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CHAPTER 6
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The model results were of limited success, with varying levels of accuracy achieved. The model was shown to be capable of replicating the basic aspects of the conductor systems but could not adequately match the experimental results under the various conditions.
For the steady state resting displacement, the model adequately replicated the shape of a conductor hanging under the force of gravity, with the external tension causing the proper effects. The accuracy of the results was across the range of conductors was not regular though.
The constant frequency results were of limited accuracy in comparison to the experimental results. They were of a similar scale but showed little pattern in their results. It should be noted that the effectiveness of comparing individual frequency results is limited as a minor shift away from a resonance point will result in a large shift in amplitude, so comparing the model and test results could be misleading.
The swept model results showed that the model is capable of achieving the correct behaviour of a vibrating conductor when compared to the experimental results, in that the model replicated the resonance behaviour. The model lacked accuracy in the value of the frequency at which these resonance points occur, but the spread between peaks and the resonance profile matched the experimental results well.
The distributed wind results were of limited use. The model response using the determined wind force input was of a scale too small to be useful. It is unclear if the results were poor due to the model being overly stiff; the resonance peaks as determined using aeolian theory did not match the resonance patterns of the conductor; or if the wind force as applied was of insufficient strength.
The comparison between swept vs. steady frequency excitation provided an interesting result, in that it showed the difference in magnitude between the two modes of excitation. Also of interest was the uniform trend shown in the responses as they follow the same general curve.
Surprisingly the trend of which mode of excitation provided a larger response was not the same for the different conductors.
The frequency response functions were found for a conductor codenamed “Grosbeak” at different tensions and compared to the experimental results obtained by Castello & Matt. The modelled frf’s were not sufficiently accurate to be useful, but the model was shown to be capable of obtaining a frequency response function that follows the correct trend.
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The performance of the model in replicating the behaviour of the conductors was shown to be limited in the accuracy of the results. The finite element method applied in this manner is capable of reproducing the basic behavioural elements of different conductors under various loads but was not sufficiently accurate enough to properly replicate the experimental results.
The following are possible reasons for why the model did not achieve satisfactory results.
1. The application of the Rayleigh Damping method to this research was not tuned specifically to the free vibration problem before conducting the forced vibration results, in an attempt to model the vibration characteristics without prior access to a test setup to explicitly determine the damping factors. This resulted in the damping being unsatisfactory and damping the various modes incorrectly.
2. Increasing the element number of the model is limited by the availability of adequate computing power to solve large complex systems over long time frames.
3. The modelled distributed wind force input was of insufficient strength to cause a useful result, possibly due to the damping being too strong for the frequency range of the wind force
By addressing these limitations the accuracy of the model could be improved, particularly in the area of the wind-induced distributed loading.
6.4 Recommendations
Following on from this research certain recommendations can be made for future investigations.
The conductor damping should be improved, with the damping being made a function of a number of model characteristics, such as tension and stiffness of the conductor. The aerodynamic damping effects could also be considered. Model testing could also be limited to smaller sweep frequency ranges which would allow a more focused damping model for a specific frequency range.
The method of applying the external tension to the model should be improved, either by being applied within the model stiffness matrix or as part of the external forcing function. This would improve the final results of the model at rest, which would in turn improve the vibration results by ensuring the pre-loading is correct, which has a strong effect on the stiffness of the system.
The effects of non-linear conductor bending could be investigated, such as conductor slip and inter-strand friction. These result in hysteretic effects within the conductor during bending and
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become more prominent as overall conductor bending increases, such as when pre-loaded under gravity.
A more rigorous application of the wind force input to the model system should be developed and implemented. The force input could be modelled such that it includes the effects of wind in the transverse direction of the conductor to more closely model the effects of aeolian vibrations.
Improving the accuracy of the model by increasing the number of elements is not necessarily the best proposition, as the current choice of 60 elements for a conductor 86 metres long resulted in simulations that ran for multiple 24 hour periods. Increasing the number of elements would result in a slight improvement of accuracy at the expense of substantially increased run- time as well as exposing the possibility of the simulation failing due to insufficient computer memory. Increasing the element number by a significant amount would require access to substantially more powerful computer resources.
6.5 Research Summary
This research demonstrated that a finite element model could be derived to investigate the wind- induced vibration of overhead conductors in various loading conditions. The model was designed and used to investigate the behaviour of the overhead conductors under a range of different loading conditions and to analyse the effects of using a single-point shaker when testing an overhead conductor for resonance behaviour. The model was compared to results obtained by other researchers in similar research conditions.
Limitations in scope of the research reduced the models effectiveness in obtaining results that could be used to draw firm conclusions about the behaviour of conductors under the various loading conditions. It was shown however that the model successfully replicated the basic behavioural components of the conductors under test.
The results of the model and test results were discussed and the limitations in the model were considered. Possible avenues to improve and continue the research were presented and detailed.
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