Η παρούσα διπλωματική εργασία πραγματεύεται την ανάλυση και τον σχεδιασμό έλεγχο ενεργού και αέργου ισχύος επαγωγικής γεννήτριας διπλής τροφοδοσίας (Doubly-Fed Induction Generator, DFIG), που χρησιμοποιείται σήμερα κατά κόρον σε συστήματα ανεμογεννητριών. Ο έλεγχος επιτυγχάνεται μέσω του διανυσματικού ελέγχου με προσανατολισμό στην τάση του δικτύου (Voltage-Oriented Control, VOC χρησιμοποιώντας αναλογικο-ολοκληρωτικούς (Proportional-Integral, PI) ελεγκτές, επιβάλλοντας με αυτόν τον τρόπο την επιθυμητή μεταβατική συμπεριφορά. / The following diploma thesis addresses the analysis and design of an active and reactive power control system for a Doubly-Fed Induction Generator (DFIG), vastly used in wind power systems. Control is being achieved under voltage orientation (Voltage-Oriented Control, VOC) and through the use of proportional-integral (PI) controllers, who impose the desirable transient behavior.

Modeling of wind farms to determine their short circuit contribution in response to faults is a crucial part of system impact studies performed by power utilities. Short circuit calculations are necessary to determine protective relay settings, equipment ratings and to provide data for protection coordination. The plethora of different factors that influence the response of wind farms to short circuits makes short circuit modeling of wind farms an interesting, complex, and challenging task. Low voltage ride through (LVRT) requirements make it necessary for the latest generation of wind generators to be capable of providing reactive power support without disconnecting from the grid during and after voltage sags. If the wind generator must stay connected to the grid, a facility has to be provided to by-pass the high rotor current that occurs during voltage sags and prevent damage of the rotor side power electronic circuits. This is done through crowbar circuits which are of two types, namely active and passive crowbars, based on the power electronic device used in the crowbar triggering circuit. Power electronics-based converters and controls have become an integral part of wind generator systems like the Type 3 doubly fed induction generator based wind generators. The proprietary nature of the design of these power electronics makes it difficult to obtain the necessary information from the manufacturer to model them accurately. Also, the use of power electronic controllers has led to phenomena such as sub-synchronous control interactions (SSCI) in series compensated Type 3 wind farms which are characterized by non-fundamental frequency oscillations. SSCI affects fault current magnitude significantly and is a crucial factor that cannot be ignored while modeling series compensated Type 3 wind farms. These factors have led to disagreement and inconsistencies about which techniques are appropriate for short circuit modeling of wind farms. Fundamental frequency models like voltage behind transient reactance model are incapable of representing the majority of critical wind generator fault characteristics such as sub-synchronous interactions. The Detailed time domain models, though accurate, demand high levels of computation and modeling expertise. Voltage dependent current source modeling based on look up tables are not stand-alone models and provide only a black-box type of solution. The short circuit modeling methodology developed in this research work for representing a series compensated Type 3 wind farm is based on the generalized averaging theory, where the system variables are represented as time varying Fourier coefficients known as dynamic phasors. The modeling technique is also known as dynamic phasor modeling. The Type 3 wind generator has become the most popular type of wind generator, making it an ideal candidate for such a modeling method to be developed. The dynamic phasor model provides a generic model and achieves a middle ground between the conventional electromechanical models and the cumbersome electromagnetic time domain models. The essence of this scheme to model a periodically driven system, such as power converter circuits, is to retain only particular Fourier coefficients based on the behavior of interest of the system under study making it computationally efficient and inclusive of the required frequency components, even if non-fundamental in nature. The capability to model non-fundamental frequency components is critical for representing sub-synchronous interactions. A 450 MW Type 3 wind farm consisting of 150 generator units was modeled using the proposed approach. The method is shown to be highly accurate for representing faults at the point of interconnection of the wind farm to the grid for balanced and unbalanced faults as well as for non-fundamental frequency components present in fault currents during sub-synchronous interactions. Further, the model is shown to be accurate also for different degrees of transmission line compensation and different transformer configurations used in the test system.

Interest in developing a detailed understanding of jet plume aerodynamics has increased significantly in recent years, for both civil (noise reduction) and military (Infra-Red signature modelling) aerospace applications. Such flows are critically dependent on turbulence modelling of the jet plume shear layer mixing. Reynolds averaged Navier Stokes (RANS) CFD tends to overpredict while Large Eddy Simulation (LES) CFD underpredicts potential core length. Difficulties in LES begin with the challenge of providing accurate resolution of thin turbulent boundary layers at nozzle exit. Providing physically meaningful 3D unsteady LES inlet conditions is a challenge in nozzle flows since turbulence at nozzle inlet experiences relaminarisation, which determines the boundary layer state at nozzle exit. The present thesis addresses these challenges by developing and validating against benchmark measurements an LES approach for nozzle/plume flows based on an advanced inlet condition treatment and an improved level of Sub-Grid-Scale (SGS) modelling. A technique for synthetic inlet condition generation based on a rescaling/recycling method (R2M) for LES predictions of nozzle flows has been applied and validated in the present work. Results reveal the benefits of this method such that self-consistent, correlated turbulent structures were sustained throughout the high acceleration region associated with nozzle convergence, with the turbulence anisotropy developing in the expected manner. The LES results for velocity profile shape at nozzle exit are better than low Re RANS predictions. Use of the Smagorinsky SGS closure produced level of turbulence energy at nozzle exit significantly lesser than measured. A recently proposed SGS model by Piomelli and Guerts (PGSGS) that defines the SGS length scale based on local turbulence quantities using a mesh independent formulation was also applied to the nozzle flow test case with significant improvement in the turbulence energy development through the nozzle. The LES method is applied to a supersonic jet discharging from a rectangular convergentdivergent nozzle. Results show that the R2M technique was able to generate realistic turbulence conditions at nozzle inlet that were consistent with available measured data. Using a carefully designed mesh and the advanced PGSGS model, turbulent structures were sustained through the nozzle, enabling good prediction of the nozzle exit boundary layer state and near field development. The improved capture of shear layer turbulence enabled better predictions of shear layer growth, leading to improved capture of shock cell behaviour and potential core length.

Pulsars, as rotating magnetised neutron stars got much attention during the last 40 years since their discovery. Observations revealed them to be gamma-ray emitters with energies continuing up to the sub 100 GeV region. Better observation of this upper energy cut-off region will serve to enhance our theoretical understanding of pulsars and neutron stars. The H-test has been used the most extensively in the latest periodicity searches, whereas other tests have limited applications and are unsuited for pulsar searches. If the probability distribution of a test statistic is not accurately known, it is possible that, after searching through many trials, a probability for uniformity can be given, which is much smaller than the real value, possibly leading to false detections. The problem with the H-test is that one must obtain the distribution by simulation and cannot do so analytically. For such simulations, random numbers are needed and are usually obtained by utilising so-called pseudo-random number generators, which are not truly random. This immediately renders such generators as useless for the simulation of the distribution of the H-test. Alternatively there exists hardware random number generators, but such devices, apart from always being slow, are also expensive, large and most still don't exhibit the true random nature required. This was the motivation behind the development of a hardware random number generator which provides truly random U(0,l) numbers at very high speed and at low cost The development of and results obtained by such a generator are discussed. The device delivered statistically truly random numbers and was already used in a small simulation of the H-test distribution. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2005.

A multifunctional central pattern generator (CPG) can produce bursting polyrhythms that determine locomotive activity in an animal: for example, swimming and crawling in a leech. Each rhythm corresponds to a specific attractor of the CPG. We employ a Hodgkin-Huxley type model of a bursting leech heart interneuron, and connect three such neurons by fast inhibitory synapses to form a ring. This network motif exhibits multistable co-existing bursting rhythms. The problem of determining rhythmic outcomes is reduced to an analysis of fixed points of Poincare mappings and their attractor basins, in a phase plane defined by the interneurons' phase differences along bursting orbits. Using computer assisted analysis, we examine stability, bifurcations of attractors, and transformations of their basins in the phase plane. These structures determine the global bursting rhythms emitted by the CPG. By varying the coupling synaptic strength, we examine the dynamics and patterns produced by inhibitory networks.

Šiuo magistriniu darbu mėginsiu išsiaiškinti mažosios hidroenegetikos vystimosi galimybes, palyginti jas su kitomis elektrinėmis, išsiaiškinti kokius upių resursus turi mūsų šalis, kokie yra gamtosauginiai reikalavimai hidroenergetikoje, generatoriai, kokie yra hidroelektrinės prijungimo prie tinklo variantai. / The theme of Master project of Electrical engineer is The investigation of potentiolity development of small hydro energetic. I think it is very important, because Lithuania is going to close Ignalina nuclear power station as it is required by the EU. Obviously, we are forced to find solution of energy shortage. One of the solutions are renewable energy sources. Lithuania has potential power in rivers and it has opportunity to expand of small hydro energetic. Byt we have to admit, that nowadays a small hydro energetic is not expanding as fast as it could. In my Master project of electical Engineer I show, how easy is to construct hydro power station, but hard to find a place, where station could be, to conect it with electricity networks for several reasons.

Communication security is a very important part of modern life. A crucial aspect of security is the ability to identify with near 100% certainty who is on the other side of a connection. This problem can be overcome through the use of random number generators, which create unique identities for each person in a network. The effectiveness of an identity is directly proportional to how random a generator is. The speed at which a random number can be delivered is a critical factor in the design of a random number generator. This thesis covers the design and fabrication of three ring oscillator based truly random number generators, the first two of which were fabricated in 0.13µ m CMOS technology. The randomness from this type of random number generator originates from phase noise in a ring oscillator. The second and third ring oscillators were designed to have a low slew rate at the inverter switching threshold. The outputs of these designs showed vast increases in timing jitter compared to the first design. The third design exhibited improved randomness with respect to the second design.

The kinetic energy present in tidal currents and other water courses has long been appreciated as a vast resource of renewable energy. The work presented in this doctoral thesis is devoted to both the characteristics of the hydro-kinetic resource and the technology for energy conversion. An assessment of the tidal energy resource in Norwegian waters has been carried out based on available data in pilot books. More than 100 sites have been identified as interesting with a total estimated theoretical resource—i.e. the kinetic energy in the undisturbed flow—in the range of 17 TWh. A second study was performed to analyse the velocity distributions presented by tidal currents, regulated rivers and unregulated rivers. The focus is on the possible degree of utilization (or capacity factor), the fraction of converted energy and the ratio of maximum to rated velocity, all of which are believed to be important characteristics of the resource affecting the economic viability of a hydro-kinetic energy converter. The concept for hydro-kinetic energy conversion studied in this thesis comprises a vertical axis turbine coupled to a directly driven permanent magnet generator. One such cable wound laboratory generator has been constructed and an experimental setup for deployment in the river Dalälven has been finalized as part of this thesis work. It has been shown, through simulations and experiments, that the generator design at hand can meet the system requirements in the expected range of operation. Experience from winding the prototype generators suggests that improvements of the stator slot geometry can be implemented and, according to simulations, decrease the stator weight by 11% and decrease the load angle by 17%. The decrease in load angle opens the possibility to reduce the amount of permanent magnetic material in the design.

The one-dimensional distribution of pseudo-random numbers generated by the ratio of uniforms methods using linear congruential generators (LCGs) as the source of uniform random numbers is investigated in this paper. Due to the two-dimensional lattice structure of LCGs there is always a comparable large gap without a point in the one-dimensional distribution of any ratio of uniforms method. Lower bounds for these probabilities only depending on the modulus and the Beyer quotient of the LCG are proved for the case that the Cauchy the normal or the exponential distribution are generated. These bounds justify the recommendation not to use the ratio of uniforms method combined with LCGs. (author's abstract) / Series: Preprint Series / Department of Applied Statistics and Data Processing

Cryptosystems are used to provide security in communications and data transmissions. Stream ciphers are private key systems that are often used to transform large volumn data. In order to have security, key streams used in stream ciphers must be fully analyzed so that they do not contain specific patterns, statistical infomation and structures with which attackers are able to quickly recover the entire key streams and then break down the systems. Based on different schemes to generate sequences and different ways to represent them, there are a variety of stream cipher analyses. The most important one is the linear analysis based on linear feedback shift registers (LFSRs) which have been extensively studied since the 1960's. Every sequence over a finite field has a well defined linear complexity. If a sequence has small linear complexity, it can be efficiently recoverd by Berlekamp-Messay algorithm. Therefore, key streams must have large linear complexities. A lot of work have been done to generate and analyze sequences that have large linear complexities. In the early 1990's, Klapper and Goresky discovered feedback with carry shift registers over Z/(p) (p-FCSRS), p is prime. Based on p-FCSRs, they developed a stream cipher analysis that has similar properties to linear analysis. For instance, every sequence over Z/(p) has a well defined p-adic complexity and key streams of small p-adic complexity are not secure for use in stream ciphers. This disstation focuses on stream cipher analysis based on feedback with carry shift registers. The first objective is to develop a stream cipher analysis based on feedback with carry shift registers over Z/(N) (N-FCSRs), N is any integer greater than 1, not necessary prime. The core of the analysis is a new rational approximation algorithm that can be used to efficiently compute rational representations of eventually periodic N-adic sequences. This algorithm is different from that used in $p$-adic sequence analysis which was given by Klapper and Goresky. Their algorithm is a modification of De Weger's rational approximation algorithm. The second objective is to generalize feedback with carry shift register architecture to more general algebraic settings which are called algebraic feedback shift registers (AFSRs). By using algebraic operations and structures on certain rings, we are able to not only construct feedback with carry shift registers, but also develop rational approximation algorithms which create new analyses of stream ciphers. The cryptographic implication of the current work is that any sequences used in stream ciphers must have large N-adic complexities and large AFSR-based complexities as well as large linear complexities.