Dissertations / Theses on the topic 'Currents and voltages asymmetry'

The study described in this thesis is concerned with the touch and step voltage problems due to rail potential 'rise 'and the electro-chemical corrosion problem due to stray 'current in DC powered electric I railways. The two problems are interrelated and are both affected by system design philosophies and by operational conditions. In the past, various approaches to these problems have been based on experience accumulated over many years of railway construction and operation. However, without detailed 'studies, it has not been possible to' make realistic comparisons between the different approaches on a scientific basis. In order to assess the merits and demerits of different approaches, objective parameters are proposed in this thesis: the worst-case rail potential, rail potential envelopes, the total stray current and the gross leakage charge. In order to calculate these objective parameters and obtain other detailed data, mathematical` models are constructed and a comprehensive range of computer simulation software developed. These include both static and, dynamic models. Static models are used to evaluate the objective parameters under assumed conditions. Dynamic models, which are integrated simulators that combine the static models with the multi-train simulator, are used to simulate 'practical railway operations. Finally, the theoretical models are validated by practical tests, both in laboratory and on-site. Favourable agreement is obtained between simulation and test results.

This thesis presents an alternative method for measuring the rotor position of a synchronousgenerator, without the use of sensors to measure the rotor position. The method uses thecurrent and voltage measurements on the stator and rotor terminals from the protectionrelays. The rst step is to investigate how the rotor position can be estimated basedon the available measurement data. It was found out that the rotor position couldn'tbe estimated during dynamic state operation, when directly solving it from measurementdata. This is due to the presence of the eld and d-axis damper winding current dynamics,including the q-axis damper winding current. In order to achieve this, multiple stepsconsisting of dierent methods for estimating the rotor position were utilized. The rststep is to estimate the rotor position during steady state. The second step estimates therotor position iteratively based on initial values. This method is useful during a dynamicstate operation. The initial values are obtained from the rst step at the start of a dynamicstate operation. The nal step predicts the rotor position based on previous data. It isused during cases when the second step fails to estimate the rotor position accurately.Due to insucient data for estimating the rotor position during saturation, the eects ofsaturation for the simulated synchronous machine were not considered.
Detta projekt presenterar en alternativ metod för estimering av rotorvinkeln för en synkronmaskin med utpräglade poler, utan användning av sensorer. Metoden som används är att estimera rotorvinkeln genom uppmätt data av strömmar och spänningar från stator och rotor terminaler inom skyddsreläer. Första steget är att undersöka ifall rotorvinkeln kan bestämmas direkt ur mätdata. Det visade sig att rotorvinkeln kunde inte estimeras vid en dynamisk fas, genom att lösa rotorvinkeln direkt från mätdata. Detta på grund av strömmderivatan i fält och dämplindningen i d-led, samt strömmen i dämplindningen i q-led. För att uppnå detta, flera olika steg baserad på olika metoder användes för att estimera rotorvinkeln. Det första steget går ut på att estimera rotorvinkeln i ett stabilt läge. Det andra steget estimerar rotorvinkeln iterativt baserad på initialvärden. Denna metod är användbar vid ett icke-stabilt läge. Initialvärdena erhålls från steg ett vid början av ett icke-stabilt läge. Det slutliga steget förutsäger rotorvinkeln baserad på tidigare data. Den används då steg två misslyckas att estimera rotorvinkeln noggrant. På grund av otillräcklig data för att estimera rotorvinkeln under mättnad så togs inte mättnadsfenomen till hänsyn för den simulerade synkronmaskinen.

Дисертація на здобуття наукового ступеня доктора технічних наук за спеціальністю 05.09.03 – електротехнічні комплекси та системи. – Національний технічний університет "Харківський політехнічний інститут", Харків, 2018. Розглядаються методи оптимізації режимів систем електропостачання з несиметричними і нелінійними навантаженнями з метою підвищення енергетичних показників і розрахунку симетро-компенсуючих пристроїв. Обґрунтовано необхідність і можливість застосування комп'ютерних засобів, розроблено узагальнені алгоритми реалізації пошукової оптимізації. Розроблені методи успішно застосовані для оптимізації режимів в трипровідних і чотирипровідних системах, в тому числі в системах з взаємно-зв'язаними індуктивностями, в системах залізничного електропостачання, в системі з нейтралером, в системах живлення асинхронних двигунів, освітлювальних приладів високого тиску, в системах з силовими активними фільтрами, а також для випадків кількох навантажень з урахуванням вкладу кожного з них в зниження енергетичних показників системи.
Thesis for a Doctor’s degree in Engineering Science by specialty 05.09.03 – Electrical Engineering Complexes and Systems. – National Technical University "Kharkov Polytechnic Institute", Kharkiv, 2018. The dissertation is devoted to the development and research of optimization methods of power supply systems modes in electrotechnical systems with asymmetric and nonlinear loads in order to increase the energy indices and calculate the parameters of symmetry-compensating devices using mathematical and computer models, and using search engine optimization implemented with the use of modern software of computer mathematics. The necessity and the possibility of using computer tools for solving set tasks are substantiated. The generalized algorithms for implementing search engine optimization using modern software packages are developed. The possibilities of applying different optimization criteria for solving the problems of increasing the energy indices of power supply systems with asymmetric and nonlinear loads are shown. The properties of the search engine optimization system have been found to extrude the inappropriate element of the synthesized device, as well as the possibility of releasing the optimization variables by increasing their amount, which allows us to get ahead of a faster locating of the local minimum and then recalculate the parameters corresponding to the global minimum are shown. The developed method of search optimization using the models of power supply systems has been successfully applied for optimization of regimes and synthesis of symmetry-compensating devices in three-phase three-wire and four-wire systems of power supply. The possibilities of using the optimization tools for Mathcad and Matlab software packages are considered, in particular, using zero-order methods that do not require the calculation of derivatives, such as the deformed polyhedron method and the conjugate gradient method. The algorithm of load equivalence is proposed, with the help of which the symmetrical and asymmetric parts of the load are allocated. After this, the parameters of the balancing device are determined with sufficient accuracy by means of the Steinmetz and Kennely formulas. The method of currents direct symmetric component rotation with the preservation of symmetry and the mode of full reactive power compensation is proposed. For four-wire systems, the use of a generalized reactive element in a symmetry-compensating device is proposed, which accelerates the process of achieving the optimal solution. The method of determining the optimal mode based on the decomposition of the power supply system, which improves the convergence of the solution processes, is developed. Systems of power supply containing inductively coupled elements are considered. The calculation of the symmetry-compensating device of the traction system of the alternating current railway power supply is considered. A four-wire system with a neutralizer was studied, with the help of search engine optimization the parameters of the symmetry-compensating device were determined which allows to balance and counterbalance such a system. The possibilities of optimization of the regime in the power system of asynchronous motors, including the asymmetry of the supply network, are shown. Compensation of reactive power allows here to reduce the consumed currents and increase the efficiency of the system. To find the optimal modes of systems with an arc discharge, visual models have been developed that are adapted for use with the SimPowerSystem library elements. With the help of these models, the possibilities of increasing the power indices of arc discharge power supply systems, including high-pressure lighting devices, are investigated. It was shown that the optimization of the power factor alone, calculated with the help of the pro-posed methods, leads to decrease in the current consumed by the fundamental harmonic, which substantially reduces the losses in the transmission lines. For a thyristor compensator with single-stage switching, the advantage of symmetric control is proven, which greatly improves the spectrum of harmonics of supply currents. The use of the search optimization method to increase the power factor is shown without the use of traditional rather complicated control systems by power active filters. Comparison signals are used as control signals, synchronized with the phase voltages of the supply system. The amplitudes of these signals are accepted as optimization variables, and the optimization criterion is determined by the balance of active power in the system, which is characterized by the stabilization of the periodic voltage on the storage capacitor of the power active filter. The problems of synthesis of symmetric-compensating devices for several asymmet-rical loads in parallel and cascade connection are considered. The task is to determine the parameters of the symmetric-compensating devices for each of the loads separately, and the contribution to the creation of asymmetry and the generation of reactive power of each connected load must be taken into account. This problem is solved by the method of search optimization, and it is shown that, in forming the objective function, currents in the feeders, supplying energy from the point of connection of the load to the network to the common point of connection of the load and the symmetric-compensating device. It is effective to use the developed decomposition method, which makes it possible to simplify and accelerate the determination of the optimal regime of the system under study, taking into account the contribution of each load to the reduction of the energy parameters of the system as a whole. The case is also analyzed when two loads consisting of both unbalanced linear and nonlinear loads are simultaneously connected to the network. Optimization of the regime with increasing power factor is achieved by using a parallel power active filter with control over the proposed optimization algorithm. Methods and algorithms of search optimization developed for the purposes of increasing the energy indicators of power supply systems with asymmetric and nonlinear consumers developed and presented in the thesis work are characterized by high accuracy, the maximum possible use of computer technology, low computer time and the possibility of complete automation of design and research procedures in solving theoretical and practical tasks related to increasing energy performance and quality of electrical energy in power supply systems.

Дисертація на здобуття наукового ступеня доктора технічних наук за спеціальністю 05.09.03 – електротехнічні комплекси та системи. – Національний технічний університет "Харківський політехнічний інститут", Харків, 2018. Розглядаються методи оптимізації режимів систем електропостачання з несиметричними і нелінійними навантаженнями з метою підвищення енергетичних показників і розрахунку симетро-компенсуючих пристроїв. Обґрунтовано необхідність і можливість застосування комп'ютерних засобів, розроблено узагальнені алгоритми реалізації пошукової оптимізації. Розроблені методи успішно застосовані для оптимізації режимів в трипровідних і чотирипровідних системах, в тому числі в системах з взаємно-зв'язаними індуктивностями, в системах залізничного електропостачання, в системі з нейтралером, в системах живлення асинхронних двигунів, освітлювальних приладів високого тиску, в системах з силовими активними фільтрами, а також для випадків кількох навантажень з урахуванням вкладу кожного з них в зниження енергетичних показників системи.
Thesis for a Doctor’s degree in Engineering Science by specialty 05.09.03 – Electrical Engineering Complexes and Systems. – National Technical University "Kharkov Polytechnic Institute", Kharkiv, 2018. The dissertation is devoted to the development and research of optimization methods of power supply systems modes in electrotechnical systems with asymmetric and nonlinear loads in order to increase the energy indices and calculate the parameters of symmetry-compensating devices using mathematical and computer models, and using search engine optimization implemented with the use of modern software of computer mathematics. The necessity and the possibility of using computer tools for solving set tasks are substantiated. The generalized algorithms for implementing search engine optimization using modern software packages are developed. The possibilities of applying different optimization criteria for solving the problems of increasing the energy indices of power supply systems with asymmetric and nonlinear loads are shown. The properties of the search engine optimization system have been found to extrude the inappropriate element of the synthesized device, as well as the possibility of releasing the optimization variables by increasing their amount, which allows us to get ahead of a faster locating of the local minimum and then recalculate the parameters corresponding to the global minimum are shown. The developed method of search optimization using the models of power supply systems has been successfully applied for optimization of regimes and synthesis of symmetry-compensating devices in three-phase three-wire and four-wire systems of power supply. The possibilities of using the optimization tools for Mathcad and Matlab software packages are considered, in particular, using zero-order methods that do not require the calculation of derivatives, such as the deformed polyhedron method and the conjugate gradient method. The algorithm of load equivalence is proposed, with the help of which the symmetrical and asymmetric parts of the load are allocated. After this, the parameters of the balancing device are determined with sufficient accuracy by means of the Steinmetz and Kennely formulas. The method of currents direct symmetric component rotation with the preservation of symmetry and the mode of full reactive power compensation is proposed. For four-wire systems, the use of a generalized reactive element in a symmetry-compensating device is proposed, which accelerates the process of achieving the optimal solution. The method of determining the optimal mode based on the decomposition of the power supply system, which improves the convergence of the solution processes, is developed. Systems of power supply containing inductively coupled elements are considered. The calculation of the symmetry-compensating device of the traction system of the alternating current railway power supply is considered. A four-wire system with a neutralizer was studied, with the help of search engine optimization the parameters of the symmetry-compensating device were determined which allows to balance and counterbalance such a system. The possibilities of optimization of the regime in the power system of asynchronous motors, including the asymmetry of the supply network, are shown. Compensation of reactive power allows here to reduce the consumed currents and increase the efficiency of the system. To find the optimal modes of systems with an arc discharge, visual models have been developed that are adapted for use with the SimPowerSystem library elements. With the help of these models, the possibilities of increasing the power indices of arc discharge power supply systems, including high-pressure lighting devices, are investigated. It was shown that the optimization of the power factor alone, calculated with the help of the pro-posed methods, leads to decrease in the current consumed by the fundamental harmonic, which substantially reduces the losses in the transmission lines. For a thyristor compensator with single-stage switching, the advantage of symmetric control is proven, which greatly improves the spectrum of harmonics of supply currents. The use of the search optimization method to increase the power factor is shown without the use of traditional rather complicated control systems by power active filters. Comparison signals are used as control signals, synchronized with the phase voltages of the supply system. The amplitudes of these signals are accepted as optimization variables, and the optimization criterion is determined by the balance of active power in the system, which is characterized by the stabilization of the periodic voltage on the storage capacitor of the power active filter. The problems of synthesis of symmetric-compensating devices for several asymmet-rical loads in parallel and cascade connection are considered. The task is to determine the parameters of the symmetric-compensating devices for each of the loads separately, and the contribution to the creation of asymmetry and the generation of reactive power of each connected load must be taken into account. This problem is solved by the method of search optimization, and it is shown that, in forming the objective function, currents in the feeders, supplying energy from the point of connection of the load to the network to the common point of connection of the load and the symmetric-compensating device. It is effective to use the developed decomposition method, which makes it possible to simplify and accelerate the determination of the optimal regime of the system under study, taking into account the contribution of each load to the reduction of the energy parameters of the system as a whole. The case is also analyzed when two loads consisting of both unbalanced linear and nonlinear loads are simultaneously connected to the network. Optimization of the regime with increasing power factor is achieved by using a parallel power active filter with control over the proposed optimization algorithm. Methods and algorithms of search optimization developed for the purposes of increasing the energy indicators of power supply systems with asymmetric and nonlinear consumers developed and presented in the thesis work are characterized by high accuracy, the maximum possible use of computer technology, low computer time and the possibility of complete automation of design and research procedures in solving theoretical and practical tasks related to increasing energy performance and quality of electrical energy in power supply systems.

Electrohydrodynamics (EHD) is a promising research field with several trending applications. Even though the phenomenon was first observed centuries ago, there is very little research until the middle 20th century, as the mechanisms behind it were very poorly understood. To this date, the majority of research is based on the development of empirical models and the presentation of laboratory experiments. This work begins with an extensive literature review on the phenomenon, clarifying conflicts between researchers throughout the history and listing the findings of the latest research. The literature review reveals that there are very few mathematical models describing even the most important parameters of the EHD fluid flow and most are either empirical or greatly simplified. As such, practical mathematical models for the assessment of all primary performance characteristics describing EHD fluid accelerators (Voltage Potential, Electric Field Intensity, Corona Discharge Current and Fluid Velocity) were developed and are begin presented in this work. These cover all configurations where the emitter faces a plane or another identical electrode and has a cylindrical surface. For configurations where the emitter faces a plane or another identical electrode and has a spherical surface, Corona Discharge Current and Fluid Velocity models have been presented as well. Laboratory experiments and computer simulations were performed and are being thoroughly presented in Chapter 4, verifying the accuracy and usability of the developed mathematical models. The laboratory experiments were performed using two of the most popular EHD electrode configurations - wire-plane and needle-grid. Finally, the findings of this research are being summarized in the conclusion, alongside with suggestions for future research. The step-by-step development of the equipotential lines mathematical model is presented in Appendix A. Appendix B covers the mathematical proof that the proposed field lines model is accurate and that the arcs are perpendicular to the surface of the electrodes and to all of the equipotential lines.

Three-phase voltage source converters(VSCs) have become the converter of choice in many ac medium and high power applications due to their many advantages, including low harmonics, high power factor, and high efficiency. Modular VSCs have also been a popular choice as building blocks to achieve even higher power, primarily through converter paralleling. In addition to high power ratings, paralleling converters can also provide system redundancy through the so-called (N+1) configuration for improved availability, as well as allow easy implementation of converter power management. Interleaving can further improve the benefit of paralleling VSCs by reducing system harmonic currents, which potentially can increase system power density. There are many challenges to implement interleaving in paralleled VSCs system due to the complicated relationships in a three-phase power converter system. In addition, to maximize the benefit of interleaving, current knowledge of symmetric interleaving is not enough. More insightful understanding of this PWM technology is necessary before implement interleaving in a real paralleled VSCs system. In this dissertation, a systematic methodology to analyze and design a paralleled three-phase voltage source converters with interleaving is developed. All the analysis and proposed control methods are investigated with the goal of maximizing the benefit of interleaving based on system requirement. The dissertation is divided into five sections. Firstly, a complete analysis studying the impact of interleaving on harmonic currents in ac and dc side passive components for paralleled VSCs is presented. The analysis performed considers the effects of modulation index, pulse-width-modulation (PWM) schemes, interleaving angle and displacement angle. Based on the analysis the method to optimize interleaving angle is proposed. Secondly, the control methods for the common mode (CM) circulating current of paralleled three-phase VSCs with discontinuous space-vector modulation (DPWM) and interleaving are proposed. With the control methods, DPWM and interleaving, which is a desirable combination, but not considered possible, can be implemented together. In addition, the total flux of integrated inter-phase inductor to limit circulating current can be minimized. Thirdly, a 15 kW three phase ac-dc rectifier is built with SiC devices. With the technologies presented in this dissertation, the specific power density can be pushed more than 2kW/lb. Fourthly, the converter system with low switching frequency is studied. Special issues such as beat phenomenon and system unbalance due to non-triplen carrier ratio is explained and solved by control methods. Other than that, an improved asymmetric space vector modulation is proposed, which can significantly reduce output current total harmonic distortion (THD) for single and interleaved VSCs system. Finally, the method to protect a system with paralleled VSCs under the occurrence of internal faults is studied. After the internal fault is detected and isolated, the paralleled VSCs system can continue work. So system reliability can be increased.
Ph. D.

Thesis (MTech (Electrical Engineering))--Cape Peninsula University of Technology, 2009
The industrial and population growth of a nation can cause power delivery problems to localized areas of a national grid through their increased demand for electrical energy. One reason for these power shortages is the insufficient current carrying capacity of existing high voltage alternating current, (HVAC), transmission lines supplying the area. High voltage direct current (HVDC) transmission lines are a possible solution as they provide better power delivery than HVAC lines. New or upgraded HVAC lines, or HVDC lines or combinations of HVAC and HVDC lines are possible solutions to improve power delivery. This research investigates the various line possibilities using theory. and cutting edge frequency and time domain software tools. The challenge is how to approach this problem. What methodology or structure should be used? Thus one of the contributions of this work is the development of a strategy (flow chart), for solving power delivery problems to localized areas of a national grid through individual or combinations (e.g. parallel operation) of HVAC and/or HVDC transmission lines. The main contribution is the evaluation of a HVDC system as a solution to overcoming power delivery shortages to a localized area of a national grid. Three different software packages (two industrial and one academic) namely, PSCAD/EMTDC (time domain), DlgSILENT PowerFactory (frequency domain) and MathCAD software are evaluated for their capability to perform the simulation studies necessary to prove the possible solutions given in the developed flow chart. The PSCAD/EMTDC software package is evaluated for integrated HVAC/HVDC load flow analyses, DlgSILENT for individual and parallel combinations of HVAC lines and MathCAD to prove hand calculations to software results. Five case studies are conducted. The first case study demonstrates a healthy system with no delivery shortcomings, the second case study portrays the delivery shortcoming due to increased localized area demand, and the remaining three case studies explore possible solutions to solve the problem. The first possible solution is to construct an identical HVAC line in parallel to the existing line.

Induction motors are commonly used in numerous industrial applications. To maintain a reliable operation of the motor, it is important to identify the potential faults that may cause the motor to fail. Bearing failures are one of the main causes of motor breakdown. The causes of bearing damage have been studied in detail for a long time. In some cases, bearing failed due to the current passing through them. In this thesis, bearing currents in an inverter driven five-phase induction motor are studied and a new solution is proposed. First, theory of shaft voltage and bearing current are presented. The causes are identified and current solutions are discussed. Then, new switching patterns are proposed for the five-phase induction motor. The new schemes apply a modified algorithm for the space vector pulse-width-modulation (SVPWM). The system is simulated and the results of the new switching patterns are compared with the conventional switching pattern. Finally, the new schemes are experimentally tested using a digital signal processor (DSP) to drive the five-phase IGBT inverter. The experimental results verified that the new switching pattern could reduce shaft voltages and bearing current without affecting the performance.

碩士
國立中興大學
電機工程學系所
98
The hydrogenated amorphous silicon thin-film transistors (a-Si:H TFTs) were important electronic devices in many electronic products. The a-Si:H TFT technology in active-matrix liquid-crystal displays (AM-LCD) was applied to new products recent years, the active-matrix organic light emitting displays (AM-OLED). We investigated the asymmetric TFT electrical characteristics under different voltage stress and constant current stress because TFTs would be used as current driver. The instability of voltage stress: We made the experiments about DC stress and AC stress. The gate bias was larger, the more degradation in TFT electrical characteristics. Because larger gate bias could induce more electrons, the electrons broke the weak bond, Si-H, at the interface between a-Si and the gate insulator, and induced Si dangling bond defect. Based on this model we add the stressed bias on drain electrode to reduce the voltage between gate and drain electrodes, and the degradation of the TFTs electrical characteristics would become smaller. In AC stress, the bias stress experiment has the same result. But the degradation of AC stress showed no frequency dependence. Because the electrons would rapidly accumulate in the channel, the AC signal was not fast enough to react. Because there were different patterns of drain electrode between and source electrode in the asymmetric TFTs, the U type terminal, which has large channel width, could induce more defects. When U type terminal provided electrons, the electric characters of asymmetric TFTs had larger degradation. The instability of constant current stress: The symmetric TFT under 1µA constant current stress, the degradation of TFT operated in linear region was serious than that operated in saturation region. The electrons would break the Si-H bond at the interface between a-Si:H and the gate insulator when TFT operated in linear region, however, the depletion region near drain terminal deduced the electrons to impact the Si-H bond, therefore, it led to the result. We investigated that 1.3µA constant current stress had larger degradation than 10µA on electrical characteristics under the same gate bias in linear region. For the asymmetric TFT, we would explain the degradation mechanism of the constant current stress baseon the DC stress model.

Thesis (M.S.)--University of Wisconsin--Madison, 1990.
Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 194-199).

碩士
國立交通大學
電子物理學系
85
In this thesis, we mainly discuss the relationships between the single electron tunneling currents and tunneling voltages of YBCO high temperature superconducting thin films with various oxygen contents by a scanning tunneling microscope(STM). YBCO films were fabricated on several substrates by pulse laser deposition(PLD) method. Their oxygen contents were controlled with a homemade system. The tunneling currents I and tunneling voltages V were measured with the STM. The tunneling currents were then differentiated with respect to the voltages and the resulting dI/dV-V curves were used to calculate the superconducting energy gaps.

The HVDC, High Voltage Direct Current, is important when it comes to upgrading the energy system to a renewable, sustainable and efficient system. This master thesis is investigating what is happening during the most decisive fault cases when the HVDC station is operating with inherent overload. An inherent overload operating area is defined and simulations are performed in PSCAD/EMTDC to study the transient behavior of the fault currents and overvoltages.