Selective Harmonic Elimination in Multimodule Three-Phase Current-Source Converters

Modular current-source converter (MCSC) structures can enhance the current capacity and improve the power quality of converters in medium-voltage pulsewidth-modulated applications. An MCSC uses individual three-phase modules in a shunt connection to share the total power in a symmetrical manner. The modulation of these units is normally done using optimized patterns such as selective harmonic elimination patterns. However, the optimization is usually performed at the module level. This paper proposes a modulation technique that optimizes the operation of a complete MCSC taking into account all $P$ modules. Thus, from the injected ac current, we remove $P$ times the number of harmonics as compared with the conventional approach using the same switching frequency. A complete mathematical formulation and experimental results validate the proposed approach.      

Appraisal and optimization of agricultural water use in large irrigation schemes: II. Applications

In this paper, the theoretical approach presented in Part I is demonstrated by means of case studies on the irrigation schemes of Rio Mendoza and Rio Tunuyán in the Province of Mendoza, Argentina. The object of the case studies was the determination of optimal allocation of surface water to reduce the use of groundwater. Current and optimal conjunctive allocation of ground and surface water is studied by means of the developed simulation and optimization models.The second case study was designed on the basis of the experience gathered during the first one: each step of the case studies is compared. The comparison between the two case studies illustrates how to apply the proposed approach when the amount and quality of available data are different. For the more detailed Rio Tunuyán study, we determined the following physical characteristics of all terminal nodes: on-farm rotational intervals, mean water application depth, actual soil water storage capacity, crop water requirements, depth of groundwater table, aquifer transmissivity, and efficiency of groundwater use.The performance of the entire system is sensitive to changes in the water application depth, as it was shown by a simulation study.The application of our optimization approach to the conjunctive use of ground- and surface water showed that the total water requirements over a year can be met by a reassignment of water that reduces the total costs of a great amount.     

Hypersensitivity to airborne allergens common in the central region of Coahuila

A retrospective review of 247 clinical files of patient that have responded to the office allergy and immunology since 1990 to 1993 in Monclova, Coah., for allergy illness. This study was to determine the frequency of Aeroalergens and foods sensitization in Monclova, Coahuila, mediated Allergy Test Skin (intradermo reactions, and Prick) and MAST immuno Systems (Multiple Allergy System Test). The patients were highly sensitive to house dust (13.5%) others aeroalergen pollen grains, were; rusian thistle (7.1%), Pigweed (5.4%), Cynodon Dactylon (4.9%), Pecan (4.8%), Ragweed Mix (4.8%), Rye grass (4.8%) other polen (32.4%). The molds: Candida (3.2%), Alternaria (2.7%), Rhizopus (3.6%), Penicillium (2.1%), Fusarium (2.1%), other molds (8.7%). The foods more important were: Wheat (21.3%), Cheese (10.0%), Shellfish Mix (10.0%), Pork (8.8%), Egg Yolk (7.5%).     

The action of local anesthetics on myelin structure and nerve conduction in toad sciatic nerve

X-ray scattering and electrophysiological experiments were performed on toad sciatic nerves in the presence of local anesthetics. In vitro experiments were performed on dissected nerves superfused with Ringer's solutions containing procaine, lidocaine, tetracaine, or dibucaine. In vivo experiments were performed on nerves dissected from animals anesthesized by targeted injections of tetracaine-containing solutions. In all cases the anesthetics were found to have the same effects on the x-ray scattering spectra: the intensity ratio of the even-order to the odd-order reflections increases and the lattice parameter increases. These changes are reversible upon removal of the anesthetic. The magnitude of the structural changes varies with the duration of the superfusion and with the nature and concentration of the anesthetic molecule. A striking quantitative correlation was observed between the structural effects and the potency of the anesthetic. Electron density profiles, which hardly showed any structural alteration of the unit membrane, clearly indicated that the anesthetics have the effect of moving the pairs of membranes apart by increasing the thickness of the cytoplasmic space. Electrophysiological measurements performed on the very samples used in the x-ray scattering experiments showed that the amplitude of the compound action potential is affected earlier than the structure of myelin (as revealed by the x-ray scattering experiments), whereas conduction velocity closely follows the structural alterations.     

Low-resolution structures of proteins in solution retrieved from X-ray scattering with a genetic algorithm

Small-angle x-ray solution scattering (SAXS) is analyzed with a new method to retrieve convergent model structures that fit the scattering profiles. An arbitrary hexagonal packing of several hundred beads containing the problem object is defined. Instead of attempting to compute the Debye formula for all of the possible mass distributions, a genetic algorithm is employed that efficiently searches the configurational space and evolves best-fit bead models. Models from different runs of the algorithm have similar or identical structures. The modeling resolution is increased by reducing the bead radius together with the search space in successive cycles of refinement. The method has been tested with protein SAXS (0.001 < S < 0.06 A(-1)) calculated from x-ray crystal structures, adding noise to the profiles. The models obtained closely approach the volumes and radii of gyration of the known structures, and faithfully reproduce the dimensions and shape of each of them. This includes finding the active site cavity of lysozyme, the bilobed structure of gamma-crystallin, two domains connected by a stalk in betab2-crystallin, and the horseshoe shape of pancreatic ribonuclease inhibitor. The low-resolution solution structure of lysozyme has been directly modeled from its experimental SAXS profile (0.003 < S < 0.03 A(-1)). The model describes lysozyme size and shape to the resolution of the measurement. The method may be applied to other proteins, to the analysis of domain movements, to the comparison of solution and crystal structures, as well as to large macromolecular assemblies.     

An approximate nonlinear design procedure for partially prestressed concrete beams

The theoretical basis and the main results of an approximate nonlinear design procedure which attempts to predict the behavior at ultimate of prestressed and partially prestressed concrete members is presented. Difficulties encountered in simulating the actual behavior of concrete and steel are discussed; particular emphasis is put on comparing the various available stress-strain relationships for prestressing steels, and observed variability in their actual properties. Recommended equations and corresponding coefficients are given. Typical results showing the influence on ultimate behavior of major variables such as the type and amount of prestressing steel, the amount of nonprestressed tensile and compressive reinforcement, and the ultimate concrete compressive strain are presented and compared with code derived results. They help point out where the code recommendations, if used, may lead to an unsafe design and what modifications may be needed to make them more representative of observed trends.     

Examination of Aluminum Corrosion Products in Marine or Industrial Marine Atmosphere

A method has been developed to characterize the passivation film formed on aluminum in marine atmosphere, whether polluted or not by sulphur oxides and fluorides The information thus obtained was integrated by X-ray diffractometric analyses. The following corrosion products were found in the marine atmosphere: Al₂O₃· 3H₂O + Al (oxychoride) + Al (hydroxide) (The latter two compounds had not been revealed by X-ray analysis). The corrosion products found in the polluted atmosphere were: AlF₃; Al₂(SO₄)₃ · H₂SO₄; Al₁₁(OH)₃₀Cl₃; AlF_1.96(OH)_1.4; 16Al(OH,F)₃ · 6H₂O; AlF_1.65(OH)_1.35 · H₂O. Since the developed method provides useful quantitative information on the aluminum distribution between the various anions, it is deemed to be a useful tool to study the corrosion kineties and mechanism.     

Contribution to the discussion on Localized Corrosion of Stainless Steels in natural sea water

Electrochemical hysteresis methods are employed to develop experimental potential-pH diagrams for some commercial austenitic and ferritic stainless steels in 3% sodium chloride solutions at room temperature. The results were compared with the electrochemical and the gravimetric behaviour of the single stainless steels in natural sea water. The comparison suggested some considerations about the mechanism of initiation and growth of pitting and crevice corrosion prevailing in natural environment. The higher probability of pits observed in field exposure was correlated to the deposit of a microbiological slime on metal surface that ennobles the free corrosion potentials of passive surfaces up to +400 ÷ +450 m V SCE. The crevice corrosion initiation was in turn attributed to a moderate local acidification originating in conditions of reduced diffusion under the action of passivity current that makes possible pit nucleation which is otherwise unlikely to occur. The pitting growth speeds up the acidification process in the interstice until it causes general corrosion of the walls.     

Review of technology in small-scale biomass combustion systems in the European market

This work studied the importance of wood pellets, chips and wood logs for small- and medium-scale heat production. Pellet use can contribute substantially to reaching the renewable heat and electricity goals set by the European Union (EU) Renewable Energy Directive. Consequently, to integrate into European energy markets, pellet use must be a key piece of the EU energy policy.This study provides a wide perspective on the state-of-the-art small-scale biomass combustion units, particularly those that use pellets for fuel. Small-scale combustion units include stoves and boilers with capacities less than 200 kW. A wide market review has been conducted, including a review of the literature and information from manufacturers and test institutes. A database has been created, which includes 186 companies and offers 995 different models, providing an extensive view of the European market. The large number of new companies shows that the solid-fuel boiler market is continuously increasing across Europe. The technologies that are currently the most widely used are described and compared.