CHAPTER 3: Experimental equipement & Set-up
3.4 Method of testing
3.4.3 Test procedure
To conduct a full-scale mechanical load (Figure 3-19),strain or bending strain test on Pelican and Tern conductors, several tests were conducted according to the following (Figures 3-5, 3-6, 3-21 and 3-22):
Line Post insulator tower Anchor
Tower
Test
Articulated clamp
Non-articulated clamp Suspension
clamp only
Suspension insulator
Articulated Non-articulated
Mechanical loads Mechanical loads & strains Bending strain Output measurement
Clamp configurations
Clamp configurations Support
configuration
Figure 3-19: Flowchart of the testing
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the static tension of the conductor (Table 3-5);
the type of support tower at the mid-span tower (suspension and line post) (Figures 3-2, 3-21, 3-22); and
the type of clamp configurations (non-articulated or articulated shown in Figures 3-5, 3-6, 3-22a and 3-22b).
It took a relatively long time to obtain conclusive results from all tests. Below is the flowchart of the testing at all four different static tensions (% UTS):
3.4.3.1 Installation of the conductors and insulators
The two ACSR conductors (Pelican and Tern) tested were pre-tensioned up to 35% of UTS before being relaxed for three days before the tests. As described previously, the pistol-grip strain-clamp was used on the Pelican conductor for the attachment to the dead-termination block and the loading arm.
Figure 3-20: End-termination block with pistol-grip strain clamp (a) of the Pelican and the dead-end clamp of the Tern (b)
3.4.3.2 Choice of the static tension of the conductor
Four levels of static tension from 15% of UTS with a 5% increment were applied to conductors (See Table 3-5). The static tension of 15% UTS was added to the range that is commonly used by Eskom in order to further investigate the influence of this tension of the conductor. The choice of these static tensions was motivated by everyday stress (EDS) used by ESKOM in transmission lines recommended by a procedure named allowable stress design (ASD). The following formula is recommended by ESKOM for the choice of the static loading (%UTS) [54]:
= (3.1)
Pistol grip Dead-end clamp
(a) (b)
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where: (N) represents the everyday tension, while (N/m) is the conductor weight per unit length, (m) referring to the catenary equation of the conductor [58]. ESKOM strongly recommends a maximum value of 1800m which results to 26.50 % UTS for ACSR Pelican, and 24.40 % UTS for ACSR Tern.
Table 3-7: Static tensions (kN) of the conductors used in relation to the UTS ACSR
Conductor
Ultimate Tensile Strength (UTS) (%)
15 20 25 30
Pelican 8.010 10.500 14.450 16.610 Tern 18.405 19.740 24.675 29.610 3.4.3.3 Support configurations
The anchor tower at mid-span allowed the possibility of having different clamp configurations (Figure 3-21). This was achieved by a movable fixation system which allowed maximum flexibility either to block or hold the clamp to/or by the force transducer. The following measurements were taken from the anchor tower:
Measurement of mechanical loads of the vibrating conductor with an articulated and/or a non-articulated clamp mounted on the tower via a force transducer mounted under the middle plate of the anchor tower.
Measurement of mechanical loads of the vibrating conductors and their effects on the suspension insulator in the strains were collected. The suspension insulator was mounted to the anchor tower via the force transducer. The middle plate was removed.
For the line post insulator mounted in cantilever, bending strains were only collected while the conductor was vibrating (Figure 3-22). Both tower set-ups used the LVDT (See Figure 3-21 & 3-22).
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Figure 3-21: Set-up of the anchor tower with composite insulator and suspension clamp (articulated and non-articulated)
Figure 3-22: Line post insulator tower and the insulator with two clamp configurations.
3.4.3.4 Mechanical loads and strain measurement
As explained previously, the mechanical loads of the conductor, the strain of the suspended insulator, and the bending strain of the line post insulator were measured as
Flange of post insulator
Articulated clamp
Non-articulated clamp
Dummy gauges Active strain gauges LVDT Stand
Line post support Line post insulator Suspension clamp
(a) (b)
(c)
Top plate of anchor tower Force transducer
Suspension insulator
Suspension clamp
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functions of the bending amplitude of the LVDT placed on the conductors at 89 mm away from the last point of contact of the suspension clamp.
a. Choice of the natural frequency
The sweep method (performed at 0.3 m/s), which is an experimental searching resonance frequencies method using an elastic connection, was applied, in order to find and select the stable measurement frequencies near natural frequencies of the conductor which would be used during the steady-frequency method deploying a rigid connection.
Thus, this was achieved for each conductor, each static tension, each clamp configuration and for each mid-span support. In accordance with the Strouhal formula, the frequency range was chosen between two limits. The experimental resonance frequencies found were checked by the VIP software, which is a resonance calculator based on the conductor parameters, i.e. linear mass, static tension, and the diameters.
Table 3-6 shows the frequency range used for both conductors.
Table 3-8: Frequency range of conductors tested during the sweep method
ACSR Conductor
Frequency (Hz) Minimum Maximum
Pelican 5 70
Tern 5 50
b. Choice of bending amplitude
The bending amplitude method was used according to the IEEE and IEC standards and CIGRE recommendations [1]. The bending amplitude measurement of the conductors measured by the LVDT was taken at 89 mm from the last point of contact (LPC) of the suspension clamp with the conductor (Figure 3-23).
Figure 3-23: Bending Amplitude method
Thus, a range of acceptable limits between 0.1mm and 1.0 mm constant peak-to-peak bending amplitudes was chosen, while the conductor was vibrating at steady frequency.
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