Les écoulements diphasiques en fracture se produisent dans de nombreuses applications industrielles et environnementales. Pour modéliser ces écoulements, il est important de connaître le rôle des caractéristiques géométriques des surfaces constituant les fractures.L’objectif de ce travail de thèse est d’étudier expérimentalement les écoulements diphasiques et la dispersion de traceurs dans des fractures réelles dont on connaît la cartographie des surfaces et d’en proposer une modélisation adéquate.L’écoulement diphasique inertiel a été modélisé à l’aide de la généralisation de la loi de Darcy couplée à l’approche de la fonction F. Le modèle proposé permet de prédire les perméabilités relatives et les saturations en fonction des débits des deux fluides avec deux paramètres d’ajustement.Les profils de concentration obtenus lors des expériences de dispersion ont été interprétés avec différents modèles afin d'évaluer leur capacité à décrire la dispersion dans les fractures et donc l’hétérogénéité de celles-ci. Les résultats montrent que le modèle stratifié, avec un seul paramètre estimé, donne des résultats satisfaisants et permet de quantifier l'hétérogénéité.Enfin, avec cinq paramètres estimés, à partir de la courbe de la concentration de traceur à la sortie des fractures, le modèle CTRW permet de décrire avec une bonne précision les profils de concentration de toutes les abscisses / The two-phase flows through fractures occur in many industrial and environmental processes.Modelling these flows needs understanding the role of geometric characteristics of the fractures surfaces. The objective of this thesis is to study experimentally the two-phase flow and the dispersion of tracers in fractures which the aperture map is known and propose a suitable model.The inertial two-phase flow model is based on the generalization of the single-phase full cubiclaw that accounts for non-Darcian effects by using the F function approach. On this approach,the presence of a second fluid is taken into account through a multiplier function introduced into the superficial velocity of each fluid. This model let us predict the relative permeability and the saturation as a function of the two fluid flows with two adjustment parameters.The concentration profiles obtained during dispersion experiments were interpreted with different models to study their ability to describe the dispersion in fractures and hence the heterogeneity of them. The results show that the classical convection-dispersion is not appropriate due to the heterogeneity of the fractures. On the other hand, the stratified model,with only one estimated parameter, gives satisfactory results and allows quantifying heterogeneity. Finally, with five estimated parameters, from the concentration curve at the outlet of the fractures, the CTRW model can describe the concentration profiles of all abscissas with good accuracy

Interconnection of electric power systems is becoming increasingly widespread as part of the power exchange between countries as well as regions within countries in many parts of the world. There are numerous examples of interconnection of remotely separated regions within one country. Such are found in the Nordic countries, Argentina, and Brazil. In cases of long distance AC transmission, as in interconnected power systems, care has to be taken for safeguarding of synchronism as well as stable system voltages, particularly in conjunction with system faults. With series compensation, bulk AC power transmission over very long distances (over 1000 km) is a reality today. These long distance power transfers cause, however, the system low-frequency oscillations to become more lightly damped. As a result, many power network operators are taking steps to add supplementary damping devices in their systems to improve the system security by damping these undesirable oscillations. With the advent of thyristor controlled series compensation, AC power system interconnections can be brought to their fullest benefit by optimizing their power transmission capability, safeguarding system stability under various operating conditions and optimizing the load sharing between parallel circuits at all times. This thesis reports the results of digital time-domain simulation studies that are carried out to investigate the effectiveness of a phase imbalanced hybrid single-phase-Thyristor Controlled Series Capacitor (TCSC) compensation scheme in damping power system oscillations in multi-machine power systems. This scheme which is feasible, technically sound, and has an industrial application potential, is economically attractive when compared with the full three-phase TCSC which has been used for power oscillations damping.<p> Time-domain simulations are conducted on a benchmark model using the ElectroMagnetic Transients program (EMTP-RV). The results of the investigations have demonstrated that the hybrid single-phase-TCSC compensation scheme is very effective in damping power system oscillations at different loading profiles.

Interconnection of electric power systems is becoming increasingly widespread as part of the power exchange between countries as well as regions within countries in many parts of the world. There are numerous examples of interconnection of remotely separated regions within one country. Such are found in the Nordic countries, Argentina, and Brazil. In cases of long distance AC transmission, as in interconnected power systems, care has to be taken for safeguarding of synchronism as well as stable system voltages, particularly in conjunction with system faults. With series compensation, bulk AC power transmission over very long distances (over 1000 km) is a reality today. These long distance power transfers cause, however, the system low-frequency oscillations to become more lightly damped. As a result, many power network operators are taking steps to add supplementary damping devices in their systems to improve the system security by damping these undesirable oscillations. With the advent of thyristor controlled series compensation, AC power system interconnections can be brought to their fullest benefit by optimizing their power transmission capability, safeguarding system stability under various operating conditions and optimizing the load sharing between parallel circuits at all times. This thesis reports the results of digital time-domain simulation studies that are carried out to investigate the effectiveness of a phase imbalanced hybrid single-phase-Thyristor Controlled Series Capacitor (TCSC) compensation scheme in damping power system oscillations in multi-machine power systems. This scheme which is feasible, technically sound, and has an industrial application potential, is economically attractive when compared with the full three-phase TCSC which has been used for power oscillations damping.<p> Time-domain simulations are conducted on a benchmark model using the ElectroMagnetic Transients program (EMTP-RV). The results of the investigations have demonstrated that the hybrid single-phase-TCSC compensation scheme is very effective in damping power system oscillations at different loading profiles.

This thesis is about the improved design of small size brushless DC fan motor driving circuit. Two main improvements in the new design are increase the stability and decrease the size of motor fan at the same time. To improve the stability, there are two major parts added to the original driving circuit. The delay circuit that protects the H-bridge and the output low current limit circuit. Furthermore, it is believed that the speed control also can improve the stability. With regard to the rotation speed control, two circuits are attached to the motor, 1) speed feedback controller and 2) speed and current feedback controller. Both controllers are attached in the close loop rotation speed control circuit. They are used to increase the efficiency of drive circuit. In order to make the circuit more efficient, they solve problems such as disturbance in miscellaneous noise; also the power dissipation that occurs in open loop rotation speed control circuit. The second improvement in the new design is to reduce the cost and size of system. The design of sensorless control scheme is proposed to replace the Hall sensor to detect rotor position. This sensorless scheme can also supply fan motor voltage to achieve the speed control.

This thesis presents a novel modulation scheme, called feedback dithering modulation, for DC to AC power converters. The feedback dithering modulator consists of a quantizer and a recently reported feedback dithering circuit, performing multilevel modulation with improved linearity and signal quality as opposed to the conventional modulation schemes. By combining the feedback dithering modulation and optimal control, a single-phase DC to AC power converter is built and tested. The resulting total harmonic distortion can be as low as 0.38% for a 25£[ load, or 0.47% when the output is open. Under the various operating conditions with DC voltages source varying from 190 V to 300 V and output powers from 0 to 600 W, the power converter always maintains a total harmonic distortion less than 1%, exhibiting high performance and excellent robustness.

No description available.

The environmental awareness has led to many political decisions and initiated laws that regulate the market towards responsible energy usage. The demand of sustainable power has led to an increasing integration of renewable energy sources to the electric grid. Solar power is the 3rd largest renewable power source after wind and bio-power. One of the main reasons to this fast expansion is the German renewable energy act that has motivated households to install PV systems in their houses. This has led to a large amount of producers on the low-voltage network. The small scale producers receive compensation for electricity generated from the PV systems, both when it is used directly in the producer’s home and when it is sold to the grid due to low usage. The systems can be more profitable by storing the energy instead of selling it on the grid. In this way the amount of bought electricity can be reduced. There are concerns regarding the connection of renewable sources to the grid. This project aims to examine the impact from single-phase PV systems on the low-voltage grid. The focus of this bachelor thesis is understanding problems such as harmonic distortion and grid asymmetry. Simulations have been carried out using the software MATLAB in order to study harmonic distortion in the output of a single-phase PV system. Grid asymmetry is examined through calculations and simulations of a worst case scenario in the software NEPLAN. This scenario contains a low voltage grid with a star-star connected transformer, where all PV-systems are connected to the same phase. The simulations in combination with a literature review have provided the conclusion that harmonic distortion caused by the inverter becomes higher when the voltage supply is too low. Integration of battery energy storage systems together with PV systems does not cause additional harmonic distortion. The results also show how single-phase systems contribute to the asymmetry in the grid. When the production from the PV systems is high, and all systems are connected to a certain phase, the current and voltage will also have an impact on the other phases in the worst case scenario.

The main purpose of this research - to investigate the mechanical and working characteristics of single - phase induction motor with asimmtrical stator. The other purpose - to evaluate single phase motor use with frequensy transformer.

A single-phase, buck-boost based, dual-output AC-DC converter is studied in this thesis. The converter has two DC outputs with opposite polarities, which share the same ground with the input power line. The power stage performance, including the input filter, is studied and procedure to select power components is given. The circuit model is analyzed to develop appropriate control. Zerocrossing distortion of the source input current is addressed and a solution is proposed. Experimental results are satisfactory in that a high power factor line current results for steady-state operation.

The removal of heat from the containment building is an important consideration in the design of a nuclear power plant. In this investigation a simple rectangular natural circulation loop was simulated to determine whether it could possibly be used to remove usable quantities of heat from a containment building. The loop had a vertical pipe on the inside and outside of the containment building. These pipes acted as heat exchangers. Single phase and two phase cases were simulated by imposing a temperature on the respective vertical leg pipe walls and determining the heat absorption from the containment building. The heat was conveyed from the inside of the building to the outside via the natural circulation phenomenon. A literature study was done to cover topics relevant to this investigation. A theoretical model using conservation equations and control volumes was derived. This model was based largely on knowledge gleaned from the literature study. The theoretical model was a simple homogenous model, which was sufficiently detailed for a conceptual investigation. The theoretical model was then manipulated into a form suitable for use in a computer simulation program. Simplifications were made to the simulation model and underlying theory due to the nature of the investigation. The simulation model was validated against published experimental results. During the simulation phase a number of cases were investigated. These cases were divided into base cases and parametric studies. During the base case simulations the change of key fluid variables along the loop was examined. During the parametric studies the hot and cold leg inside wall temperatures, loop geometry and pipe diameter were varied. The effect of these parameters on the heat absorption from the containment was determined. The simulations showed that with the current assumptions about 75 to 120 of the natural circulation loops are needed depending on their geometry and containment conditions. The heat removal rates that were calculated varied from 50 kW to 600 kW for a single loop. As explained in the final chapter, there are many factors that influence the results obtained. The natural circulation concept was deemed to be able to remove usable quantities of heat from the containment building. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2012.