PERMANENT MAGNET MULTIPHASE MACHINE MODELING AND CONTROL FOR MV WIND ENERGY APPLICATIONS

Due to the rapid development of the power electronics in the second half of the twentieth century, a significant research effort has been put into the modelling of electrical machines to provide mathematical models for control purposes. As the power electronics isolate the machine from the grid, the number of phases on both sides no longer needs to be the same, thus allowing for use of multiphase machines. Several studies have shown that multiphase machines can yield lower torque ripple, provide higher torque per phase current, and that they can continue to operate with one or more faulty phases, thus increasing the robustness of the power stage. This, amongst other benefits, has led to increased interest in multiphase machine employment for critical applications, such as more-electric aircraft, electrical propulsion systems for ships and offshore wind, etc. Amongst the different multiphase machine constructions, the multiple three-phase winding structure with isolated neutral points is of special interest. It can be operated using multiple three-phase converters, so that almost no modification of hardware is needed. Furthermore, with high power machines (above the 5 MW class), several converters in parallel should be used when increased availability is desired. This is where multiple three-phase winding machines show an additional benefit, galvanic isolation between the windings. By connecting one three-phase converter to each of the three-phase windings of the machine, the increased availability of paralleling converters is obtained while the problem of the circulating current between paralleled converters is practically eliminated thanks to said galvanic isolation. The control schemes of three-phase machines should not be directly applied to multiple three-phase winding machines, since these show internal cross couplings between the different three-phase windings that may affect dynamic performance. To examine the behaviour and design control schemes for multiple three-phase winding machines, modelling approaches based on vector space decomposition, multiple dq modelling approach and a novel approach, specifically developed in this thesis for the independent power flow control in individual three-phase windings, are studied. It is demonstrated that, by including appropriate decoupling terms in the traditional three-phase control structure, a completely decoupled operation can be obtained in all the three-phase windings in the machine when control scheme is based on the multiple dq modelling approach. With this control approach, the control of these machines is accomplished using control structures and model transformations familiar to those skilled in the art of the three-phase machines. For six-phase machines the existing transformations are sufficient for all control purposes, while the novel transformation becomes a useful tool when there are three or more three-phase windings. The influence of a low switching to fundamental frequency ratio on behaviour of the controlled object is also covered in this work. This has a great impact on the modelling of current control loops, especially when using the synchronously rotating reference frame in variable fundamental frequency applications, such as motor drives. The precise modelling of the actual control loops is of vital importance since it allows development of faithful control tuning techniques. With these, the regulator parameters, which ensure certain specified dynamic performance of the loops, are obtained and their behaviour can be precisely described and predicted by simulations. The machine’s parameter identification has also been approached in this work; accurate parameter knowledge is of essential importance to ensure the correct match between experimental and simulation results. All the experimental work has been done using a 150 kW permanent magnet synchronous generator in six-phase configuration with two three-phase winding placed spatially in phase. Unequal power sharing between different three-phase windings is studied further, including the simultaneous operation of one winding in motoring and the other in generation for a six-phase machine. This particular mode of operation has been found as very useful in development of a novel testing method for the machines with multiple three-phase windings, of synthetic loading type, which is fully verified by experimentation. A corresponding theoretical/simulation work has been performed for a nine-phase (triple three-phase) machine

Shocks, Superconvergence, and a Stringy Equivalence Principle

We study propagation of a probe particle through a series of closely situated gravitational shocks. We argue that in any UV-complete theory of gravity the result does not depend on the shock ordering - in other words, coincident gravitational shocks commute. Shock commutativity leads to nontrivial constraints on low-energy effective theories. In particular, it excludes non-minimal gravitational couplings unless extra degrees of freedom are judiciously added. In flat space, these constraints are encoded in the vanishing of a certain "superconvergence sum rule." In AdS, shock commutativity becomes the statement that average null energy (ANEC) operators commute in the dual CFT. We prove commutativity of ANEC operators in any unitary CFT and establish sufficient conditions for commutativity of more general light-ray operators. Superconvergence sum rules on CFT data can be obtained by inserting complete sets of states between light-ray operators. In a planar 4d CFT, these sum rules express (a-c)/c in terms of the OPE data of single-trace operators.Comment: 93 pages plus appendice

A Hybrid Environment for Syntax-Semantic Tagging

The thesis describes the application of the relaxation labelling algorithm to NLP disambiguation. Language is modelled through context constraint inspired on Constraint Grammars. The constraints enable the use of a real value statind "compatibility". The technique is applied to POS tagging, Shallow Parsing and Word Sense Disambigation. Experiments and results are reported. The proposed approach enables the use of multi-feature constraint models, the simultaneous resolution of several NL disambiguation tasks, and the collaboration of linguistic and statistical models.Comment: PhD Thesis. 120 page

Effects of blade configuration on flow distribution and power output of a zephyr vertical axis wind turbine.

Worldwide interest in renewable energy systems has increased dramatically, due to environmental concerns like climate change and other factors. Wind power is a major source of sustainable energy, and can be harvested using both horizontal and vertical axis wind turbines. This thesis presents studies of a vertical axis wind turbine performance for applications in urban areas. Numerical simulations with FLUENT software are presented to predict the fluid flow through a novel Zephyr vertical axis wind turbine(VAWT). Simulations of air flow through the turbine rotor were performed to analyze the performance characteristics of the device. Major blade geometries were examined. A multiple reference frame (MRF) model capability of FLUENT was used to express the dimensionless form of power output of the wind turbine as a function of the wind freestream velocity and the rotor's rotational speed. The simulation results exhibit close agreement with a stream-tube momentum model

Stabilising Semiconducting Polymers Using Solid State Molecular Additives

Solution processed organic semiconductors contain extrinsic environmental species that cause device instabilities as they are difficult to remove during low temperature processing and are able to penetrate into organic electronics after fabrication. This dissertation is centered on the search for and development of solid state molecular additives to improve device stability associated with atmospheric defects in organic field effect transistors. To achieve this, an extended study was undertaken of the influence of over 95 different molecular additives expected to improve the stability characteristics of organic field effect transistors based on the literature. This dissertation demonstrates that positive bias and light stress stability can be improved in both p-type diF-TES ADT and IDT-BT organic field effect transistors by incorporating solid state small molecular additives. Simulations predict that the additives improve stability by introducing a competitive recombination pathway in order to prevent the trapping of electrons in the LUMO of the semiconductor by atmospheric species. Improvements with the solid state molecular additives are achieved by controlling the LUMO and morphology of the additive polymer blend. Secondly, improvements in the environmental and negative bias stress stability of organic field effect transistors are also observed with molecular additives. The solid state small molecular additives that significantly improve device characteristics are limited to a subset of molecules with similar structure to tetracyanoquinodimethane. It is demonstrated that this subset of solid state molecular additives correlates with a chemical reaction between the molecules and water. The chemical reaction appears to change the molecular additives into a new chemical species, plausibly consuming water, modifying the pH and doping the semiconductor, resulting in improved organic field effect transistor characteristics. Thirdly, machine learning techniques are used to accurately predict which solid state additives are capable of improving device performance. The machine learning algorithm uses neural passing networks for feature generation, due to its ability to capture physical plausible features such as functional groups. The algorithm screened over 1.5 billion molecular structures and found plausible molecular structures based on expert knowledge. Fourthly, novel analogue neuromorphic computer architectures based on anti-ferromagnetic and analogue transistors are modeled. The proposed architecture presents both trainable anti-ferromagnetic based synapses for learning and non-trainable voltage controlled synapses for computationally demanding inferences. This dissertation suggests that combining both the fully controllable and trainable networks is a promising route forward for analog neuromorphic computers.FlexEnable Christ's College Canadian Centennial Scholarship Fun

Optimal Test Inputs for Helicopter System Identification

The test input applied to a helicopter, or any other system, for the purpose of system identification can have a substantial effect on the parameter estimates obtained. It is therefore important that an appropriate input is chosen. Inputs must take account of the requirements, and restrictions, of the application. For example, in the rotorcraft case studied a linearised model is being identified, and it is therefore essential that the input produces a linear response. A straightforward method has been developed for the design of multi-step inputs. This method is based in the frequency-domain, and involves tailoring the auto-spectra of the inputs to give long, linear test records, and parameter estimates with reasonably low variances. In flight trials using the Lynx helicopter at RAE (Bedford), the double-doublet input, designed with this method, has been found to be a significant improvement over more traditional inputs. Using the data from the flight trials of the double-doublet, both equation-error and output-error identification have been carried out. Several discrepancies were found between the theoretical and identified models. More work is required to clarify this. Numerical difficulties were encountered during the output-error identification, and these were attributed to ill-conditioning resulting from the use of an unstable system. The design of optimal inputs has also been investigated. In particular, constraints have been developed which are suitable for ensuring that the optimal inputs produce linear responses, and are robust. Conventional energy constraints were found to be of little use for these purposes. Algorithms have been developed for the design of optimal inputs with a variety of constraints, and simulation studies have been made to gain an understanding of the effect of these constraints on the form of the inputs. With the constraints obtained from this work, an optimal input has been designed for use with the Lynx helicopter. This input is robust, and yet is predicted to give significantly improved parameter estimates. Unfortunately, at the time of writing, flight trials of this input could not be performed

Chemical and physical modification of optical transitions in perovskite quantum dots

Quantenpunkte (QP) sind faszinierende Halbleitermaterialien und scheinen für neuartige Dis- plays, Sensoren oder Quantencomputer wie gemacht zu sein. Als Material hat dabei besonders Bleihalidperowskit (BHP) im letzten Jahrzehnt mit herausragenden Eigenschaften für Furore gesorgt. BHP QP weisen etwa eine intrinsische Defekttoleranz, hohe Photolumineszenz- Quantenausbeute und spektrale Durchstimmbarkeit auf. Maßgeschneiderte Anwendungen erfordern jedoch – bedingt durch die geringe Größe von QP – das Wissen um und die Kontrolle von Wechselwirkungen mit der Umgebung. Eine wichtige Rolle spielen dabei chemische und physikalische Umgebungsmodifikationen, welche im Rahmen dieser Arbeit anhand der indikativen Photolumineszenz (PL) von BHP-QP analysiert wurden. Zum einen wurde ein mikroskopisches Modell der Interaktion zwischen Metallsalzen und QP entwickelt. Solche Interaktionen werden etwa postsynthetisch zur Defektpassivierung oder Modulation der Zusammensetzung angewandt. Die Kombination von in-situ-spektroskopischen sowie zeitaufgelösten Methoden ermöglichte es, die Bildung kontrollierter, reversibler Oberflächendefekte bei Zugabe von BiBr3 zu Kolloiden von sphärischen CsPbBr3 QP nachzu- weisen. Dies trägt zur Präzisierung der bislang unklaren Wirkung der Bismutheinlagerung in BHP-QP bei. Anhand des ungewöhnlichen Auslöschungsverhaltens wurde ein statistischer Wechselwirkungsprozess identifiziert und mit einem Aktionssphären-Modell beschrieben. Dabei offenbarte sich die Tragweite der gebildeten Defekte, von denen bereits ein einziger zur vollständigen PL-Löschung ausreicht. Durch Einführung eines Affinitätsfaktors ergab sich schließlich ein detailliertes mikroskopisches Bild des Adsorptionsprozesses. Relevante Prozessparameter, wie etwa Orte erhöhter Reaktivität oder der Einfluss einer Ligandenmonolage, konnten als Basis für künftige gezielte Oberflächenmodifikationen ermittelt werden. Weiterhin wurde das Nahfeld von Metaoberflächen (MO) als mögliche physikalische Umgebungsmodifikation hinsichtlich BHP-basierter polarisierter Photodetektoren untersucht. Lösungsbasiert wurde eine Monolage kubischer CsPbBr3 QP auf einer z-förmigen dielektrischen MO abgelagert. So wurde eine hybride, chirale BHP-MO gebildet, welche bislang nur mit BHP-Dünnschichten untersucht wurde. Durch PL-Mikroskopie wurde zirkularer Dichroismus unter nichtlinearer Anregung in den abgelagerten intrinsisch achiralen BHP-QP erzeugt. Das z-förmige MO-Design ermöglichte dabei starke polarisationsabhängige Resonanzen mit ausgeprägter Unterscheidung von links- und rechtszirkularem Licht im roten Spektralbereich. Dies eignete sich besonders für einen nichtlinearen Anregungsprozess im hybriden System. Die beobachtete Steigerung der PL-Intensität sowohl bei direkter als auch nichtlinearer Anregung konnte durch verstärkte Absorption in Folge von Streuung des Anregungslichts an der MO erklärt werden. Insbesondere wurde so eine Abhängigkeit von der Anregungswellenlänge sowie -polarisation mit einer chiralen Auflösung von bis zu 25 % erzielt.Quantum dots (QDs) are an attractive class of semiconductor material for a plethora of applications, ranging from novel display concepts via sensing to quantum computing. As a material, specifically lead halide perovskite (LHP) has shaken up the research community during the last decade. QDs made of LHP possess outstanding characteristics including intrinsic defect tolerance, near unity photoluminescence quantum yields and almost effortless spectral tunability. In view of their nanoscopic size, understanding and controlling interactions of LHP QDs with the surrounding is mandatory to enable in particular tailored applications. Especially chemical and physical modifications play an important role, which have thus been studied within this thesis by exploiting the strong photoluminescence (PL) of LHP QDs as a sensitive measure to elucidate important interaction mechanisms. On the one hand, a microscopic model of the metal salt-QD interaction was developed – a post-synthetic treatment frequently employed for defect passivation or compositional change. In-situ spectroscopy was combined with time-resolved analysis to study the addition of BiBr3 to colloidally dispersed spherical CsPbBr3 QDs. A controlled and reversible formation of surface traps was found, allowing for further clarification of bismuth incorporation thus far under debate. The unusual quenching dynamics proved the statistical nature of the process and could be well described within a modified sphere of action model. This unraveled the trap’s severity with adsorption of only one BiBr3 sufficient to completely quench the PL of a CsPbBr3 QD. By introducing a surface affinity factor, a detailed microscopic insight on the adsorption process was obtained: key parameters such as surface areas with exposed reactivity and the impact of a ligand monolayer were identified, providing a basis for future tailored surface alterations. On the other hand, physical modifications provided by the near field of metasurfaces were explored as a potential pathway to enable LHP based polarized photodetectors. A monolayer of cubic CsPbBr3 QDs was deposited on a z-shaped dielectric metasurface via a solution based process. This formed a hybrid chiral perovskite-metasurface, thus far limited to bulk perovskite films. Based on PL microscopy, the induction of nonlinear excited circular dichroism in intrinsically achiral LHP QDs was demonstrated in this hybrid system. The z-metasurface design enabled strong polarization dependent resonances in the red spectral range specifically discriminating left and right circularly polarized light. As such, it proved compatible with below band gap operation when combined with LHP QDs. Strongly enhanced PL intensities of LHP QDs for both, above and below band gap excitation were achieved. This could be assigned to an enhanced absorption in consequence of scattering of excitation light at the metasurface. Importantly, a dependence of the PL intensity on both, the excitation wavelength and polarization was achieved, yielding a chiral discrimination in PL of up to 25 %