Classification of sags gathered in distribution substations based on multiway principal component analysis

Voltage sags, whether they occur in transmission or distribution systems, may severely damage the loads connected to the power system. As these problems could cost a great deal financially, electric utilities are very interested in finding the origins of sags, that is, whether they have been originated in the transmission network (high voltage (HV)) or in the distribution system (medium voltage (MV)). In addition to the needs of utilities and regulators, many researchers have been prompted to develop reliable methods to properly classify sags. Several of these methods, based on classifying meaningful features extracted from data and waveforms, have been proposed in the literature. Unlike those methods, though, we propose a systematic transformation of data, based on multiway principal component analysis (MPCA), to develop a new voltage sag classification procedure. Sampled voltage and current waveforms of previously registered sags are used together with the MPCA technique to obtain a lower dimensional model. This model is then used to project new sags and classify them according to their origin in the power system. Different classification criteria and parameters are examined to maximize the classification rates of not yet seen sags. Applying the proposed method to real sags recorded in substations demonstrates its applicability and power.     

A semi-empirical model for steady-state simulation of household refrigerators

A semi-empirical modeling approach for simulating the steady-state behavior of vapor-compression refrigeration systems is presented in this study. The model consists of first-principles algebraic equations adjusted with experimental data obtained from the refrigeration system under analysis. Measurements of the relevant variables were taken at several positions along the refrigeration loop, supplying data for the calibration of the computer model. The predicted values of refrigeration capacity and power consumption when compared with experimental data were within ±10% error bands. The model was also employed to assess the effect of various system parameters on the refrigerator performance. Although a top-mount frost-free refrigerator was chosen for this study, the modeling methodology can be easily extended to other types of refrigeration systems, from domestic refrigerators to heat pumps.     

Direct assessment of tensile stress-crack opening behavior of Strain Hardening Cementitious Composites (SHCC)

The process of designing Strain Hardening Cementitious Composites (SHCC) is driven by the need to achieve certain performance parameters in tension. These are typically the pseudo-strain hardening behavior and the ability to develop multiple cracks. The assessment of the tensile load-deformation behavior of these materials is therefore of great importance and is frequently carried out by characterizing the material tensile stress–strain behavior. In this paper an alternative approach to evaluate the tensile performance of SHCC is investigated. The behavior of the material in tension is studied at the level of a single crack. The derived tensile stress-crack opening behavior is utilized to analyze and compare the influence of various composite parameters on the resulting tensile behavior. The deformations occurring during tensile loading are furthermore examined using a digital image-based deformation analysis technique to gain detailed insight into the crack formation, propagation and opening phases.     

Comparison of UF synthesis by alkaline‐acid and strongly acid processes

This work discusses two processes for producing urea‐formaldehyde (UF) resins. One is the alkaline‐acid process, which has three steps: usually an alkaline methylolation followed by an acid condensation and finally the addition of a final amount of urea. The other process, the strongly acid process, consists of four steps, in which the first step involves a strongly acid condensation followed by an alkaline methylolation, a second condensation under a moderately acid pH and finally, methylolation and neutralization under a slight alkaline pH. Two resins were produced using the two above described processes. The molecular weight distribution (MWD) of the resins was monitored off‐line by GPC/SEC and the final resins were characterized by GPC/SEC and HPLC. These studies showed that the two resins differ greatly in chemical structure, composition, viscosity, and reactivity. The monitoring of MWD indicated that the first condensation under a strongly acid environment leads to the production of a polymer with a distinctly different chemical structure, therefore increasing the flexibility of polymer synthesis and opening the way to the improvement of end‐use properties. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Combined simulation of central HVAC systems with a whole-building hygrothermal model

Due to the lack of building simulation programs that can simulate in details the combined heat, vapor and liquid transfer in porous elements and the HVAC systems, a generic and flexible computational algorithm has been elaborated in order to integrate models for both HVAC systems and multizone building hygrothermal model.     

Jet fires and the domino effect

The direct effects of a jet fire are among the least severe of the diverse fire accidents that can occur in industrial installations or in the transportation of hazardous materials. However, jet fires often affect equipment (e.g., a pipe or tank) that may subsequently fail and ultimately amplify the scale of the accident. This process, known as the domino effect, can have severe consequences in accidents that involve a jet fire. However, no studies have been carried out on its significance. A survey performed on data taken from several accidents databases has revealed the most frequent accident sequences. In 50% of the reported cases involving a jet fire, an additional event with severe effects also occurred.     

Reserve mobilisation in mesquite (Prosopis juliflora) seed (Leguminosae)

The seeds of Prosopis juliflora (mesquite) have a water barrier composed of the palisade layer present in the seed tegument and galactomannans in the endosperm. Changes in the morphological and physiological states of seeds are usually accompanied by changes in their metabolism. The aim of this study was to detect and characterise the main reserves and their mobilisation during and following germination of P. juliflora seeds. Sections of seeds 0, 24, 48 and 72 h after the onset of the imbibing process were evaluated by light microscopy, and seed reserves were extracted and quantified. Polysaccharides were isolated and characterised by ¹H nuclear magnetic resonance (NMR) spectroscopy. In mesquite seeds a well‐developed endosperm, consisting largely of the storage carbohydrate (galactomannans), lies between the seed coat and the cotyledons. Germination was complete after 24 h of imbibition. At this time, morphological changes were observed in stained protein bodies. Chemical analysis showed a decrease in protein levels until 48 h. After 48 h, globular structures were observed in the cotyledon cells, but after 72 h these grains were smaller and their content was reduced. Polarisation microscopy confirmed that this material was composed of starch grains. The ¹H NMR spectrum of P. juliflora seeds showed that sucrose was used following germination. Apparently, sucrose is the transport sugar mobilised from reserve stocks for seedling growth. Parallel to endosperm galactomannan degradation, starch is produced transitorily in the cotyledon. Copyright © 2007 Society of Chemical Industry     

Organic media selection for the extraction of β‐hydroxyisobutyric acid produced by microbial biotransformation

The aim of this work is to evaluate both the toxic effect of different organic media on the stereospecific oxidation of 2‐methyl‐1,3‐propanediol to R‐(−)‐β‐hydroxyisobutyric (HIBA) in two‐phase systems and the extraction ability and selectivity of these non‐water miscible phases. Apart from traditional solvents, specific organic acid‐complexing carriers like TOPO, TOA and Aliquat 336 dissolved in different diluents have been studied. Special interest has been focused on the effect of the concentration of the organic phase extractants and the pH of the aqueous phase on the extraction system. TOPO dissolved in isooctane enabled higher K_p values at lower concentrations to be attained and resulted in lower toxicity, but its extractive capacity is strongly dependent on the pH. Our results suggest that using a compromise pH value between optimum for bioconversion and extraction, TOPO dissolved in isooctane can be successfully used as an extractive phase for HIBA production in a two‐phase system. © 2000 Society of Chemical Industry     

Molecular motions and transitions in a poly(p-hydroxystyrene) derivative

The dielectric relaxation mechanisms in a poly(p-hydroxystyrene) derivative have been studied by Thermally Stimulated Depolarization Currents (TSDC) in the temperature range between −160 and 130°C. A broad relaxation was observed at low temperatures, from −160 up to 0°C, which obeyed to the zero entropy approximation. In contrast, the glass transition relaxation showed the usual behavior, a strong departure to the zero entropy prediction. Both relaxations have been studied in detail by the technique of thermal sampling. The thermal behavior of this poly(p-hydroxystyrene) derivative was also studied by Differential Scanning Calorimetry in the temperature range between 20 and 200°C. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1921–1926, 1999