2.2. Characterization

The ACCC/HW studied in this paper is only used in China now, so the test methods in this work are according to Chinese standard GB/T32502-2016 [18] “Overhead electrical stranded conductors composite core supported/reinforced”. Additionally, Figure 1 shows the ACCC/HW studied in this work is based on the physical model.

The basic mechanical property tests of both the composite core and ACCC/HW were taken in this study. The basic mechanical properties concluded tensile strength at 25 °C and 160 °C, and tensile-strain behavior at 25 °C. Deformation resistant property analysis and current-carrying capacity test of ACCC/HW were also taken. The deformation resistant properties included the sag at heating up temperature and the creep performances at 25 °C and 160 °C.

It is necessary to use special fittings, which should not only be closely connected with the composite core and the aluminum wires to effectively transfer load because of the difference between ACCC/HW and ACSR in structure, but also avoid the damage of carbon fiber composite core in the installation and use process. ACSR mainly adopts the compression connection mode, and the external pressure tightly combines the aluminum wire and the steel core because of the plastic deformation of metal materials. However, if ACCC/HW is connected in this way, the structure of the carbon fiber composite core will be destroyed, and the mechanical properties will be deteriorated. Therefore, special fittings were used, which were composed of inner cone ring, outer cone ring, steel anchor, inner liner, and outer liner, as well as current inducing plater, as shown in Figure 4. Inner cone ring is at the outside of the composite core, and outer cone ring is at the outside of inner cone ring, to protect composite core from destruction by extrusion. Inner liner and outer liner contact with heat-resistant aluminum alloy wires.

Special fittings used in this work. (a) detail sketch; (b) assembly process; (c) assembly process; (d) ready for use.

The tensile test and the stress-strain test were carried out on 500 kN tension test machine, and the strength was the average of three test results. In room temperature tensile test at 25 °C, the effective lengths of ACCC/HW and the composite core were 10 m and 1 m, respectively. In the high temperature tensile test at 160 °C, the effective lengths of ACCC/HW and the composite core were 12 m and 1 m, respectively. After ACCC/HW was heated to 160 °C through the 50 kV high current heating system and preserved for three hours, the tensile test of ACCC/HW was carried out. After the composite core heated to 160 °C through special heating furnace and preserved for 401 hours, the test of composite core was carried out.

ACCC/HW was applied load with four loading- load keeping-unloading cycles to characterize the stress-strain performance. The effective length of the ACCC/HW was 12 m, and the length of extensometer was 2 m. Firstly, 2% RTS (Rated Tensile Strength) was exerted to straighten the conductor, and the RTS is 100kN. Figure 5 shows the loading procedure. The stress-strain test of the composite core was loaded in only one cycle, and the effective length of the composite core was 5 m.

Stress-strain loading procedure of ACCC/HW.

In the sag test, ACCC/HW was heated through AC low voltage large current heating system. The temperature was measured through six pairs of thermocouples and the average temperature was taken. The test temperature was from 20 °C to 200 °C and the initial tension was 25% RTS. The sample test span was 50 m and the length of the heating span was 60 m. The sag of ACCC/HW at different temperatures was tested.

The creep test of ACCC/HW was taken on the numerically controlled creep test machine according to IEC61395-1998 [19]. The effective length of ACCC/HW and the extensometer were 14m and 2000 ± 1 mm. The elongation of ACCC/HW under the tensions of 15% RTS, 25% RTS, and 35% RTS for different time were tested at 25 °C. The total test time was 1000 h. The high temperature creep test was carried out at 160 ± 3 °C and the tension was 25% RTS.

The current-carrying capacity was tested according to IEC 1597-1995 [20]. Under the conditions of 25% RTS tension, the heating length of 13 m, no breeze, no sunshine, and natural convection, the current-carrying capacity of ACCC/HW was tested. Afterwards, the current-carrying capacity in different environments was calculated according to the calculation parameters that were recommended by China Electric Power Industry (wind speed was 0.5 m/s, sunshine intensity was 1000 W/m, conductor surface absorption coefficient was 0.9, conductor radiation coefficient was 0.9, ambient temperature was 20~45 °C, and conductor working temperature was 70~160 °C).