ACSS and ACSS/TW

Many millions pounds of Aluminum Conductor Steel Supported (ACSS) have been installed and are operat-ing successfully in the United States. However, for many it is still considered a relatively new conductor, and its performance is not well understood. As such, it is an integral part of EPRI’s HTLS conductor field test project. Most of the initial concerns about installation and surface roughness problems due to the use of annealed aluminum strands have passed. The main limi-tation with ACSS is its relatively low strength and mod-ulus that may limit its application in regions experiencing high ice loads. The use of ACSS/TW can offset this problem to some extent, as can the use of extra-high-strength steel core wires. The conductor and special connectors designed for it allow continuous

operation at temperatures up to 200^oC with conven-tional galvanized steel core wires. The conductor can be operated above 200^oC if Alumoweld or special zinc

“Galfan” coated steel is used.

ACSS is described in ASTM B 856-95. It consists of fully annealed strands of aluminum (1350-H0) stranded around stranded steel core. The steel core wires may be aluminized, galvanized, or aluminum clad, and are nor-mally “high strength,” having a tensile strength about 10% greater than standard steel core wire. In appear-ance, ACSS conductors are essentially identical to stan-dard ACSR conductors (see Table 2.6-1).

By using annealed aluminum, the rated strength of ACSS is reduced by an amount dependent on the strand-ing (e.g., 35% for 45/7, 18% for 26/7, and 10% for 30/7).

In fact, a 45/7 ACSS conductor has about the same rated breaking strength as a conventional all-aluminum con-ductor (e.g., 16,700 lbs (71.4 kN) for 954 kcmil (487 mm²) 45/7 ACSS versus 16,400 lbs (73.2 kN) for 954 kcmil (487 mm²) 37 strand AAC [Magnolia]). The ther-mal elongation coefficient, creep rate, and maximum operating temperature are, however, quite different.

ACSS Conductor Designs

ACSS is typically available in three different designs:

“Standard Round Strand ACSS,” “Trapezoidal Wire of Equal Area,” and “Trapezoidal Wire of Equal Diame-ter.” In addition, it is possible to obtain all three ACSS conductor designs with any of the standard types of steel core wire (galvanized, aluminized, and Alumoweld).

Advantages and Disadvantages of ACSS

ACSS provides a number of advantages in reconductor-ing. The combined effect of these factors can make it economically attractive in thermal uprating applica-tions. It has higher self-damping than conventional ACSR. It has lower thermal elongation over a wide range of conductor temperatures. It can be operated at temperatures as high as 250 °C without damage. It can be installed at smaller initial sags without dampers if it is prestressed. With reference to the preceding

discus-Table 2.6-1 ACSS Equivalents to Standard Type 16, 795 kcmil, 26/7 ACSR (Drake)

Conductor Drake ACSR 1.108 28.14 795 0.1170 0.0727 æ Drake/ACSS 1.108 28.14 795 0.1137 0.0707 -3%

Suwannee/

ACSS/TW 1.108 28.14 960 0.0939 0.0584 -17%

Drake/ACSS/TW 1.010 25.65 795 0.1132 0.0704 -3%

sion of sag clearance, the conductor properties make it attractive for reconductoring applications as well as cer-tain new line designs.

In reconductoring existing lines, in comparison to con-ventional ACSR conductors, ACSS can yield a much larger increase in thermal capacity while minimizing the need for expensive structure modifications. In new lines, this conductor can yield designs with less environmental impact (shorter and/or fewer structures) with greatly increased thermal capacity for essentially no increase in cost. As discussed in the following, the key advantages of ACSS are:

• Operate to 250 °C with no loss in strength

• No creep elongation over time

• High self-damping (which yields low levels of Aeo-lian vibration)

• Lower thermal elongation than conventional conduc-tor

• 63% IACS conductivity, not 61.2%

• Equal OD and equal AREA options.

Higher Maximum Temperature

Typically aluminum stranded conductors can be oper-ated at temperatures up to 95°C without significant loss of tensile strength. Aluminum conductors with a steel-reinforcing core can be operated at temperatures of up to 150° C for limited periods. Because the aluminum in ACSS is fully annealed at the factory, it can be operated continuously at temperatures up to 250°C or, with spe-cial high-temperature-tolerant galvanizing coatings such as “Galfan,” even higher.

Table 2.6-2 shows a comparison of continuous ampacity (with 2 ft/sec (0.61 m/sec) crosswind, 40 °C air tempera-ture, and full sun) for ACSS and ACSR conductors.

Note that the ampacity of an ACSS conductor

operat-ing at 250°C is nearly twice that of an ACSR of the same cross-sectional area operating at 100°C.

Thermal Elongation

Aluminum strands elongate thermally at twice the rate of steel. The sag increase of ACSR conductor is, there-fore, less than it is for AAC. In the case of ACSS, the tension level in the aluminum strands is very small, and the conductor elongates thermally as though it were steel. Thus, the sag increase in going from 15°C to 150°C with ACSS may be the same as the sag increase from 15°C to 95°C with ordinary ACSR.

As an example of this lower thermal elongation of ACSS, consider the data in Table 2.6-3. The ACSS con-ductor has the same sag at 150°C as the ACSR conduc-tor of the same diameter has at 100°C. Therefore, for a clearance-limited line, by reconductoring with ACSS, the thermal capacity of the line increases by about 30%

without the need to raise or reinforce structures.

Self-Damping

The tension of conductors in overhead lines is normally determined by concern about Aeolian vibration-induced fatigue. It is normal to limit initial tension to no more than 20% of the rated breaking strength in order to limit vibration levels. Because it has higher self-damping than ordinary ACSR, ACSS may be installed to smaller initial sags, and because it has a lower modu-lus, it yields lower maximum tensions than ACSR.

Low Creep Elongation

When reconductoring, one must allow for creep elonga-tion over time with ordinary ACSR. In addielonga-tion, except for ACSR conductors with a high steel content, one must consider the possibility of accelerated creep at high operating temperatures. ACSS does not creep at any temperature, high or low. Thus, its final and initial sags are the same as shown in Figure 2.6-3.

Not only is there little or no difference between the ini-tial and final sag, but also the iniini-tial sag is less, and the change in sag due to temperature is less than it is for standard ACSR.

Table 2.6-2 Continuous Ampacity of Equivalent ACSR and ACSS Conductors as a Function of Maximum Allowable Conductor Temperature

100 990 1110 980

150 -- 1490 1320

200 -- 1770 1560

250 -- 2000 1740

a. For continuous loads, ACSR is normally limited to about 100^oC to avoid annealing of the aluminum strands.

Table 2.6-3 Illustration of the Lower Thermal Elongation of ACSS Conductor

--100 37.6 11.5 35.3 10.8 1110

150 -- -- 37.8 11.5 1490

The novel characteristics of ACSS make it attractive as a replacement conductor for HV lines where thermal capacity is inadequate. ACSS can be substituted for existing ACSR of the same diameter. Although having nearly the same resistance and diameter as the conduc-tor it replaces, ACSS can be operated at a much higher temperature without exceeding the original high-tem-perature sag levels. Since the aluminum strands of ACSS are fully annealed, it has a somewhat lower rated strength than the same stranding in ACSR. In areas where ice and wind loads permit, ACSS may be speci-fied with a reduced steel content. The result is that, with ACSS, the maximum tension loads on angle and dead-end structures may be no higher than those generated by the ACSR conductor that it replaces.

As an example of the advantages of ACSS in reconduc-toring, consider Figure 2.6-4, which shows ampacity and sag as a function of maximum allowable

tempera-ture. The original conductor in the existing line is assumed to be 477 kcmil (243 mm²) ACSR (Hawk). The proposed replacement conductors are 565.3 kcmil (288 mm²) ACSS/TW (Calumet), which has the same diame-ter as the original and 795 kcmil (405 mm²) ACSR (Drake), which has a diameter that is 30% higher. For continuous operation, the 565.3 kcmil (288 mm²) ACSS/TW (Calumet) conductor at 200°C has an ampacity about 25% higher than Drake at 100°C and lower maximum sag than the original or replacement ACSR conductors.

ACSS/TW: Field Trial in the EPRI HTLS Conductor Project

As part of the EPRI HTLS conductor project, an ACSS/TW conductor was spliced into a line segment of an operating 138-kV transmission line. This test seg-ment consists of four spans, and is approximately 2880 ft in length, and includes five structures (two dead-end and three suspension towers). The test conductor was spliced into all three phases of one circuit of a double-circuit vertical line. Various field data associated with conductor performance are intended to be collected over an extended period of time (about three years).

The conductor is classified as “Trapezoidal Shaped Wire Concentric-Lay Aluminum Conductor Steel Supported”

(ACSS/TW). It is designated by the name “Suwannee,”

and is 1.108 in. (2.814 cm) in diameter. Figure 2.6-5 shows photos of the conductor—the outside aluminum strands and steel center strands are indicated.

Figure 2.6-3 Typical behavior of ACSS conductor, illustrating that initial and final sags are nearly identical.

Figure 2.6-4 Ampacity and sag of original Drake ACSR and Calumet ACSS/TW replacement conductor as a

function of maximum allowable temperature. Figure 2.6-5 ACSS/TW cable, manufactured by SouthWire, installed on operating test line.

During the course of the project (ongoing for just over two years at this time), measurements and observations were made of the following quantities:

• Sag and Tension

• Weather Parameters

• Average Conductor Temperature

• Current

• Splice Resistances

• Hardware Temperatures via Infrared Measurements

• Corona

• Electric and Magnetic Fields

• Visual Inspections

This project is still ongoing, and final results are not yet available

2.6.4 High-Temperature Aluminum Alloy