The study of the self-damping properties of overhead transmission line conductors subjected to wind-induced oscillations.

Doctoral Degree. University of KwaZulu-Natal, Durban.Conductors are flexible, elastic structural components of power lines. The relatively high flexibility of the conductors, coupled with the long spans and the axial tension, makes conductors to be highly prone to dynamic excitation such as wind loading. The problem of the dynamic behavior of overhead power transmission line conductors under the action of wind and other forms of excitations is very important, since it proffers the optimal design of the line in terms of its dynamic characteristics. Thus, mechanical vibration of power lines needs to be mitigated, especially from aeolian vibration as they can lead to damage of the lines causing power interruptions. The dynamic behaviour of conductors can be influenced by its damping. However, available tools for the analysis of this phenomenon is scarce. The objective of this study is to evaluate the conductor self-damping. The goal is to characterize and ascertain the influence of various conductors’ parameters on the amount of energy dissipation. In this study, a numerically based investigation of the response of conductors was carried out i.e. finite element analysis (FEA or FEM). This was used to model the conductor using a new modeling approach, in which the layers of its discrete structure of helical strands were modelled as a composite structure. Due to the helical structure of the conductor strands, this give rise to inter-strands contacts. During bending caused by external loading, the stick-slip phenomenon does occur around the contact region resulting in damping of energy out of the system. Characterizing the damping mechanism as hysteresis phenomenon, this resulted from coulomb’s dry-friction with the stick-slip regime at contacts points between the conductor strands. Employing contact mechanics to characterize and the use of FEM to discretize these contact regions, parameters such as the contact forces, strain and stress were established. When the conductor experiences a dynamic excitation in a sinusoidal form, a hysteresis loop is formed. The use of contact region parameters, to evaluate the area of the hysteresis loop and the area of the loop determines the amount of self-damping. Experimental studies were conducted to validate the FEM model. Two forms of experiment were done. The first was the sweep test, done at a specified axial tension i.e. as a function of its ultimate tensile strength. This was used to determine the resonance frequencies for the conductors. In the second test, using the determined resonance frequencies from the first test were used to vibrate the conductors at these frequencies to establish the hysteresis loop at the same specified axial tension. The experiment was conducted with four different conductors with different number of layers. This was used to establish the relation between the numbers of layer and the amount of damping from the conductor. The conductors’ vibration experimental results obtained at a defined axial tension (as percentage of its UTS) correlate with that of FEM model. The results obtained showed a general increase in the resonance frequencies of vibration and a decrease in damping as the axial tension of the conductor is increased. The establishment of the hysteretic constitutive behaviour of strands under specific loading conditions as described in the thesis, using this FEM model, an algorithm was developed to evaluate the conductor self-damping. Based on this algorithm, computer programs have been developed to evaluate the conductor’s dynamic behaviour and implemented in MATLAB environment. Due to the very close relation between damping and conductor fatigue, this model can also be extended to investigate fatigue failure of conductors

High Voltage Transmission Line Vibration: Using MATLAB to Implement the Finite Element Model of a Wind-Induced Power-Line Conductor Vibration

Wind-induced vibration affects the performance and structural integrity of high voltage transmission lines. The finite element method (FEM) is employed to investigate wind-induced vibration in MATLAB. First, the FEM model was used to develop the equation of motion of the power line conductor. In addition, dampers, conditions for damping, free and forced vibrations of the overhead conductor were considered in the FEM model. Wind-induced experiments were conducted in the laboratory using an actual overhead power conductor. The developed FEM models were simulated in the MATLAB computing environment. The results from the MATLAB simulation, finite element and experimental recordings were compared in order to evaluate the efficacy of models simulated in MATLAB and developed using the FEM

Dynamic characteristics of bare conductors.

Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2011.The dynamic characteristic of transmission line conductors is very important in designing and constructing a new line or upgrading an existing one. This concept is an impediment to line design and construction because it normally determines the tension at which the line is strung and this in respect affects the tower height and the span length. Investigations into the phenomenon of mechanical oscillation of power line conductors have been extensively looked into by many researchers using concepts from mechanics and aerodynamics to try and predict the conductor dynamic behaviour. Findings have shown that precise prediction of conductor windinduced vibration is very difficult i.e. non-linearity. Over the years, various analytical models have been developed by researchers to try and predict the mechanical vibration of transmission line conductors. The first part of this dissertation considers the analysis of the model describing the transverse vibration of a conductor as a long, slender, simply supported beam, isotropic in nature and subjected to a concentrated force. The solution of this beam equation was used to obtain the conductor natural frequencies and mode shapes. Conductor self-damping was obtained by the introduction of both external and internal damping models into the equation of motion for the beam. Next, also using the same beam concept was the application of the finite element method (FEM) for the dynamic analysis of transmission line conductors. A finite element formulation was done to present a weak form of the problem; Galerkin‟s method was then applied to derive the governing equations for the finite element. Assembly of these finite element equations, the equation of motion for the transverse vibration of the conductor is obtained. A one dimensional finite element simulation was done using ABAQUS software to simulate its transverse displacement. The eigenvalues and natural frequencies for the conductors were calculated at three different tensions for two different conductors. The damping behaviour of the conductors was evaluated using the proportional damping (Rayleigh damping) model. The results obtained were then compared with the results from the analytical model and the comparison showed a very good agreement. An electrical equivalent for the conductor was developed based on the concept of mechanicalelectrical analogy, using the discrete simply supported beam model. The developed electrical equivalent circuit was then used to formulate the transfer function for the conductor. Matlab software was used to simulate the free response of the developed transfer function. Finally, the experimental study was conducted to validate both the analytical model and the FEM. Tests were done on a single span conductor using two testing methods i.e. free and force vibration. The test results are valid only for Aeolian vibration. From the test results the conductor‟s natural frequencies and damping were determined. The experimental results, as compared with the analytical results were used to validate the finite element simulation results obtained from the ABAQUS simulation

A Comparative Assessment of Conventional and Artificial Neural Networks Methods for Electricity Outage Forecasting

The reliability of the power supply depends on the reliability of the structure of the grid. Grid networks are exposed to varying weather events, which makes them prone to faults. There is a growing concern that climate change will lead to increasing numbers and severity of weather events, which will adversely affect grid reliability and electricity supply. Predictive models of electricity reliability have been used which utilize computational intelligence techniques. These techniques have not been adequately explored in forecasting problems related to electricity outages due to weather factors. A model for predicting electricity outages caused by weather events is presented in this study. This uses the back-propagation algorithm as related to the concept of artificial neural networks (ANNs). The performance of the ANN model is evaluated using real-life data sets from Pietermaritzburg, South Africa, and compared with some conventional models. These are the exponential smoothing (ES) and multiple linear regression (MLR) models. The results obtained from the ANN model are found to be satisfactory when compared to those obtained from MLR and ES. The results demonstrate that artificial neural networks are robust and can be used to predict electricity outages with regards to faults caused by severe weather conditions