CHAPTER 5: Conclusion
5.1 Research summary
Since their advent in the 1950s in the power line industry, the mechanical properties of the FRP used for the core of the composite insulator are still under investigation, even though much progress has been made in the manufacture of these materials. Actually, the mechanical standard tests proposed for composite insulators are limited mainly to static load conditions. This could be insufficient in making decisions on the mechanical loading considerations of these insulators. This is because, while in service, insulators are exposed to dynamic loads generated by the conductor’s oscillation, namely, Aeolian vibrations.
It was thus the intention of this research to determine experimentally the mechanical loads generated by overhead conductors stretched at different tensions. Once those loads were known and applied to the composite insulators, the mechanical behaviour of these insulators (suspension and line post) was investigated. The wind effect simulated on the overhead line conductors was under Aeolian vibration conditions. Two major types of tests were conducted: the swept frequency method, and the steady frequency method. Three support configurations were used.
The swept frequency method conducted by the Puma control system provided the dynamic response of the bending amplitude and the vibrational loads of the overhead conductors under a range of frequency provided by the Strouhal formula.
Interesting results are found on the relationship between the dynamic loads and the bending amplitude, which reached highest peak value at the lowest frequencies (under 10 Hz). The heaviest conductor has a smaller range of frequency (5-50 Hz) where peak value of loads and bending amplitudes are higher before the constant damping state, while the lighter conductor presented a long and valuable frequency range (5-70 Hz).
The steady frequency method was conducted at multiple values of the alternating peak- to-peak bending amplitudes from 0.1 to 1.0 mm as the standard values. Thus, the
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dynamic loads of the overhead conductors at different tensions (15, 20, 25 and 30%
UTS) were measured by a force transducer, while the LVDT was measuring the peak- to-peak bending amplitudes.
In order to study more in-depth the dynamic load effects, the articulated clamp used was blocked, as described in [33]. The blocked clamp configuration presented high loads compared with the articulated clamp. These experiments showed that for all clamp configurations, the axial compressive and tensile loads are of nearly the same intensity.
The vibrational loads of the vibrating conductor collected by the force transducer while using the suspension insulator were much lower compared to the blocked and articulated clamp configuration. It has been noticed that the suspension insulator was also subjected to compression or axial tensile loads, which may be compared with a rod subjected to longitudinal vibrations [61]. Like the presence of both composite insulators in the line (suspension and line post) reduced considerably the oscillation force of the conductor at the suspension point, this has led to conclude that the composite insulator have some damping properties which reduce the impact of mechanical load generated by the vibrating conductor. The dynamic behaviour of the suspension insulator collected by strain gauge measurements on the FRP rod of the insulator showed that the inferior part of the suspension insulator closed to its end fittings was more exposed to the vibrational loads of the conductor.
The dynamic loads on the line post insulator were even lower than using a suspension insulator. This conclusion arose while comparing the static measurement with the dynamic. The mechanical behaviour of the line post insulator with the strain gauges attached to the FRP rod indicated that at low frequency only vertical mechanical loads were considered. While the frequency was increased, this brought more stress to the insulators by increasing the longitudinal vibrational loads, which was nearly insignificant at low frequency.
The axial strain of the suspension insulator or the bending strain of the FRP rod or the line post insulator subjected to vertical loads measured by the strain gauges showed that strains of those insulators were proportional to the bending amplitudes of the vibrating conductor.
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The combination and orientation of insulator’s mechanical loading depend on the type of insulators used (suspension or line post) and the effects generated by the two types of conductors used (ACSR) were slightly the same as mentioned. It is well known that composite material (Fiberglass reinforced plastic) resist badly to the torsion but once mounted on the overhead line, composite insulators experience less torsion in service because of their assembly configurations:
For the suspension insulator: there was a rotation freedom of the long rod insulators according to its axis allowed by the insulator end fittings.
For the line post insulator, the suspension clamp mounted to the insulator is free to rock according to its pin axis. This makes the insulator to avoid torsion. The configuration of the line post with the suspension clamp blocked was careful studied in this research. It was found that the twisted movement (torsion forces) created by the vibrating conductor around the line post insulator was mainly avoided by the cantilever position of the insulator. This action was reacting like a spring beam that was going up and down.
The vibrational loads created by the vibrating conductor showed that the lowest part of the suspension insulator (from the middle to the end-fitting close to the suspension clamp of the conductor) was more exposed to high mechanical stresses as indicated by the strain gauge measurement. For the line post insulator, the stressed area of the insulator is the area close to the end-fitting that is mounted to the support tower. This scenario is comparable to a cantilever beam subjected to a vertical point load at its free end. The static tests done confirmed the Bernoulli’s theorem: The strains on the top and the bottom of the insulator were nearly symmetrical to the x-axis (Figure 4-22). Also both gauges on the neutral axis had the same value (theoretically zero) [59].
The movement of the suspension clamp mounted to the line post insulator showed that there is no major difference either the suspension clamp is articulated or blocked. The torsion forces on the line post insulator which could be caused by greater longitudinal loads were minimised by the rocking movement of the suspension clamp and the bending movement of the whole line post insulator. All restrictions on the suspension clamp seemed to be converted more in bending of the insulator which acted like a spring beam.
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