Browsing by Author "Loubser, Richard Clive."

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The computational investigation of the wind-induced vibration of overhead conductors.
(2013) Athol-Webb, Avern Malcolm.; Bright, Glen.; Loubser, Richard Clive.
The reliable transmission of high-voltage electricity through overhead electrical conductors is a cornerstone of the modern industrialised world. Any situation or phenomenon that impedes the functioning of this network warrants investigation. An example of which is the occurrence of aeolian vibration. This is caused by airflow over the conductor breaking off into alternating turbulent vortices. These vortices can cause an alternating lift force on the conductor, resulting in unwanted vibrations and damaging fatigue loading. The Vibration Research Testing Centre (V.R.T.C.) of the University of Kwa-Zulu Natal is investigating the effects of this problem experimentally by oscillating an overhead conductor in a test facility. An electrodynamic shaker is used in a frequency and amplitude range equivalent to that produced by measured wind power input. This method is limited because only a single point force input to the conductor is possible as well as the limited span length. The aim of this research was to investigate the effects of aeolian vibrations and to develop a model that can verify the results of the V.R.T.C. This model can also be used to analyse scenarios that cannot be experimentally tested. A mathematical simulation of an overhead conductor subject to various wind power and single point oscillator inputs was developed. The mathematical simulation was performed using the MATLAB computing environment in the form of a finite element model. The model consists of a number of beam elements, arranged linearly to form a cable model, with suitable end conditions and driving inputs. The system was solved using a varying time-step 4th order Runge-Kutta solving method. The results of the model were compared to tests performed at the V.R.T.C. on a sample conductor length.
Condition monitoring of a rotor bearing system.
(2011) Grobler, Herbert Alfred.; Bright, Glen.; Loubser, Richard Clive.
The key objective for this research was to construct an experimental test rig along with a finite element model. Both had to accommodate a certain extent of misalignment and unbalance to provide induced vibrations in the system. Misalignment and unbalance was then varied in magnitude to identify the effect it has on the system. The next variable was the rotor speed and its effects. Finally the experimental and theoretical results were compared and the slight differences have been outlined and described. A rotor supported by two bearings with a disk attached to the middle and a three jaw coupling at the one end was considered for this research. The three jaw coupling consists out of two hub elements with concave jaws and a rubber element that fits in-between the jaws. The rotor-bearing system was subjected to unbalance at the disk and both angular and parallel misalignment at the coupling. Misalignment was achieved by offsetting the centre of rotation of the rotor and the motor shaft. Finite element analysis, along with Lagrange method, was used to model the behaviour of the system. A mathematical model for the three jaw coupling was derived to simulate its behaviour. The second order Lagrange model was reduced to a first order and solved using the Runge-Kutta method. Experimental results were obtained from a test rig and used to validate the theoretical results. Time domain and frequency spectrum were used to display the results.
Determination of dynamic loads caused by aeolian vibration on composite insulator : experimental approach.
(2015) Ntambwe, Kabwit Alain.; Loubser, Richard Clive.; Papailiou, Konstantin.
High voltage insulators have to fulfil high reliability criteria by being one of the main components for the operational safety and operating efficiency of the transmission system of electrical power. Various advantages and the attractive price of composite insulators have made them the most frequently used insulators in the power industry and have generally proven to be extremely reliable. Nevertheless, there are certain variable loads which occur on the overhead conductor in the field, such as mechanical loads caused by wind-induced vibrations, so-called Aeolian vibration, which might affect the insulators. It should be noted, that standardized tests of composite insulators are providing information only on their static mechanical strength This thesis describes the experimental investigation of a realistic model to determine the dynamic mechanical loads of the overhead line conductor undergoing Aeolian vibrations. In addition, the dynamic behaviour of the composite insulator was investigated. The tests were performed using a test span in an indoor laboratory. The test procedures used are the swept sine and the steady frequency tests. A shaker connected to a single conductor has simulated Aeolian vibration. Two types of ACSR (Aluminium Conductor Steel-Reinforced) conductor (Tern 45Al. /7St. and Pelican 18Al. /1St.) were tested on two different types of support configurations (suspension and line post), placed at mid-span at four different ranges of static tensile loads (15- 30% Ultimate Tensile Strength – UTS with 5% of increment). For both types of support configurations, the suspension clamp was blocked or articulated in order to know the influence of the oscillation motions on vibrational loads of the vibrating conductors. The measurements were done using the bending amplitude range of 0.1 to 1.0 mm on the conductor at 89 mm away from the last point of contact between the suspension clamp and the conductor. In accordance with the stick-slip bending model of overhead conductors, which predicts a certain degree of non-linearity at low vibration amplitudes, the results obtained were statistically analysed by graphing the vibrational loads versus the bending amplitude. It was found that the magnitude of dynamic mechanical loads on a composite insulator caused by Aeolian vibration is relatively low (10% less) compared to its mechanical strength and can thus most probably not induce fatigue failures on the insulator.
The effect of fretting marks introduced during strand winding on the fatigue performance of transmission line conductors.
Botha, Larry Chama.; Loubser, Richard Clive.; Stephen, Neil Robert.; Davidson, Innocent Ewean.
In this research, the Elasto-plastic interlayer contact of the TERN ACSR conductor used on Eskom’s 400kV transmission lines is investigated. Characterization of the elliptical contact marks using eucentric tilting in SEM stereomicroscopy for depth measurement is conducted. The size of these manufacturing defects (fret marks) resulting from the strand winding process is quantified. The already established observation that two geometrically dissimilar contacting surfaces may exhibit the same contact mechanics is extensively used to explain how the variations in stranding lay ratios affects the size of the fret marks and hence the surface quality of strands. From this supposition, an equivalent contacting sphere radius is calculated and used for both nonlinear and linear elastic finite element Analysis (FEA) in MSC Marc mentat. An inner conductor contact mechanics model for determining the normal contact force per defect is also presented and used with existing inner conductor mechanics models for tension determination. The calculated equivalent radius shows strong linear correlation with the defect size, contact force, plastic strain and stress and can therefore be used in the design of conductors. Fatigue testing was then conducted on two (02) TERN conductors. Fractographic analysis of the samples exposed to fatigue cycles was conducted using the Scanning Electron Microscopy (SEM) and Field Emission SEM (FEGSEM). Energy Dispersive Spectroscopy was also used for Surface Elemental analysis before and after the fatigue testing to analyse the changes in material composition on the fret mark surfaces.
Influence of wagon structure on the vertical response of freight.
(2002) Loubser, Richard Clive.; Kaczmarczyk, Stefan.; Adali, Sarp.
Historically, wagons have been designed according to the American Association of Railroads specifications. These require that wagons be designed to withstand a static load between the couplers of 350 tons. This implies that the structure has a certain stiffness. In order to improve load to tare ratio, there has been talk of reducing the end load specifications. This implies that the stiffness of the wagon will reduce. Using more flexible wagons implies that the freight will probably be exposed to a harsher dynamic environment. There is a trade off between the cost of packaging and the cost of protection devices installed in the vehicle. If handling damage can be prevented then an understanding of the dynamic environment will assist in reducing the packaging requirement. This research looked at the dynamic characteristics of an existing design of wagon using modal analysis. The results from the modal analysis were extended to be inputs to the time domain freight model. Various analytical models of the freight were developed depending on the configuration and dynamic properties. Special consideration was given to a cylinder with its axis transverse to the wagon. The modal model was modified to accommodate the change in mass imposed by the freight. The various sources of dynamic excitation were explored, namely inputs from the coupler and from the bogie. Data from shunting yard simulations were used to generate spectra as input to the wagon model. The objective was to use modal techniques to be able to take individual components, form them into a complete model and make informed decisions about the suitability of a certain configuration for traffic.
Life prediction of power line damper.
(2012) Badibanga, Kalombo Remy.; Loubser, Richard Clive.
Power line function is to transfer electrical power. Power lines represent a major component in the transport process of electricity and they are subjected to various types of failures. Causes of failure include wind-induced oscillations or Aeolian vibrations. Wind causes transmission line conductors to undergo oscillatory motions which cause failure. To mitigate oscillations of line conductors, Stockbridge dampers are used. It has been observed that dampers are subjected to the same undesirable and destructive effects from vibrations as the conductors they are meant to protect. In the case of a damper, the cyclic bending as well as the friction between its wire cables are caused by vibrations leading to failure. The mathematical model describing the bending stress of the symmetrical Stockbridge damper’s messenger cable near the clamped end is analyzed. The reliability of the mathematical model is assessed using experimental data obtained from the forced response test conducted at the VRTC laboratory at the University of KwaZulu-Natal, Durban. Data from the experiment has been compared with the MatLab model established by the researcher. Due to friction between the wires of the messenger cable, variation of temperature is observed in the messenger cable during operation. Change of temperature of the messenger cable was investigated, as a function of time, at constant velocity and constant displacement. Experimental data were generated during dynamic characteristic tests on Stockbridge dampers and thereafter the prediction of the variation temperature was undertaken. There are various mechanical characteristics of a damper that can be affected with time. To reach the aim of this study, three types of vibration test were conducted on the Stockbridge damper: the fatigue test, the forced vibrations test and the free vibrations test. Tests were conducted on a shaker machine with new and used Stockbridge dampers to determine the remaining life of those dampers by looking at their different mechanical properties. The frequency domain and time spectrum were used to display the results. The fatigue test investigated one of the commonest types of Stockbridge damper failure, namely, loss of the small mass because of sustained high frequency resonance. Ultimately, data correlated well and two mathematical models were developed: one for predicting damage in the life of a Stockbridge damper (based on the highest resonance frequency of the damper), and one for predicting the temperature of the messenger cable.
The relationship between the bending amplitude and bending stress/strain at the mouth of a so-called square-faced clamp for different conductor sizes and different tensile loads : experimental approach.
(2013) Kubelwa, Yatshamba Daniel.; Loubser, Richard Clive.
In this research, realistic models were developed using experimental approach and statistical or deterministic analysis in the relationship between bending amplitudes and bending stress (strain) of the overhead line conductor. This was rigidity clamped and subjected to Aeolian vibration (1Hz-150 Hz). The experiments were performed at the Vibration Research and Testing Centre (VRTC) laboratory of the University of KwaZulu-Natal. A shaker connected to the conductor was used to simulate the Aeolian vibration and transducers (accelerometers, thermocouples and strain-gauges) to control the shaker and collect data. For almost half a century, in transmission lines, bending stress which is a key factor in determining the life expectancy prediction of overhead conductor is assessed by using an idealized model the so-called Poffenberger-Swart formula based on cantilever beam theory and many assumptions [2]. Four overhead line ACSR (Aluminum conductor steel-reinforced) conductors i.e. Rabbit (6 Al. /1St.), Pelican (18 Al. /1St.), Tern (45 Al./7St.) and Bersfort(48 Al./7St.) were investigated at three different ranges of tensile load i.e. 20 %, 25%, and 30% Ultimate Tensile Strength (UTS). Bending amplitudes (0.0 1mm -1.2mm) and bending stresses measurements were collected and plotted as bending stress σb versus bending amplitude Yb, curve-fitting with polynomial function of the third order in terms of four parameters (where curve fitting coefficients B0, B1, B2, and B3) provided excellent simulations (predictions) of the experimental data for the conductors. However, it was found that the accuracy of the fit is not improved by the inclusion of higher order terms. Therefore, only four-parameters (for all cases high order than 3 are ambiguous, in spite of the Regression parameter or predictor were R2 ≥ 0.998 but Standard errors were large). It was noticed that the precedent model is the simplest polynomial to be employed for the characterization of all conductors investigated (for all three wires). Other ways to obtain the best curve fittings were explored and discussed such as power model. The experimental results were compared to the Poffenberger-Swart model. In all cases, it was observed that the deviation from the results to the above model is significant for small bending amplitudes and is small for high bending amplitudes and good correlations were observed when associated this with the bending stiffness model developed by Papailiou [17].