Implementation of a Converter in Sequence Domain to Counter Voltage Imbalances

This paper demonstrates the basic problem of the conventional d-axis and q-axis control applied to a voltage source converter (VSC) during a system imbalance. The specific imbalances studied are a balanced fault and an unbalanced fault at the terminals of the converter. The dc link voltage of the VSC is disturbed during the unbalanced fault due to negative sequence components in the system. The sequence characteristics of the system during an imbalance are further analyzed. A novel controller in the sequence domain is proposed. The positive sequence components of the control are implemented in the positive synchronous reference frame and the negative sequence components of the control are implemented in the negative synchronous reference frame. The negative sequence components are commanded to zero in the control. The approach demonstrates the stabilization of the voltage to a greater extent. The control also points out the need for a storage element when the VSC is to be used for application with that of a Doubly-Fed Induction Generators (DFIG) in wind turbines

Ejection and impact angles of saltating particles measured with a high-speed camera

3D and 2D trajectory data of sand grains saltating over a bed are presented from highspeed camera measurements. They were obtained at Zingst peninsula and in laboratory using a wind tunnel. Trajectories, calculated with a Runge-Kutta procedure, using values of the mean wind profile and the air flow were fitted to the measured ones. The trajectory with the lowest RMSE against the measured one was used to estimate the grain diameter of the saltating grain. Also ejection and impact angle, ejection and impact speed of the grain were determined. The results confirm earlier findings that ejection angles decreases with increasing grain diameter. Ejection angles between 57° and 27° for fine (63-200 μm) and middle (200-630 μm) ejecta and between 38° and 20° for coarse grains (630-2000 μm) were found. The impact angle β increases with increasing grain diameter. Impact angles between 8° and 15° for fine impactors and between 12° and 36° for middle and coarse grains were found. Additionally the ratio between the mean ejection angle α and mean impact angle β, which decrease with increasing grain diameter (Rice et al., 1995), could be confirmed. The ration between the ejection speed ue and impact speed ui was found nearly the same for all determined grain sizes, but the grains ejected from the bed had an average speed of one order of magnitude less than the impact speed

Sheet Flows, Avalanches, and Dune Evolution on Earth and Mars

This investigation is a collaboration between researchers at Cornell University, the University of Florida, and the University of Rennes 1, France. Flow modeling at Cornell University focused on mechanisms for the suspension and transport of wind-blown sand that are important in both terrestrial and Martian environments. These mechanisms include the saltation (or jumping) of grains, collisions between grains, and the interaction of grains with the velocity fluctuations of the turbulent wind. Of particular interest are sheet flows; these are relatively thin, highly concentrated regions of grains flowing near the ground under the influence of a strong turbulent wind. In them, the grains are suspended by interparticle collisions. Sheet flows may be relatively rare events, but they have the capacity to move great amounts of sand. In order to describe sheet flows, a turbulent mixture theory was formulated for particles in a fluid in which fluctuations in the volume fiaction of the particles take place on the scale of the turbulent eddies. Ensemble averaged equations for particle and fluid mass, momentum, and energy and fluid rate of dissipation were expressed in terms of Farve (concentration) averaged velocities and the associated velocity fluctuations. Correlations that describe the turbulent suspension of particles and dissipation of turbulent energy of both phases due to fluid particle interactions were modeled and boundary conditions at the bed and at the upper surface of the collisional flow were formulated. The boundary conditions at the upper surface were tested in a numerical simulation developed at the University of Florida. Steady and unsteady solutions for steady and unsteady fully-developed flows were obtained over a range of wind speeds fiom the lowest for which collisional between particles occurred to at which turbulent suspension is found to dominate collisional suspension. Below the value of the wind speed at which collisions between particles were unimportant, numerical solutions were obtained for the velocity distribution function and the resulting fields of concentration, particle and gas mean velocity, and particle shear stress for the steady two-dimensional saltation of spherical sand particles driven by a turbulent wind over a bed characterized by a simple relationship (the splash function) between the properties of incoming particles and those of the rebounding particles and other particles ejected fiom the bed. At the University of Rennes 1, experiments devoted to the characterization of the splash function for beds consisting of either random or ordered arrays of spheres in two- dimensions were completed. These indicated the role played by the packing geometry in the rebound and ejection of grains. Preliminary experiments on response of a three- dimensional collision bed to a collision with a single particle were performed. Data was taken with a single camera focused on the plane of collision. Here, for example, the decrease of the effective coefficient of restitution of the bed with an increase of the angle of incidence of the incoming particle has been measured. Other experiments on avalanches at Rennes studied the properties of the flows of particles that are responsible for the motion of the leeward side of a dune. In these, the dependence of the initiation of avalanches on the packing and depth of the particles was measured. Particle migration was studied in inclined flows of a binary mixture of disks and the mechanisms of diffision and segregation were isolated and characterized. The influence of side wall on dense, rapid inclined flows was measured and shown to be the reason why the angle of the free surface in such flows can exceed the static angle of repose. Future research will be devoted to a better understanding the transition between saltating (collisionless) and collisional flows as the wind speed the increases. This will involve the understanding of the evolution of the splash function as clisions with the bed become more numerous, more frequent, and more violent

A Control Method in dq Synchronous Frame for PWM Boost Rectifier under Generalized Unbalanced Operating Conditions

Abstract-This paper proposes a new control scheme for regulating the instantaneous power for pwm boost type rectifiers under generalized unbalanced operating conditions. By nullifying the oscillating components of instantaneous power at the poles of the converter instead of the front-end through solving a set of nonlinear control equations in real time, the harmonics in the output dc voltage can be eliminated more effectively under generalized unbalanced operating conditions in the ac input side. The control scheme allows the pwm rectifier to generate a dc output without substantial even-order harmonics and to maintain nearly unity power factor under generalized unbalanced operating conditions, which makes it possible to reduce the size of the dc-link capacitor and ac inductors leading to reduced total cost. Simulation results along with experimental results for the open-loop control using a laboratory prototype converter confirm the feasibility of the new control method

Минимизация сопротивления путем распределенного отсоса пограничного слоя

4-

Automatic classification and characterization of power quality events

Copyright © 2008 IEEEThis paper presents a new technique for automatic monitoring of power quality events, which is based on the multiresolution S-transform and Parseval's theorem. In the proposed technique, the S-transform is used to produce instantaneous frequency vectors of the signals, and then the energies of these vectors, based on the Parseval's theorem, are utilized for automatically monitoring and classification of power quality events. The advantage of the proposed algorithm is its ability to distinguish different power quality classes easily. In addition, the magnitude, duration, and frequency content of the disturbances can be accurately identified in order to characterize the disturbances. The paper provides the theoretical background of the technique and presents a wide range of analyses to demonstrate its effectiveness.Ameen M. Gargoom, Nesimi Ertugrul and Wen. L. Soon

Le manuel scolaire en classe d’histoire de l’art : étude exploratoire et comparative sur ce support de médiation

peer reviewedDepuis l’apparition du manuel au sein du monde scolaire au XVe siècle en France et sa prolifération au XIXe siècle, son statut a évolué (Choppin, 1980). Tantôt il a été considéré comme un outil indispensable à la mise en œuvre des programmes (Education nationale française) tantôt comme « une fausse évidence historique » (Choppin, 2008). Ainsi périodiquement, la communauté scientifique débat-t-elle du bien-fondé des manuels scolaires. Dans le cadre de notre recherche en didactique de l’histoire de l’art, nous avons constaté qu’aucun des enseignants interrogés en Fédération Wallonie-Bruxelles (FWB) n’utilisaient de manuels scolaires en classe. Certains regrettent l’inexistence d’un tel support pédagogique susceptible de fournir une aide précieuse lors de la construction de leur cours. Ces manuels sont davantage présents en Italie, en Allemagne ou au Luxembourg. Cela pourrait s’expliquer en partie par l’origine du manuel d’histoire de l’art. Le premier manuel d’histoire de l’art Handbuch der Geschichte der Malerei de Franz Kugler (1837) est apparu en Allemagne (Martin, 2008). A l’issue de cette mise en contexte, la question guidant l’ensemble de notre réflexion est la suivante : L’absence de manuels d’histoire de l’art en FWB : quelles pistes de recherche pourraient émerger de la comparaison entre deux manuels étrangers pour créer un outil correspondant aux attentes des enseignants

Difficulté de communication et hôpital

La communication : un enjeu de santé COMMUNICATION, « le mot est à la mode, souligne Raymond BOUDON, la chose est parée de toute les vertus, politiques, sociales, cognitives ». Mais, comme il le précisait dans un article intitulé « petite sociologie de l’incommunication », le meilleur service à rendre à la cause de la communication, c’est peut-être d’en rappeler les difficultés, de manière à mieux les traiter ». C’est la principale justification du propos que je voudrais tenir ce matin devant..

Rancang bangun sistem pendeteksian dan monitoring harmonisa menggunakan metode DFT

Salah satu isu penting dalam analisis kualitas daya adalah untuk mendeteksi gangguan kualitas daya secara efisien dan akurat sehingga dapat menyediakan pelayanan suplai kualitas daya yang lebih baik kepada pelanggan. Keberadaan alat ukur saat ini menggunakan fast fourier transform (FFT) dalam menganalisis harmonik tetapi FFT tidak sesuai digunakan untuk mendeteksi sinyal yang non stasioner. Oleh karena itu, pada penelitian ini mengusulkan metode Fast S-Transform (FST) untuk menganalisis kualitas daya terutama harmonik karena FST memiliki komputasi yang lebih cepat. FST digunakan untuk mengubah sinyal arus dan tegangan dari daerah waktu ke daerah frekuensi untuk memperoleh komponen harmonik. Pada penelitian ini, diimplementasikan secara hardware menggunakan mikrokontroller STM32F407. Eksperimen dibandingkan dengan metode discrete fourier transform (DFT), Stockwell transform (ST), alat ukur standar FLUKE 43B dan Osiloskop DL850. Hasil yang diperoleh dari pengujian THDi pada beban non linier dan beban motor induksi memiliki kesalahan relatif sebesar 0,374284 % dan 0,548021 % sedangkan hasil yang diperoleh dari pengujian THDv pada beban non linier dan beban motor induksi memiliki kesalahan relatif sebesar 3,052288 % dan 5,073938 % dibandingkan dengan FLUKE 43B. Metode FST memiliki waktu komputasi masingmasing 0,0855 detik, 0,731 detik dan 3,7001 detik lebih cepat dengan jumlah data sebesar 2000 data, 5000 data dan 10000 data dibandingkan dengan ST. ========== One of the important issues in the analysis of power quality is to detect power quality disturbance efficiently and accurately so can provide service of power supply better quality to customers. In general, the presence of measurement tools currently using fast fourier transform (FFT) to analyze harmonics but FFT is not suitable for the detection of nonstationary signals and often occurs of leak signal. Therefore, this paper proposes a method of Fast S-Transform (FST) to analyze the power quality especially harmonics because FST has good accuracy and faster computing. FST is used to change the current and voltage signals from the voltage domain to the frequency domain to obtain the harmonic components. In this paper, implemented in hardware using microcontroller STM32F407. Experiment is compared with the method of discrete fourier transform (DFT), standard measuring tools is FLUKE 43B and oscilloscope DL850. The results obtained from testing of THDi on the non-linear load and the induction motor load has a relative error is 0.374284 % and 0.547021%, while the results obtained from testing of THDv on the non-linear load and the induction motor load has a relative error is 3,052288 % and 5,073938 % compared with FLUKE 43B. FST method has the computing time respectively is 0.0855 seconds, 0.731 seconds and 3.7001 seconds faster with the amount of 2000, 5000 and 10000 data is compared with Stockwell transform