Application of the Front-Fixing Method for Numerical Modelling of Field Diffusion in Non-linear Systems

Application of a finite difference front fixing method for modelling magnetic field and associated power loss in high temperature superconductors or other strongly non-linear phenomena is considered. Advantages of the scheme are discussed and implementation problems highlighted. Particular attention is paid to conservation properties of the algorithm and accurate solutions close to the transition boundaries. The algorithm is tested using a well-known solution of the spherical diffusion problem with complex conditions at the moving interface

Analytic approximations for the broadening of the spectral lines of hydrogen-like ions

Broadband approximate expressions for calculating the broadening of the spectral lines of hydrogenlike ions in a multicomponent plasma are derived taking into account both the influence of the interaction between plasma particles on the distribution function of the plasma microfield and the effect of the microfield dynamics on the broadening of the central component of the spectral line. With the approximate expressions proposed, the calculation of the shape of a given spectral line of a certain ion in a plasma with a given ion composition requires only a few seconds of computer time. The approximate expressions provide a good computational accuracy not only for the central component of the spectral line but also for the spectral line wings

Application of the Front-Fixing Method for Numerical Modelling of a Thermistor Problem

Application of a finite difference front fixing method to a thermistor problem with strongly non-linear material properties is discussed. Advantages and implementation problems of the method are highlighted. Particular attention is given to conservation properties of the algorithm and accurate solutions close to the moving transition region. The algorithm is tested using a well-known solution of the plane diffusion problem with complex conditions at the moving interface

Efficient Numerical Modelling of Field Diffusion in High-Temperature Superconducting Wires

Multidimensional field diffusion problems with front-type behaviour, moving boundaries and non-linear material properties are analysed by a finite volume front fixing method. Advantages and implementation challenges of the method are discussed with special attention given to conservation properties of the algorithm and achieving accurate solutions close to the moving boundaries. The technique is validated using analytical solutions of diffusion problems with cylindrical symmetry

Two Dimensional Numerical Model to Predict the Thermal-Chemical Degradation of a piece of Carbon Fibre Composite (CFC) due to Laser Ablation

There is a growing interest in using carbon fibre composites (CFC) as a high tech construction material. The reason for this is that CFCs have similar mechanical performance to that of the more traditionally used materials like aluminium alloys, whilst being considerable lighter. The benefits of using a lighter material are vast. However whilst CFC have similar structural properties to that of aluminium its electrical and thermal properties are very different. This becomes important if CFCs are placed in an environment where the pieces of CFC could be struck by lightning as this interaction will damage the panels [1]. Previous studies published by N. Jennings and C. J. Hardwick [2] and F Lago et. al. [3] have attempted to model the damage caused to a piece of CFC due to a lightning strike. However these models have only considered very simple degradation methods and also did not include gas transport. The study presented here is an expansion of what has been discussed previously [4]. A two dimensional numerical model has been built which is designed to predict the damaged caused to a piece of CFC due to a lightning strike. Initial verification of the model is conducted by decoupling the thermal physics from the electrical effects and damaging the pieces of CFC by using laser ablation. The two dimensional numerical model (2D) includes thermal chemical degradation of the polymer via pyrolysis, the resultant gas transport through the decomposing material and carbon fibre vaporisation. An image of the x-ray tomography results of the laser ablated CFC samples are shown in figure 1. The predictions from the 2D model provide a reasonable agreement with the experimental results. Although further expansion of the model, into three dimensions, is required before a true validation of the numerical predictions can be achieved

Mathematical Modelling on Bridge Formation in Contaminated Transformer Oil

Oil is an essential insulating and cooling medium used in a vast range of high voltage equipment from cables to transformers and switchgears. Analysis on power transformer failures has revealed that insulation/oil contamination is a major factor, accounting for nearly 30% of the total failures. As a result there is a great deal of research interest in understanding the composition, insulating performance, ageing processes and breakdown mechanisms in such oils. In the present project we are focussing on the effect of particle contamination of transformer oil on electrical performance of the power transformer using mathematical modelling. The model is based on the current knowledge of dielectrophoresis (DEP) whereby a neutral body placed in an electric field becomes polarized and is equivalent to an electric dipole with an excess of positive charge on one end and negative charge on the other. The forces acting on the two ends do not balance in non-uniform electric field region and the particle moves. This phenomenon was first described by Pohl [1]. Our initial model is based on fibrous particles using equation derived by Lipowicz et al [2]. On comparison with the experimental data, the model is a successful means of predicting bridge formation rate. The electric current does not match with that observed in the experiment as the contamination level used in experiment is by weight rather than volume as for simulation. Furthermore, the simulation model is based on several assumptions of unknown variables in our initial attempt. Nevertheless, the rate of change in terms of current increase is similar to the experiment. Continual improvements to the model will allow us to make more accurate predictions regarding the current

Experimental Investigation on Bridge Formation in Contaminated Transformer Oil

Oil is an essential insulating and cooling medium used in a vast range of high voltage equipment from cables to transformers and switchgears. Analysis on power transformer failures has revealed that insulation/oil contamination is a major factor, accounting for nearly 30% of the total failures. As a result there is a great deal of research interest in understanding the composition, insulating performance, ageing processes and breakdown mechanisms in such oils. In this project we are focussing on the effect particle contamination of transformer oil on electrical performance of high voltage power transformers. Transformer oil fulfils purpose both as electrical insulation and coolant. During the operation it contacts with metal, iron core and pressboard insulation inside a transformer. Contaminants such as metal filings or cellulosic residual can be formed in the oil, especially for transformers with aged paper insulation. During normal operation non-uniform fields are present within the transformer. These contaminants tend to move towards high field regions due to dielectrophoresis (DEP) forces and could form a bridge over a period of time. The bridge may potentially act as a conducting path between two different potentials within the transformer structure, leading to partial discharges or insulation failure. Initial experiment on pressboard cellulosic particles has demonstrated that pre-breakdown phenomena are closely related to the level of contamination [1]. Current work focuses on experiments with different sizes of pressboard particles under dc voltages. Three different levels of contaminants (0.0025, 0.0050 and 0.0075% by weight) have been used for the experiment. Current is also monitored whilst a dc voltage applied across a known contaminated oil sample. It has been found that at higher voltages the rate of bridge formation is increased along with an associated current increase