Courant monophasé | EPFL Graph Search

Un courant monophasé est un courant électrique alternatif fourni au moyen d'une ligne bifilaire. Il s'oppose aux courants polyphasés, tels que le courant triphasé, pour lesquels plusieurs lignes sont utilisées et déphasées entre elles. Le courant monophasé est principalement utilisé pour l'éclairage et le chauffage, lorsque l'emploi de moteurs de forte puissance n'est pas nécessaire. Conversion Un courant monophasé peut être produit à partir d'un courant triphasé en connectant une des trois phases et le neutre, ou en connectant deux des trois phases. Les tensions obtenues varient d'une région à l'autre. L’Amérique du Nord se partage le système à , qui coexiste avec un deuxième système (de aux États-Unis et de au Canada). L’Europe quant à elle utilise presque exclusivement le système à . Dans les systèmes de haute tension (quelques kilovolts), un transformateur monophasé peut être utilisé pour générer une basse tension à partir du triphasé. En Amérique du Nord et dans c

Precipitation and transformation of nanostructured copper oxalate and copper/cobalt composite precursor synthesis

Lucica Soare

The aim of this thesis was to synthesise and control a nanostructured composite of copper/cobalt. Both the copper and cobalt oxalate exhibit a nanostructure. Attempts to produce nanocomposite particles in the size range of 10-70 nm were made via an oxalate co-precipitation route followed by the appropriate thermal treatment to finally obtain the metal nanocomposite. Much attention was focussed on the understanding of the copper oxalate precipitation and its further transformation into metal before the investigation of the co-precipitation system. One major challenge of this thesis work was to achieve a better understanding of the copper oxalate precipitation mechanism. This was made by following kinetic parameters in order to shed light on the various steps of the precipitation from a supersaturated solution (nucleation, growth and aggregation of nanocrystals). Following the pH as a function of time and using the thermodynamic solubility data it was possible to propose a kinetic model of copper oxalate precipitation, with the co-precipitation of slight amounts (around 0.40% wt) of malachite (CuCO3·Cu(OH)2). The copper oxalate nanostructured particle growth mechanism, from the self-assembly of nanosized buildings blocks, was confirmed for intermediate precipitation times (1-15 minutes). Evidence for such organisation of the particles was shown by a combination of XRD diffraction, SEM and AFM measurements showing the presence of steps at the particle surface with a height that corresponds to a multiple of the mean crystallite size in that particular crystallographic orientation. Further investigations were performed for the early steps of the precipitation by SAXS but either the precipitated volume fraction was too low for detection of the particles or nucleation and growth kinetics were to fast (less than one second) to be followed. The TEM cross-section analyses showed a possible core-shell assembly mechanism. The core showed a random organisation of the crystallites with a size of around 25 nm, while the crystallite in the shell with a size of around 40 nm presented certain order along the 110 axis, particularly towards the particle surface. All these details provided the opportunity to propose a new and more detailed mechanism of the copper oxalate polycrystalline particle formation. In order to master the conditions of the Cu/Cu oxalate decomposition, a good understanding of the simple copper oxalate decomposition was necessary. All along this thesis, much attention was paid to the transformation of the copper oxalate cubic particles into the metal. The objective was thus to conserve the particles cubic macrostructure morphology and their internal nanoscale spatial organisation. To this goaltwo routes were investigated: a direct transformation and another, an indirect one, that required the formation of an oxide. Both the copper oxalate and the oxide showed an anisotropic behaviour during the transformation into the metal. It is shown experimentally that the anisotropy, nanostructure and inhomogeneity of the initial nanocrystallites of both the oxalate and the oxide have an important influence on the mechanism and evolution of transformation into the metal. The particle morphology was shown to be lost for a transformation yield of α>0.80 in the case of the direct transformation from the oxalate whereas the morphology was kept up to the metallic state when passing via the oxide. A kinetic model was proposed for both systems using the method of the sudden change in temperature and pressure. The kinetics analysis did not permit a total understanding of the transformations studied, as several stages were shown to have complex and concurrently competing mechanisms. However, a geometrical model was proposed using the ex-situ analysis of the samples as a function of the reaction yield for both routes. For the initial stages of the copper oxide reduction under He/H2 atmospheres, the kinetic analysis showed hydrogen dissociation as a rate-limiting step. With a view to producing a cobalt/copper composite, preliminary experiments were carried out for the co-precipitation of the Co/Cu oxalate with different cationic ratios. Thermodynamic calculations showed the formation of the two solids was possible independent of the ratio Co/Cu. Experimentally, however a co-precipitation was obtained only for a ratio Co/Cu of 1, whereas for other ration only one single phase (either copper oxalate or cobalt oxalate) was formed. A second route was investigated using cobalt oxalate or cobalt oxide seeds. The amount of the cobalt detected by TEM in the precipitate using either the oxalate or from the oxide seeds was around 3%wt, which is lower than the value of 10%wt necessary to provide desired magnetic properties in the resulting precipitate. The exact nature and spatial distribution of the cobalt was not ascertained and further analysis of the nanostructure by TEM needs to be carried out to confirm the premise of the seed route. This thesis has shown that it is possible to get to a deeper understanding of the kinetics and the mechanism of the copper oxalate precipitation using the appropriate techniques. The cubic macrostructure can be conserved from the initial CuO nanoparticles (13 nm) to metallic Cu (42 nm) by a controlled transformation in a reducing atmosphere. Preliminary experiments made on the possible formation of a Co/Cu composite via the use of the cobalt oxide seeds as heterogeneous nuclei for the copper oxalate precipitation seems most promising.

EPFL2004

Symmetrical multilevel converters with two quadrant DC-DC feeding

In the technology sector of power electronics and control, the multilevel converter technology is still a rather new research area, but the application possibilities in the field of power drives and energy will demand more solutions with this promising technology. In the future, more converter systems will be realized with the multilevel topology. Up to now, multilevel converters have only been used in very particular applications, mainly due to the high costs and complexity of the multilevel converter system. The high costs are due to the fact that the latest technology on semiconductors, magnetic material for inductor and transformer cores and control system technology had to be used. But nowadays new developments in the fields of power semiconductors such as the IGBT, IGCT and perhaps in the future SiC switches as well as improvements of the performance of magnetic cores used in medium frequency transformers will favor the multilevel converters for many other application fields. It can be noted that the industrial trend is moving away from heavy and bulky passive components towards power converter systems using more and more semiconductor elements controlled by powerful processor systems integrating intelligent multi-task control algorithms. The presented work is a contribution to the large field of multilevel converters. It shows a certain kind of multilevel converter in a single phase and a three-phase configuration, called the series-connected four-quadrant converters (SCFQ). The two specialties of the presented converter type are a) that all the multilevel converter steps are fed by an identical DC voltage and b) that every multilevel converter step is realized with an individual AC-DC converter or four-quadrant converter. This type of multilevel converter is called multilevel converter with symmetrical feeding. In this work, a general theoretical development has been done for the use of this multilevel converter type. A special type of DC-DC converter is presented, in order to feed the individual four-quadrant converters of the multilevel converter with a constant DC voltage. All the developments and methods used are based on mathematical expressions. Various simulations using the latest software simulation tools are accomplished and are used to study different cases. The feasibility of the developments is underlined with a series of experimental results with all types of the used converters, which have been realized in the framework of this thesis. The main application for the multilevel converter presented in this work is the frontend power converter in locomotives. Instead of using a heavy low-frequency transformer to reduce the high-voltage from the catenary to a supportable voltage for the semiconductors, a multilevel converter concept is used. The multilevel converter is directly coupled to the catenary. There are many advantages compared to the existing solutions. In the same context, a novel solution of a multilevel converter has been developed for a locomotive usable on different power lines. The converter allows not only the operation on the high AC voltage power line (15kV), but also can be coupled to a medium-voltage DC power line (3kV). Three different configuration types of the locomotive converter have been developed and tested in a complex simulation environment. Besides the locomotive application, there are many more interesting applications for the symmetrical multilevel converter, e.g. in the fields of energy transmission (FACTS, static VAR compensators, electronic high-voltage transformers, etc.) and industrial drives. But certainly in the future with the availability of cheap semiconductors adapted to the needs of the multilevel converter, even more applications in lower power fields will be realized.

EPFL2000

A Modified Formula for Distance Relaying of Tapped Transmission Lines with Grounded Neutrals

Sadegh Azizi, Mario Paolone

Protection of tapped transmission lines with grounded neutrals has been always a challenging problem, usually resulting in compromise solutions. This type of lines may be protected by pilot relaying requiring dedicated and reliable telecommunication links. This paper proposes a modified distance relaying for protection of tapped lines with grounded neutrals under single-phase-to-ground (1-ph-g) faults. The proposed method uses only local measurements similar to conventional distance relays. Nonetheless, it facilitates fast fault clearing from both line ends over a wider portion of the protected line length. Regardless of whether or not the tapping transformer is in service, the method performs desirably with no need for signaling. The proposed method is extensively tested on a large number of test-systems using the sinusoidal steady-state analysis. It is also implemented into the National Instrument Compact-RIO embedded hardware that has been suitably coupled with the RT-LAB real-time digital simulator by Opal-RT. This setup is used to conduct extensive hardware-in-the-loop (HIL) testing. The improved performance together with simplicity of the proposed distance relaying makes it an attractive option to include in numerical protective relays.

2018

Precipitation and transformation of nanostructured copper oxalate and copper/cobalt composite precursor synthesis

Lucica Soare

EPFL2004

Symmetrical multilevel converters with two quadrant DC-DC feeding

EPFL2000