and processing of extruded, medium voltage power cables in the s has led to cables that can be expected to function for 30,40, or perhaps even 60 years when all of the proper choices are utilized. Sine moisture penetmtion is the key to inducing crosslinking, it is apparent that the process becomes more efficient as the wall thickness is reduced. It should be noted that not all peroxides will decompose and induce crosslinking over the same temperatureranges. The use of shielded cable permits using cables that are more economic to manufacture and install as compared to non-shielded cables that would require very heavy insulation thickness. The absorption current normally is relatively small and decreases with time.
In Detroit, a cable had been installed in Dorsett conduit, but later abandoned. Some of these mains were still in use at the original voltage after more than 50 years.
The cables consisted of two concentric conductors insulated with wide strips of paper applied helically around the conductor and saturated with a rosin based oil. The insulated conductors were forced into a lead pipe and installed in 20 foot lengths.
These mains were not flexible and were directly buried in the ground. Soon after, cables insulated with narrow p p e r strips saturated in a rosin compound and covered with a lead sheath very similar in design to those in use at the present time were manufactured in the United States by the Norwich Wire Company.
These were the fhsl flexible paper-insulated cables, and all subsequentprogress has been made through improvementsin the general design. Paper insulated cables were improved considerably with: This cable is still known as Type H. This permitted the voltages to be raisedto 69 kV and higher. Fisher and R. Atkinson revealed that the dielectric strength of impregnated paper-insulated cable could be greatly increased by maintaining it under pressure. This system was not used until the commercial installation of a psi cable in London.
Impregnated paper became the most common form of insulation for cables used for bulk Vansmission and distribution of electrical power, particularly for operating voltages of Impregnated paper insulation consists of multiple layers of paper tapes, each tape from 2. The total wall of paper tapes is then heated, vacuum dried, and impregnated with an insulating fluid. Originally, most of the paper used was made from Manila-rope fiber. This was erratic in its physical properties and not always susceptible to adequate oil penetration.
Increased knowledge of the chemical treatment of the wood in order to obtain pure cellulose by the adjustment of the fiber content and removal of lignin , the control of tear resistance, and the availability of long fiber stock resulted in the almost universal use of wood pulp paper in cables after The impregnating compound was changed from a rosin-based compound to a pure mineral oil circa , or oil blended to obtain higher viscosity, until polybutene replaced oil circa Paper insulated, lead-covered cables were the predominant primary cables of all the large, metropolitan distribution systems in the United States, and the rest of the world, throughout the twentieth century.
Their reliability was excellent. It was. A shift towatds extruded dielectric cables began about in those metropolitan areas, but the majority of the distribution cables of the large cities remain paper insulated,lead-covered cables as the century ends.
Impregnated paper insulation has excellent electrical properties, such as high dielectric strength, low dissipation factor, and dielectric loss. Because of these properties, the thickness of impregnated paper insulation was considerably less than for rubber or varnished cambric insulations for the same working voltages.
There are two major types of extruded dielectric insulation in wide use today for medium voltage cables: Thermoplastic polyethylene PE , which was widely used through the , was introduced during World War I1 for high-frequency cable insulation. PE was furnished as 15 kV cable insulation by Post-warURD systems were basically the same as the earlier systems except that there were two directions of feed the loop system. The pre systems were very expensive because they utilized such items as paper insulated cables, vaults, and submersible transformers.
Expressed in terms of buying power at that time, you could buy a luxury car for the same price! Underground service was, therefore, limited to the most exclusive housing developments. But for three developments in the ,the underground distribution systems that exist today might not be in place.
First, in , a large midwestem utility inspired the development of the pad-mounted transformer; the vault was no longer necessary nor was the submersible transformer. Second, the polyethylene cable with its concentric neutral did not require cable splicers, and the cable could be directly buried. While possibly not as revolutionary, the loadbreak elbow separable connector allowed the transformer to be built with a lower, more pleasing appearance. The booming American economy and the environmental concerns of the nation made underground power systems the watchword of the Great Society.
In a decade, URD had changed from a luxury to a necessity.
The goal for the utility engineer was to design a URD system at about the same cost as the equivalent overhead system. This time it did not have the same meaning as with paper insulated cables. See for additional information on treeing. Crosslinked polyethylene exhibited a much lower failure rate that was not escalating nearly as rapidly. Data from Europe confirmed the same facts . The realization of the magnitude and significance of the problem led to a series of changes and improvements to the primary voltage cables: It was known to be superior to butyl rubber for moisture resistance, and could be readily extruded.
It was used with tape shields, which achieved their semiconducting properties because of carbon black. By , virtually all of the URD installations consisted of polyethylene-insulated medium voltage cables.
The higher the molecular weight, the better the electrical properties. The highest molecular weight PE that could be readily extruded was adopted.
Jacketed cotlsttllction was seldom employed at that time. Extruded thermoplastic sluelds were introduced between and leading both to easier processing and better reliability of the cable Crosslinked polyethylene XLPE was first patented in for a filled compound and in for unfilled by Dr.
Frank Precopio. It was not widely used because of the tremendous pressure to keep the cost of URD down near the cost of an overhead system. This higher cost was caused by the need for additives crosslinking agents and the cost of manufacturing based on the need for massive, continuous vulcanizing CV tubes.
EPR ethylene pmpylene rubber was introduced at about the same time. The significantly higher initial cost of these cables slowed their acceptance for utility purposes until the s. In order to facilitate removal for splicing and terminating, those early era XLPE cables were m a n d m with thermoplastic insulation shields as had been used over the HMWPE cables. A reduction in ampcity was required until deformation resistant and then crosslinkable insulation shields became available during the later pact of the s.
A two-passextrusion process was also used where the conductor shield and the insulation were extruded in one pass. The unfinished cable was taken up on a reel and then sent through another extruder to install the insulation shield layer.
This resulted in possible contamination in a very critical zone. This had limited commercial application and never became a major factor in the market.
This approach was abandoned when a true thermosetting shield material became available. Jackets became increasingly popular by By ,40 percent of the cables sold had a jacket.
EPR cables became more popular in the s. In , another significant change took place: Because water was a problem for long cable life, the ability to virtually eliminate water became imperative.
Half the cable sold had a jacket by that time. During the second half of the s, a major change in the use of filled strands took place. Vast improvements in the materials and processing of extruded, medium voltage power cables in the s has led to cables that can be expected to function for 30,40, or perhaps even 60 years when all of the proper choices are utilized. Thue, J. Bankoske, and R. Balaska and Carl C. Landinger , 2 4 1. In order to accomplish this, a conductor is provided which is adequate to convey the electric current imposed.
Equally important is the need to keep the current from flowing in unintended paths rather than the conductor provided. Insulation is provided to largely isolate the conductor from other paths or surfaces through which the current might flow.
Therefore, it may be said that any conductor conveying electric signals or power is an insulated conductor. It also presents an apportunitY to easily visualize the parameters involved. Fikm Location of Voltage and Current In Figure , clearly the voltage is between the conductor and the ground [, Also,because of the charge separation, there is a capacitor and a large resistance between conductor and ground.
We know that all bend to ultimately terminate at ground. Air is not a very good insulating material since it has a lower voltage breakdown strength than many other insulating materials. It is low in cost if space is not a constraint. As the voltage between the conductor and ground is increased, a point is reached where the electric stress at the conductor exceeds the breakdown strength or air. At this point, the air literally breaks down producing a layer of ionized, conducting air surrounding the conductor.
It represents power loss and can cause interference to radio, TV, and other signals. It is not uncommon for this condition to appear at isolated spots where a rough burr appears on the conductor or at a connector. This is simply because the electric stress is locally increased by the sharpness of the irregularity or protrusion from the conductor. In air or other gasses, the effect of the ionized gas layer surrounding the conductor is to increase the electrical diameter of the conductor to a point where the air beyond the ionized boundary is no longer stressed to breakdown for the prevailing temperature, pressure, and humidity.
The unlimited supply of fresh air and the conditions just mentioned, precludes the progression of the ionization of air all the way to ground. It is possible that the stress level is so high that an ionized channel can breach the entire gap from conductor to earth, but this generally requires a very high voltage source such as lightning.
Imagine the space requirements to wire a house or apartment using bare conductors on supports with air as the insulation. In Figure , we see that the voltage from conductor to ground is the same as before. A voltage divider has been created that is made up the impedance from the covering surface to ground.
The distribution of voltage from conductor to the surface of the covering and from the covering surface to ground will be in proportion to these impedances.
It is important to note that with ground relatively far away from the covered conductor, the majority of the voltage exists from the covering surface to ground. Voltage from d a c e of covering to ground So little current is available at the covering d a c e from a low voltage covering volts or less , that it is imperceptible.
The question arises as to what is considered to be low voltage. The voltage rating of insulated cables is based on the phase-to-phase voltage. Low voltage is generally considered to be less than volts phase-to-phase. See Chapters 4 and 9 for additional information.
Because of the proximity and contact with other objects, the thickness of indating materials used for low voltage cables is generally based on mechanical requirements rather than electrical. The surrounding environment, the need for special properties such as sunlight, or flame resistance, and rigors of installation often make it dBicult for a single material satisfy all related requirements.
Designs involving two or more layers are commonly used in low voltage cable designs. When the ground plane is brought close or touches the covering, the electric field lines become increasingly distorted. Recognizing that equipdential lines are perpendicular to the field lines, the bending results in potential difference on the covering surface. At low voltages, the effect is negligible. If this condition is allowed to continue, eventually the erosion may progress to failure.
It is important to note that the utilization of spacer cable systems and heavy walled tree wires depend on this ability of the covering to reduce current flow to a minimum. When sustained contact with branches, limbs, or other objects occurs, damage may result hence such contacts may not be left permanently.
Unfortunately, surface erosion and personnel hazards are not linear functions of voltage versus thickness and this approach becomes impractical. This layer creates some complications, however.
In Figure , it is clear that a capacitor has been created from the conductor to the surEace of semiconducting layer. A great deal of charge can be contained in this capacitor.
This charging current must be controlled so that a path to ground is not established along the surface of the semiconducting layer. This path can lead to burning and ultimate failure of that layer. Accidental human contact would be a very serious alternative. It is clearly necessary to provide a continuous contact with ground that provides an adequate path to drain the capacitive charging current to ground without damage to the cable.
This is done by adding a metallic path in contact with the semiconducting shield. Once a metallic member has been added to the shield system, there is simply no way to avoid its presence under ground fault umditions.
This must be considered by either providing adequate conductive capacity in the shield to handle the fault currents or to provide supplemental means to accomplish this. This is a critical factor in cable design. Electric utility cables have fault current requirements that are sufficiently large that it is common to provide for a neutral in the design of the metallic shield. It is important that the functionsof the metallic shield system are understood since many serious errors and accidents have cmumd because the functions were misunderstood.
The maximum stress occus at the conductor. The grounded insulation shield results in the entire voltage stress being placed across the insulation. Just as in the case of the air insulated conductor, there is concern about exceeding maximum stress that the insulating layer can withstand.
The problem is magnified by stranded conductors or burrs and scratches that may be present in both stranded and solid conductors.
In Figure , a semiconducting layer has been added over the conductor to smooth out any irregularities. This reduces the probability of protrusions into the insulating layer.
Protrusions into the insulation or into the semiconducting layer increase the localized stress stress enhancement that may exceed the long term breakdown strength of the insulation.
This is especially critical in the case of extruded dielectric insulations. Unlike air, there can be no fresh supply of insulation. Any damage will be progressive and lead to total breakdown of the insulating layer. First, protrusions, whether by material smoothness or manufacturing, must be minimized. Such protrusions defeat the very purpose of a shield by enhancing electrical stress.
The insulations shield layer has a further complication in that it is desirable to have it easily removable to facilitate splicing and terminating, This certainly is the case in the medium voltage 5 to 35 kv. Contamination results in stress enhancement that can increase the probability of breakdown.
Voids can do the same with the additional possibility of capacitive-resistive CR discharges in the gas-filled void as voltage gradients appear across the void. Such discharges can be destructive of the surrounding insulating material and lead to progressive deterioration and breakdown.
Chemical attack includes corrosion of underlying metallic layers for shielding and armoring. In multi-conductor designs, overall jackets are common for the same purposes. For medium and high voltage cables, jackets have been almost universally used throughout the history of cable designs.
They are used for the same purposes as for low voltage cables with special emphasis on protecting underlying metallic components from corrosion.
Both experiments resulted in elevated failure rates for these designs. Jackets are presently used for these designs. It does electrical work but there are no parts that move, at least no discernible movement to the naked eye. Do not be misled.
This cable is a sophisticated electrical machine, even though it looks commonplace. For simplicity, we shall contine this discussion to single conductor cable. There are only two components in this cable, a conductor and its overlying insulation.
The conductor may be solid or stranded and its metal usually is either copper or aluminum. An attempt to use sodium was short-lived. The strand can be concentric, compressed, compacted, segmental, or annular to achieve desired properties of flexibility, diameter, and current density. Assuming the same cross-sectional area of conductor, there is a difference in diameters between solid and the various stranded conductors. This diameter differential is an important consideration in selecting methods to effect joints, terminations, and fill of conduits.
This provides d i c i e n t separation between the conductor and the nearest electrical ground to preclude dielectric failure. For low voltage cables, 2, volts and below , the required thickness of insulation to physically protect the conductor is more than adequate for required dielectric strength. The conductor and the insulation are visible to the unaided eye.
However, there is a third component in this cable. It is invisible to the unaided eye. This third component is what contributes to sophistication of the electrical machine known as cable.
Alternating curtent fields will be discussed, not direct current In all cables, regardless of their kV ratings, there exists a dielectric field whenever the conductor is energized.
This dielectric field can be visualized by lines of electrostatic flux and equi-potentials. Electrostatic flux lines represent the boundaries of dielectric flux between electrodes having different electrical potentials. Eaui-mtential lines represent points of equal potential difference between electrodes having Werent electrical potentials. They represent the radial voltage stresses in the insulation and their relative spacing indicates the magnitude of the voltage stress.
The closer the lines, the higher the stress. See Figures , , and If the cable is at an infinite distance from electrical ground ideal situation , there will be no distortion of this dielectric field. The electrostatic flux lines will radiate between the conductor and the surface of the cable insulation With symmetrical spacing between them.
However, in actual practice this ideal situation does not exist. In actual practice, the fluface of the cable insulation is expected to be in contact with an electrical ground. This actual operating condition creates distortion in the dielectric field.
The lines of electtostatic flux are crowded in the area of the insulation closest to ground. The lines of equi-potential are eccentric with respect to the conductor and the surface of the cable insulation. This situation is tolerated if the dielectric strength of the cable insulation is suflicient to resist the flow of electrons lines of electrostatic flux , and the surface discharges and internal voltage stresses by that are due to cuncentrated voltage gradients stresses that are replhes of equi-potential.
Low voltage, non-shielded cables are designed to withstand this condition. Service performance of non-shielded cables is generally considered acceptable. If we use the same volts per mil wall thickness of volt cable to determine higher voltage walls, we achieve a wall of at least 4.
A similar approach using 5 kV cable voltage stress as the basis for extrapolationprovides at least a 0. It is apparent that the bulk dimensions created by extrapolationof non-shielded cable walls are unacceptable. To overcome this situation of bulk dimensions,generally shielded cable is used. However, where is it placed, what materials are used, and what does it do to the dielectric field? Let us start from the conductor again and move outward from the center of the cable.
Nothing unusual as compared to a non-shielded cable. A conducting material is placed over the conductor circumfmnce to screen shield out irregularities of the conductor contours. The differences between insulation for a non-shielded cable as compared to a shielded cable are in material, quality, cleanliness, and application. The thickness applied is primarily influenced by considerations of electrical stress voltage gradients.
Insulation Shield. This is a two-part system, consisting of an auxiliary shield and a primary shield. A conducting material that is placed over the outer diameter of the cable insulation.
A metallic layer of tapes, wires, or a tube that is placed over the circumference of the underlying auxiliary shield. A dielectric field, composed of electrostatic flux and equi-potential lines, exists when the conductor is energized. There is no distortion in this dielectric field because of the shielding of insulation and conductor, Electrostatic flux lines are symmetrically spaced and equi-potential lines are concentric. See Figure However, observe features not previously noted; the electrostatic flux and equipotential lines are spaced closer together near the conductor shield as compared to their spacing near the insulation shield.
This is why we are cognizant of maximum stresses at areas of minimum radii and diameters. Insulation voids at the conductor shield are more critical than voids at the insulation shield. Also these lines are spaced closer together at the minimum diameter or radii.
This substantiatesthe maximum radial stress theory. The use of shielded cable permits using cables that are more economic to manufacture and install as compared to non-shielded cables that would require very heavy insulation thickness.
Table provides a oomparison. Over the insulation shielding system, the cable contains components that provide environmental protection for the cable.
Clapp, C. Landinger, and W. Landinger 1. The choice of the conductor material, size, and design must take into consideration such items as: Examples of these as ranked by low resistivity at 20 "C are shown in Table As such, they are the dominant metals used in the power cable industry today.
The choice between copper and aluminum conductors should carefilly compare the properties of the two metals, as each has advantages that outweigh the other under certain conditions. The properties most important to the cable designer are shown below. Therefore, an aluminum conductor must have a cross-sectional area about 1. This difference in area is approximately equal to two AWG sizes.
A unit length of bare aluminum wire weighs only 48 percent as much as the same length of copper wire having an equivalent dc resistance. However, some of this weight advantage is lost when the conductor is insulated, because more insulation volume is required over the equivalent aluminum wire to cover the greater circumference.
See Chapter 9 for details and references, but obviously more aluminum cross-sectional area is required to carry the same current as a copper conductor as can be seen from Table 3- 1.
Equivalent voltage drops result with an aluminum conductor that has about 1. Not only do they tend to creep, but they also oxidize rapidly. When aluminum is exposed to air, a thin, corrosion-resistant, high dielectric strength film quickly forms.
When copper and aluminum conductors are connected together, special techniques are required in order to make a satisfactory connection.
See the discussion in Chapter Aluminum is not used extensively in generating station, substation, or portable cables because the lower bending life of small strands of aluminum does not always meet the mechanical requirements of those cables.
However, it is the overwhelming choice for aerial conductors because of its high conductivity to weight ratio and for underground distribution for economy where space is not a consideration. Economics of the cost of the two metals must, of course, be considered, but always weighed after the cost of the overlying materials. This system is based on the following definitions: The diameter of size 36 AWG is 0.
The ratio of any diameter to that of the next smaller size is: The square of the above ratio the ratio of diameters of successive sizes is 1. Thus, an increase of one AWG size yields a An increase of two AWG sizes results in a change of 1.
The sixth power of 1. Therefore, changing six AWG sizes will approximately double or halve the diameter. Another useful short cut is that a 10 A WG wire has a diameter of roughly 0. I inch, for copper a resistance of one ohm per feet and a weight of about 10 n, or Another convenient rule is based on the fact that the tenth power of 1. Thus, for every increase or decrease of ten gage numbers starting anywhere in the table the cross-sectional area, resistance, and weight are divided or multiplied by about ten.
From a manufacturing standpoint, the AWG sizes have the convenient property that successive sizes represent approximately one reduction in die size in the wire drawing operation. This method uses an arbitrary area of a conductor that is achieved by squaring the diameter of a solid conductor. A circular mil i s a unit of area equal to the area of a circle having a diameter of one mil one mil equals 0. Such a circle has an area of 0. Thus, a wire ten mils in diameter has a cross-sectional area of circular mils.
For convenience, this is usually expressed in thousands of circular mils and abbreviated kcmil. Thus, one square inch equals 1, kcmils. The abbreviation used in the past for thousand circular mils was MCM. The SI abbreviations for million, M, and for coulombs, C, is easily confused with the older term.
An important consideration is that these are not precise sizes.
A comparison of the two systems can be seen in the tables in Chapter Compression connectors, especially for aluminum, are sensitive to size variations. In Canada, metric designations are used for all cable dimensions except for the conductor size! The variations in the two systems are too great to use any of the SI sizes as a direct substitution for standard sizes.
Stranding becomes the solution to these difficulties. The point at which stranding should be used is dependent on the type of metal as well as the temper of that metal. Copper conductors are frequently stranded at 6 AWG and greater. This consists of a central wire or core surrounded by one or more layers of helically applied wires. Each additional layer has six more wires than the preceding layer.
In the case of power cable conductors, the core is a single wire and all of the strands have the same diameter. The first layer over the core contains six wires; the second, twelve; the third, eighteen; etc.
The distance that it takes for one strand of the conductor to make one complete revolution of the layer is called the length of lay.
The requirement for the length of lay is set forth in ASTM specifications, [, to be not less than 8 times nor more than 16 times the overall diameter OD of that layer. In power cables, the standard stranding is Class B. Specifications require that the outermost layer be of a left hand lay. This means that as you look along the axis of the conductor, the outermost layer of strands roll towards the left as they recede from the observer.
More flexibility is achieved by increasing the number of wires in the conductor. The Class designation goes up to M normally used for welding cables, etc. These are covered by ASTM specifications [, , Examples of Class B standard , Class C flexible and Class D extra flexible are shown below with the number of strands and diameter of each strand: There is no reduction in conductor area.
A typical reduction is about 2. Examples of gaps in the outer layer for concentric stranded cables are shown in Table Table Gaps in Outer Layer of a Stranded Conductor Shortening the length of lay on the outer layers could solve the problem but would result in higher resistance and would require more conductor material.
The reason that compressed stranding is an excellent construction is that concentric stranding with its designated lay length creates a slight gap between the outer strands of such a conductor.
This results in surface irregularities that create increased voltage stresses and makes it more difficult to strip off that layer.
This results in a diameter nearing that of a solid conductor. Some air spaces are still present that can serve as channels for moisture migration.
This construction is used when extreme flexibility is required for small AWG sizes, such as portable cables. Examples of bunch stranded conductors are cords for vacuum cleaners, extension cords for lawn mowers, etc. Examples are: This is a combination of the concentric conductor and a bunch stranded conductor.
The finished conductor is made up of a number of groups of bunched or concentric stranded conductors assembled concentrically together. The individual groups are made up of a number of wires rather than a single, individual strand. A rope-stranded conductor is described by giving the number of groups laid together to form the rope and the number of wires in each group. Classes G and H are generally used on portable cables for mining applications.
Classes I, L, and M utilize bunch stranded members assembled into a concentric arrangement. The individual wire size is the same with more wires added as necessary to provide the area.
Class I uses 24 AWG 0. Class I stranding is generally used for railroad applications and Classes L and M are used for extreme portability such as welding cable and portable cords. A typical three-conductor cable has three " segments that combine to form the basic circle of the finished cable. Such cables have a smaller diameter than the corresponding cable with round conductors. The precise shape and dimensions varied somewhat between manufacturers.
Sector conductors that are solid rather than stranded have been used for low-voltage cables on a limited basis. There is interest in utilizing this type of conductor for medium voltage cables, but they are not available on a commercial basis at this time.
Each segment carries less current than the total conductor and the current is transposed between inner and outer positions in the completed cable. This construction has the advantage of lowering the ac resistance by having less skin effect than a conventionally stranded conductor. This type of conductor should be considered for large sizes such as kcmil and above.
This construction has the advantage of lowering the total ac resistance for a given cross-sectional area of conductor by eliminating the greater skin effect at the center of the completed cable. Where space is available, annular conductors may be economical to use for kcmil cables and above at 60 hertz and for kcmil cables and above for lower frequencies such as 25 hertz. A design frequently used for low-voltage power cables is the combination unilay where the outer layer of strands are partially composed of strands having a smaller diameter than the other strands.
This makes it possible to attain the same diameter of a compact stranded conductor. The most common unilay conductor is a compact, series aluminum alloy. Silver is an interesting possibility for a cable conductor. Its high cost is certainly one of the reasons to look for other candidates.
Silver has another disadvantage of lack of physical strength that is necessary for pulling cables into conduits. Impurities have a very deleterious effect on the conductivity of copper. Electrical conductor EC grade aluminum is also low in impurities ASTM B specifies the permissible impurity levels for aluminum [ This causes the soft temper metal to have a slightly lower conductivity as well as a higher temper.
Stranding and compacting also increases the temper of the metal. If a more flexible conductor is required, annealing the metal may be desirable. This can be done while the strand is being drawn or the finished conductor may be annealed by placing a reel of the finished conductor in an oven at an elevated temperature for a specified period of time.
ASTM specifications cover three tempers for copper conductors: Soft-drawn is usually specified for insulated conductors because of its flexibility. Medium-hard-drawn and hard-drawn are usually specified for overhead conductors, but for long or difficult pulls of underground cable, it may be desirable to use a temper greater than soft-drawn.
ASTM has five specifications for aluminum tempers as shown below. Note that some of the values overlap. Half-hard aluminum is usually specified for solid and for series alloy conductors because of the need for greater flexibility.
Three-quarter and full-hard are usually specified for stranded cables. Aluminum, in the presence of water and in the absence of oxygen, will hydrolyze. Thus, if water enters an insulated cable having an aluminum conductor, the aluminum and water combine chemically to form aluminum hydroxide and hydrogen gas.
This condition is aggravated by a deficiency in oxygen in the insulated conductor. The chemical reaction is: Aluminum hydroxide is a white, powdery material that is a good insulator. Many users of stranded aluminum conductors are now requiring filled conductors for this reason. Filled conductors prevent the passage of moisture along the conductor and thus help to retard this form of deterioration.
Regardless of the degree of compaction of a stranded conductor, there is some air space remaining. This space can be a reservoir for moisture to collect and hence a source of water for water treeing. Water blocked stranded conductors are frequently specified for underground cables to reduce the possibility of this happening.
It is generally assumed that the current is evenly divided among the strands and does not transfer from one strand to the next one.
For that reason, the dc resistance is based on: Multiply the number of strands by the cross-sectional area taken perpendicular to the axis of that strand. Be the first to like this. No Downloads. Views Total views. Actions Shares. Embeds 0 No embeds. No notes for slide. This latest addition to the CRC Press Power Engineering series covers cutting-edge methods for design, manufacture, installation, operation, and maintenance of reliable power cable systems. It is based largely on feedback from experienced university lecturers who have taught courses on these very concepts..
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Search all titles Search all collections. Your Account Logout. Electrical Power Cable Engineering. Edited By William A. Edition 3rd Edition. First Published