671 research outputs found
Low order harmonic cancellation in a grid connected multiple inverter system via current control parameter randomization
In grid connected multiple inverter systems, it is normal to synchronize the output current of each inverter to the common network voltage. Any current controller deficiencies, which result in low order harmonics, are also synchronized to the common network voltage. As a result the harmonics produced by individual converters show a high degree of correlation and tend to be additive. Each controller can be tuned to achieve a different harmonic profile so that harmonic cancellation can take place in the overall system, thus reducing the net current total harmonic distortion level. However, inter-inverter communication is required. This paper presents experimental results demonstrating an alternative approach, which is to arrange for the tuning within each inverter to be adjusted automatically with a random component. This results in a harmonic output spectrum that varies with time, but is uncorrelated with the harmonic spectrum of any other inverter in the system. The net harmonics from all the inverters undergo a degree of cancellation and the overall system yields a net improvement in power quality
Suppression of line voltage related distortion in current controlled grid connected inverters
The influence of selected control strategies on the level
of low-order current harmonic distortion generated by an inverter
connected to a distorted grid is investigated through a combination
of theoretical and experimental studies. A detailed theoretical
analysis, based on the concept of harmonic impedance, establishes
the suitability of inductor current feedback versus output
current feedback with respect to inverter power quality. Experimental
results, obtained from a purpose-built 500-W, three-level,
half-bridge inverter with an L-C-L output filter, verify the efficacy of inductor current as the feedback variable, yielding an
output current total harmonic distortion (THD) some 29% lower
than that achieved using output current feedback. A feed-forward
grid voltage disturbance rejection scheme is proposed as a means to
further reduce the level of low-order current harmonic distortion.
Results obtained from an inverter with inductor current feedback
and optimized feed-forward disturbance rejection show a THD of
just 3% at full-load, representing an improvement of some 53% on
the same inverter with output current feedback and no feed-forward
compensation. Significant improvements in THD were also
achieved across the entire load range. It is concluded that the use
of inductor current feedback and feed-forward voltage disturbance
rejection represent costâeffect mechanisms for achieving improved
output current quality
Elimination of subharmonics in direct look-up table (DLT) sine wave reference generators for low-cost microprocessor-controlled inverters
This paper investigates distortion of an inverter reference waveform generated using a direct look-up (DLT) algorithm. The sources of various distortion components are identified and the implications for application to variable speed drives and grid connected inverters are described. Harmonic and subharmonic distortion mechanisms are analyzed, and compared with experimental results. Analytical methods are derived to determine the occurrence of subharmonics, their number, frequencies and maximum amplitudes. A relationship is established identifying a discrete set of synthesizable frequencies which avoid sub-harmonic distortion as a function of look-up table length and a practical method for calculation of the look-up table indices, based on finite length binary representation, is presented. Real time experimental results are presented to verify the analytical derivations
Competing effects of mass anisotropy and spin Zeeman coupling on the upper critical field of a mixed - and s-wave superconductor
Based on the linearized Eilenberger equations, the upper critical field
of mixed d- and s-wave superconductors has been microscopically
studied with an emphasis on the competing effects of mass anisotropy and spin
Zeeman coupling. We find the mass anisotropy always enhance while the
Zeeman interaction suppresses . As required by the thermodynamics, we
find is saturated at zero temperature. We compare the theoretical
calculations with recent experimental data of
YBaCuO.Comment: To appear in PRB in Feb. 200
Magnetic levitation stabilized by streaming fluid flows
We demonstrate that the ubiquitous laboratory magnetic stirrer provides a simple passive method of magnetic levitation, in which the so-called âfleaâ levitates indefinitely. We study the onset of levitation and quantify the fleaâs motion (a combination of vertical oscillation, spinning and âwagglingâ), finding excellent agreement with a mechanical analytical model. The waggling motion drives recirculating flow, producing a centripetal reaction force that stabilized the flea. Our findings have implications for the locomotion of artificial swimmers and the development of bidirectional microfluidic pumps, and they provide an alternative to sophisticated commercial levitators
Sustainable terpene triblock copolymers with tuneable properties for pressure sensitive adhesive applications
A series of triblock copolymers in a hard-soft-hard block configuration with varying hard block α-pinene methacrylate content and molecular weight and butyl acrylate soft segment have been synthesised and investigated for viability in pressure sensitive adhesive (PSA) applications. The morphologies vary from pockets of hard phase distributed within a continuous soft matrix, through to lamellar with co-continuous phases, and finally continuous hard phase with pockets of soft phase dispersed. Uniaxial tensile properties, probe adhesion performance and cyclic adhesive behaviour are presented for seven compositions including four short chain and three long chain copolymers, alongside a commercial benchmark PSA. Structure-property relationships for the novel elastomers are evaluated, establishing that short chain materials with 20-25 wt% αPMA offer similar tensile and adhesion performance to the commercial elastomer. Raising the hard phase concentration has been observed to provide a considerable increase in ultimate tensile strength, stiffness and peak tack force, but at the expense of significant reductions in ultimate tensile strain, adhesive bond displacement and vibrational dissipation. The results suggest that the performance of these sustainable materials can be tuned to produce viable PSAs with a range of useful properties
Généralisation du diagramme de Voronoï et placement de formes géométriques complexes dans un nuage de points.
La gĂ©omĂ©trie algorithmique est une discipline en pleine expansion dont l'objet est la conception d'algorithmes rĂ©solvant des problĂšmes gĂ©omĂ©triques. De tels algorithmes sont trĂšs utiles notamment dans l'ingĂ©nierie, l'industrie et le multimĂ©dia. Pour ĂȘtre performant, il est frĂ©quent qu'un algorithme gĂ©omĂ©trique utilise des structures de donnĂ©es spĂ©cialisĂ©es.Nous nous sommes intĂ©ressĂ©s Ă une telle structure : le diagramme de VoronoĂŻ et avons proposĂ© une gĂ©nĂ©ralisation de celui-ci. Ladite gĂ©nĂ©ralisation rĂ©sulte d'une extension du prĂ©dicat du disque vide (prĂ©dicat propre Ă toute rĂ©gion de VoronoĂŻ) Ă une union de disques. Nous avons analysĂ© les rĂ©gions basĂ©es sur le prĂ©dicat Ă©tendu et avons proposĂ© des mĂ©thodes pour les calculer par ordinateur.Par ailleurs, nous nous sommes intĂ©ressĂ©s aux problĂšmes de placement de formes , thĂšme rĂ©current en gĂ©omĂ©trie algorithmique. Nous avons introduit un formalisme universel pour de tels problĂšmes et avons, pour la premiĂšre fois, proposĂ© une mĂ©thode de rĂ©solution gĂ©nĂ©rique, en ce sens qu'elle est apte Ă rĂ©soudre divers problĂšmes de placement suivant un mĂȘme algorithme.Nos travaux prĂ©sentent, d'une part, l'avantage d'Ă©largir le champ d'application de structures de donnĂ©es basĂ©es sur VoronoĂŻ. D'autre part, ils facilitent de maniĂšre gĂ©nĂ©rale l'utilisation de la gĂ©omĂ©trie algorithmique, en unifiant dĂ©finitions et algorithmes associĂ©s aux problĂšmes de placement de formes.Computational geometry is an active branch of computer science whose goal is the design of efficient algorithms solving geometric problems. Such algorithms are useful in domains like engineering, industry and multimedia. In order to be efficient, algorithms often use special data structures.In this thesis we focused on such a structure: the Voronoi diagram. We proposed a new generalized diagram. We have proceeded by extending the empty disk predicate (satisfied by every Voronoi region) to an arbitrary union of disks. We have analyzed the new plane regions based on the extended predicate, and we designed algorithms for computing them.Then, we have considered another topic, which is related to the first one: shape placement problems. Such problems have been studied repeatedly by researchers in computational geometry. We introduced new notations along with a global framework for such problems. We proposed, for the first time a generic method, which is able to solve various placement problems using a single algorithm.Thus, our work extend the scope of Voronoi based data structures. It also simplifies the practical usage of placement techniques by unifying the associated definitions and algorithms.MULHOUSE-SCD Sciences (682242102) / SudocSudocFranceF
Cyclical strain improves artificial equine tendon constructs in vitro
Tendon injuries are a common cause of morbidity in humans. They also occur frequently in horses and the horse provides a relevant, large animal model in which to test novel therapies. To develop novel cell therapies that can aid tendon regeneration and reduce subsequent reâinjury rates, the mechanisms that control tendon tissue regeneration and matrix remodelling need to be better understood. Whilst a range of chemical cues have been explored (growth factors, media etc.), the influence of the mechanical environment on tendon cell culture has yet to be fully elucidated. To mimic the in vivo environment, in this study we have utilised a novel and affordable, customâmade bioreactor to apply a cyclical strain to tendonâlike constructs generated in 3âdimensional (3D) culture by equine tenocytes. Dynamic shear analysis (DSA), dynamic scanning calorimetry (DSC) and Fourier Transform Infrared (FTIR) spectroscopy were used to determine the mechanical and chemical properties of the resulting tendonâlike constructs. Our results demonstrate that equine tenocytes exposed to a 10% cyclical strain have an increased amount of collagen gel contraction after 7 and 8 days of culture compared to cells cultured in 3D in the absence of external strain. While all the tendonâlike constructs have a very similar chemical composition to native tendon, the application of strain improves their mechanical properties. We envisage these results will contribute towards the development of improved biomimetic artificial tendon models for the development of novel strategies for equine regenerative therapies
Two nonmagnetic impurities in the DSC and DDW state of the cuprate superconductors as a probe for the pseudogap
The quantum interference between two nonmagnetic impurities is studied
numerically in both the d-wave superconducting (DSC) and the d-density wave
(DDW) state. In all calculations we include the tunnelling through excited
states from the CuO planes to the BiO layer probed by the STM tip. Compared
to the single impurity case, a systematic study of the modulations in the
two-impurity local density of states can distinguish between the DSC or DDW
states. This is important if the origin of the pseudogap phase is caused by
preformed pairs or DDW order. Furthermore, in the DSC state the study of the
LDOS around two nonmagnetic impurities provide further tests for the potential
scattering model versus more strongly correlated models.Comment: 6 pages, 6 figure
The Gribov-Zwanziger action in the presence of the gauge invariant, nonlocal mass operator in the Landau gauge
We prove that the nonlocal gauge invariant mass dimension two operator
can be consistently added to the
Gribov-Zwanziger action, which implements the restriction of the path
integral's domain of integration to the first Gribov region when the Landau
gauge is considered. We identify a local polynomial action and prove the
renormalizability to all orders of perturbation theory by employing the
algebraic renormalization formalism. Furthermore, we also pay attention to the
breaking of the BRST invariance, and to the consequences that this has for the
Slavnov-Taylor identity.Comment: 30 page
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