Procedimiento experimental

Configuration

MATERIALS

Since the first cable system, only two conductor materials have played a significant role: copper and aluminum. These materials have appeared in a variety of alloys, tempers, and configura- tions. In the late 1960s, some utilities briefly ex- perimented with sodium as a conductor

material; however, it was not cost-effective be- cause of the special precautions required during installation and maintenance.

Copper was the first material to play a major role in cable construction. With a volume resistiv- ity of 1.673 × 10-7ohm-meters (ohm-m) in its pure (99.999 percent) state, it compared favorably with

other metals. Supplies were abundant and it could be economically fabricated. Connections were simple to make and corrosion resistance was good. However, with the rapid development of the aluminum industry in the first half of the 20th century, aluminum became cost-effective for applications in which physical size was not critical. To take advantage of this economic benefit, the electric industry developed methods to overcome some of the other physical disadvantages of alu- minum. These disadvantages included higher sus- ceptibility to flexural fatigue, the high resistivity of natural surface oxides, and cold flow (creep).

For economic reasons, cables now used on underground systems are predominantly alu- minum. The use of this metal leads to a larger cross-sectional area and, consequently, greater overall cable dimensions, but, in most cases, the additional cost of other project components— such as larger size conduit—does not outweigh the present economic advantage of aluminum conductors. Aluminum conductors have a volume resistivity of 2.655 × 10-7ohm-m. Comparing this resistivity with the previously mentioned copper volume resistivity shows that, for equal cross- sectional areas, aluminum will have 1.59 times the resistance of the same-size copper conductor.

To simplify the comparison of various con- ductors, the industry uses a measure of relative conductivity that compares a particular metal to annealed electrolytic copper. This measure is re- ferred to as the International Annealed Copper Standard (IACS). The volume resistivity of an- nealed copper is defined as 1.724 × 10-7ohm-m at a temperature of 20°C (68°F).

As the tensile strength of materials increases, the conductivity decreases. As an example, hard- drawn copper has experienced an increase in

tensile strength because of work hardening dur- ing the drawing process and its conductivity has fallen to 97.2 percent IACS. By comparison, 1350H19 aluminum has a conductivity of about 61 percent IACS. The lower conductivity is mainly caused by the inherently higher volume resistivity of pure annealed aluminum. See Table 2.2 for a comparison of common conductor materials.

Because thermal capacity of conductors and cables is a function of the heat generated by in- ternal conductor losses, the ampacity of the higher conductivity copper conductors of equal size is approximately 1.6 times that of matching aluminum conductors. Of course, other signifi- cant elements determine the exact cable ampac- ity. These are discussed more extensively in Section 4of this manual.

CONDUCTOR TEMPER

Both copper and aluminum conductors are available in various tempers that designate the relative hardness of the metal. Whereas over- head conductors have generally used harder metal to increase tensile strength and reduce sags, underground conductors have tended to use the lower tempers, because high tensile strength was not usually required. Most copper power cables have used soft-drawn copper for its greater flexibility. This flexibility not only makes fabrication easier but also improves in- stallation handling, especially for larger cables. Where high tensile strength is needed for cable pulling, special installations might use harder tempers. However, this would only be where high unit stresses would be imposed on the cable conductor during installation or perhaps during cable life. Examples include mineshaft riser cables or cables for extremely long pulling

Copper Aluminum

Medium 1/2 Hard 3/4 Hard Hard Drawn

Soft Drawn Drawn Hard Drawn (H14/H24) (H16/H26) (H19)

Rated Tensile Strength — 42–60 ksi 49–67 ksi 15.0–20.0 ksi 17.0–22.0 ksi 24.5–29.0 ksi

Conductivity (% IACS) 100 96.7–97.7 97.2 61.0 61.0 61.0

Note. ksi = thousands of pounds per square inch

distances in duct. Such cables would require cus- tomized design for their particular circumstances and are beyond the scope of this manual.

Aluminum conductors in power cables are generally furnished in the 3/4-hard temper. This provides a reasonable level of tensile strength, while not introducing excessive ductility that would lead to creep problems in making durable connections. As the conductor cross section in- creases to 750 kcmil or greater, there is some ac- ceptance of aluminum conductors in the 1/2-hard temper. This gives adequate tensile strength while maintaining a higher degree of flexibility. All characteristics of aluminum conductors, es- pecially tensile strength, must be considered when specifying a cable. The specifying engi- neer must consider the mechanical stresses on the cable during installation and service.

More information on conductor characteristics can be found in reference books. Nationally ac- cepted specifications for electrical conductors are found in American Society for Testing and Materials (ASTM) standards. Copper wire is cov- ered by ASTM Specifications B-1, B-2, and B-3. Aluminum wire is covered by ASTM Specification B-230. Methods for measuring the most impor- tant characteristics of these and other materials can be found in other related ASTM standards. Aluminum conductors used in underground cable are addressed in other ASTM standards, in- cluding B-231 (concentric lay conductors) and B-400 (compact round conductors).

CONDUCTOR ALLOY

Aluminum conductor material also is designated by an alloy number. The alloy designation derives from the description of aluminum alloys in other applications in which such characteristics as high tensile strength are required. However, because high electrical conductivity (low resistivity) is the single most important aspect of underground ca- ble conductors, pure aluminum is generally used. The alloy designation for electrical alumin- um is EC. It was formerly designated as Alloy 1350. The same aluminum nomenclature system includes designations for temper. These are also shown in Table 2.2. For example, 3/4-hard tem- per has a classification of H16 or H26. The dif- ference between H16 and H26 tempers is that

the H16 alloy is only strain-hardened. The H26 alloy has the same general characteristics, but it has been partially annealed after strain hardening.

Copper conductors are almost universally sup- plied as pure copper. Pure copper provides the highest conductivity and, therefore, the highest efficiency. Because pure copper in its various tempers provides adequate mechanical strength for cable applications, there is generally no need for alloyed copper conductors.

CONDUCTOR CONFIGURATION

The wire and cable industry offers the electric utility industry a wide variety of standard con- ductor configurations, including solid conductor, various stranding arrangements, and filled-strand conductors. Each configuration has its own ad- vantages. The engineer selecting a cable design must consider these alternatives and select the option that produces the best cable for the par- ticular application. Elements significantly af- fected by the conductor configuration include the following:

• Flexibility during installation (cable bending and racking),

• Flexibility during operations (elbow switch- ing), and

• Longitudinal water migration.

Though the decision on conductor configura- tion alone will not provide the solution to any of these problem areas, it is a vital part of the larger process of selecting a cable that will pro- vide high reliability and economy.

The simplest configuration is the solid, single- strand conductor. Solid conductor is preferred in smaller cable sizes because of its absolute water- blocking capability. Because there are no voids to fill, there will be no continuing migration of water through the insulation system. Perhaps more important, if moisture does penetrate the insulation, it cannot migrate along the cable con- ductor to other areas of the cable. The inhibition of moisture migration is extremely important in reducing insulation deterioration problems so prevalent in underground cables.

As is well known, the stiffness of cable in- creases as conductor diameter increases. Cable

stiffness will increase in pro- portion to the square of the di- ameter of the solid conductor. Therefore, a point will be reached at which the cable will become unmanageable, especially where bending in confined spaces is required to

operate load-break connec- tors. The solution is the use of stranded conductors. The smaller diameter of the indi- vidual strands lowers the total force required to achieve the necessary bending. The rea- sonable upper limit for solid conductors with 3/4-hard aluminum conductors has generally been found to be 2/0 AWG. Above that size, stranded conductors are advised.

Several options in stranded conductors are available, including conventional concentric lay, compressed strand, and compact configurations. Some of these are illustrated in Figure 2.4.

The simplest stranded configuration is the conventional concentric round stranding that uses multiple layers of circular wires. Each layer of wires is laid in the opposite direction. The predominant combinations for conventional stranded cable are 1 + 6 = 7, 1 + 6 + 12 = 19, and 1 + 6 + 12 + 18 = 37. These are illustrated in Figure 2.5.

The first option, concentric round stranding, obviously produces interstices (voids) between the surfaces of the individual wires. These inter- stices have two important effects. First, for a given equivalent metallic cross section of con- ductor, the outside diameter of a stranded cable will be greater than for an equivalent solid con- ductor. Second, the voids are continuous along the cable and provide an excellent path for moisture migration. In conventional stranding, the conductor metal will occupy only 76 to 78 percent of the area enclosed by a circle drawn around the outside of the conductor.

The number of wires in a concentric stranded conductor is defined in ASTM standards as the class of the conductor. Details are contained in ASTM Standards B8 (copper) and B231 (alu- minum). An examination shows that the im- proved flexibility of higher stranding comes at the expense of larger diameter. In addition, the stranded conductors weigh more because the outer layers must be longer than the conductor axis. Table 2.3 compares the various stranding characteristics of a common single size of alu- minum conductor.

Concentric Stranded Conductor, 37-Wire

Compressed-Strand Concentric Conductor, 37-Wire

Compact Concentric-Strand Conductor, 37-Wire

Use solid or strand-