RESULTS AND DISCUSSION
4.2 Method of Investigation
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CHAPTER 4
47 4.2.1 Conductor Type
During the course of this research four different conductors were available for testing at the VRTC. All of the conductors were of the Aluminium Conductor Steel Reinforced (ACSR) type, in which the bulk of the conductor is made up of aluminium strands layered helically around a small number of steel strands at the core. A variety of different conductors are available of varying specifications, identified by code names which are standardised between the various conductor manufacturers. Table 4.1 shows the basic details of the conductors tested.
Table 4.1: Conductor Specifications Conductor Name Number of Strands
(Al/Steel)
Outside Diameter (mm)
Stiffness (N/mm^2) Density (kg/m)
Tern 45/7 27 66600 1.34
Pelican 18/1 20.7 66200 0.775
Rabbit 6/1 10.05 80400 0.214
Appendix D contains the full technical specifications of each of these conductors. The full set of tests were performed on the Pelican and Rabbit conductors, with only the sweep test having been performed on Tern due to time restrictions imposed by the availability of the VRTC and other research activities being performed.
4.2.2 Varying the Tension
Each conductor was tested at three separate tensions, from low to high tension. As per IEEE standard the tension in the conductor was measured using a load-cell in line with the tensioning load-arm. The chosen tension was set by weighting the load-arm until the desired tension was reached while the clamp that holds the conductor in place to the stationary mounting blocks was loose. Then the clamp was tightened. Table 4.2 shows the tensions for the conductors that were tested.
Table 4.2: Tension Settings per Conductor
Conductor Name Tension 1
(15% of UTS)
Tension 2 (20% of UTS)
Tension 3 (25% of UTS)
Tern 14800N 19740N 24675N
Pelican 8120N 11200N 13070N
Rabbit 2775N 3700N 4630N
48 4.3 Steady State Displacement
4.3.1 Catenary equation
For any idealised string or cable strung between two points catenary theory states that under its own weight it will take the shape of a catenary. The catenary shape is described by the equation:
( ) (4.1)
Where a is a dimensionless scaling parameter (Teichman and Mahadevan (2003)). As an example, figure 4.1 is a catenary with a scaling number of a = 20. [30]
Figure 4.1: Catenary Curve, a = 20
Comparing the results of a static simulation under gravity to a catenary of equal end points and centre span sag allows the closeness of fit between the simulated and theoretical results of steady displacement to be determined. The catenary curve however cannot exactly represent a high tension conductor cable as such a cable will have zero gradient at the fixed ends due to non-zero flexural stiffness. This will result in a discrepancy between the simulated and theoretical results near the ends when compared to the catenary curve theory.
Measurements were taken from a clamped end up to the mid-point of the conductor and then duplicated plotting against the model results to the other side to generate a full conductor span.
Table 4.3 shows the distances from a clamped end at which displacement measurements were taken.
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Table 4.3: Measurement Points Along Span
Fraction of Conductor Length 1/16 1/8 1/4 3/8 1/2
Distance From End 5.2875 10.575 21.15 31.725 42.3
The measurements were biased to more samples being taken closer to the clamped end than the mid section due to the increased relative curvature near the ends.
4.3.2 Catenary Compared To Model and Measured Results
Considering the Tern conductor as a representative example, Figure 4.2 shows a comparison between the conductor model results at a steady state displacement and a pure catenary of equal midpoint displacement.
Figure 4.2: Comparison Between Model and Catenary Curve (Tern)
The model shows an agreement with the theory of the idealised hanging cable. Figure 4.3 shows a close view of an end of the comparison. It shows that the main source of discrepancy is the lack of the catenary's support for zero gradient at the ends.
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Figure 4.3: Comparison Between Model and Catenary Curve Close View (Tern)
The model results were of a similar nature for the other conductor types.
When the catenary curve was compared in the same manner (matching the midpoint displacement and end values) to the measured displacements a different picture emerged.
Figure 4.4 shows a comparison between the measured experimental displacement and the catenary curve.
Figure 4.4: Comparison Between Measured and Catenary Curve (Tern)
The measured conductor displacement showed that the real conductor has a shallower gradient in the mid section of the conductor furthest from the ends then the catenary with a sharp increase in the gradient near the ends. These results were mirrored for all conductor types
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sampled. This indicates that the behaviour of a high tension overhead conductor does not exactly follow catenary theory.
4.3.3 Model vs. Measured Results
Using the mid-span displacement as a point of comparison, the relative error of the modelled results to the measured displacement can be determined. Using the Rabbit conductor as an example, Figure 4.5 shows the three steady state results as modelled.
Figure 4.5: Rabbit Steady Model Results
Figure 4.6 shows the three results for the steady state displacement of the Rabbit conductor as measured at the VRTC.
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Figure 4.6: Rabbit Steady Measured Results
Tables 4.4, 4.5, 4.6 list the measured and modelled results and the relative error.
Table 4.4: Rabbit Steady State Results Comparison
Tension Measured Modelled Relative Error
2775N 0.663 0.5105 23.00%
3700N 0.506 0.4341 14.21%
4630N 0.343 0.3716 8.34%
Table 4.5: Pelican Steady State Results Comparison
Tension Measured Modelled Relative Error
8120N 0.552 0.5664 2.61%
11200N 0.532 0.486 8.64%
13070N 0.447 0.4433 0.82%
Table 4.6: Tern Steady State Results Comparison
Tension Measured Modelled Relative Error
14800N 0.886 0.5555 37.30%
19740N 0.694 0.482 30.54%
24675N 0.583 0.4191 28.11%
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A wide variation in relative accuracy exists between the measured and modelled results for the three conductor types as the tension was varied. For two of the conductor types as the tension was increased the models accuracy improved relative to the measured results. The Pelican results achieved a close agreement, whilst the Tern was not a close fit.