Transient stability analysis of a power system with one generator connected to an infinite bus system

This paper presents a novel method to identify the fault that would affect the transient stability of one machine to an infinite bus. Here, the swing equation of each machine is expressed in the form of a closed-loop transfer function in Laplace ss-domain. In the transfer function, the duration of the fault and the changes in maximum electrical power output of the machine during and after the fault are considered. Then, the real value of the dominant root of each machine's characteristic equation is identified on the real axis of the s-plane. In this method, the generator has been considered unstable based on the position of the real value of the dominant root in the left half or very close to the imaginary axis or in the right half of the s-plane. The results obtained by the proposed method are compared with those of the conventional step-by-step method on the basis of different fault clearing times, and good agreements have been demonstrated.     

Vertical Transport in GaN/AlGaN Resonant Tunneling Diodes and Superlattices

Using the transfer matrix formalism, we have theoretically studied the vertical ballistic transport in GaN/AlGaN resonant tunneling diodes (RTDs) and superlattices with a small number of periods. We have calculated the transmission probability versus the longitudinal electron energy (T–E) and the current density–voltage (J–V) characteristics. Calculations of both T–E and J–V characteristics have been performed for different Al contents in the barriers. The asymmetry effects due to the internal electric field in the barriers are discussed. Applied to the RTD structure, our calculations demonstrate: (i) the increase of the peak-to-valley ratio of the negative differential resistance (NDR) with increasing Al content in the barriers, (ii) the dependence of the J–V resonance values on the current direction, and (iii) the asymmetry of the NDR with respect to the current direction due to the huge internal electric field in the structure. In the case of multiple quantum well structure (MQWS), the calculation results confirm the same trends as in the RTD case when the Al content is varied. In spite of the fact that it is more difficult to analyze the results in the case of MQWS, the obtained calculations demonstrate the applicability of the used model and of the numerical method to study GaN/AlGaN devices based on quantum well (QW) heterostructures. Furthermore, a design of an optimized 7QW structure operating symmetrically whatever the direction of the applied voltage is presented.     

Antimicrobial activity of lactic acid and copper on growth of Salmonella and Escherichia coli O157:H7 in laboratory medium and carrot juice

Outbreaks of food-borne pathogens, such as Escherichia coli O157:H7 and Salmonella, continue to draw public attention to food safety. Several reports have demonstrated the efficacy of using natural ingredients to control the growth of food-borne pathogens. The objective of this study was to investigate antimicrobial effects of lactic acid and copper, alone and in combination, on the survival and growth of Salmonella spp. and E. coli O157:H7 in laboratory medium and carrot juice. Survival and growth of 38 Salmonella spp. and six E. coli O157:H7 strains were compared when grown in brain heart infusion (BHI) broth and carrot juice under conditions including either lactic acid (0.2%) alone, copper sulfate (50 ppm) alone or the combination of the two. The growth inhibition was negligible when copper sulfate was added to BHI broth and carrot juice. Lactic acid (0.2%) retarded the growth of bacterial strains. However, the growth of bacterial strains was significantly inhibited when both lactic acid and copper were in BHI broth and carrot juice within the time frame of this study. These findings indicated that lactic acid, in combination with copper sulfate, could be used to inhibit the growth of pathogens. Natural ingredients, such as lactic acid and low dose of copper ions, can be used to improve the safety of food products.     

Effect of high temperature exposure on the microstructure of Udimet-500 super alloy

Udimet-500 alloy is a high-strength nickel-bases super alloy used for turbine blades applications. It is primarily strengthened by very fine precipitates called gamma prime (γ′) which coarsened with time when exposed to high temperature. In this study, Udimet-500 alloy in the standard aged condition was exposed at 850–1100 °C for 25–100 h in air. The change in the size of γ′ with respect to the high temperature exposure was characterized using scanning electron microscope. A systematic coarsening of the γ′ was observed with the change in the exposure temperature/time. The γ′ size in the virgin sample was 0.1 μm while after high temperature exposures it coarsened to 1 μm. The Larson-Miller parameter (LMP) of the high temperature exposure was also measured. It was found that the LMP had power relation with the γ′ size. In addition, degradation of the primary as well as secondary carbides was also observed which can be used as add-on microstructural information in high temperature exposure. Elevated temperature exposure (≥1000 °C) also lead to (a) near surface de-alloying, (b) precipitation of needle-like Ti-rich phase and (c) depletion of γ′. It may contribute in the accelerated corrosion and loss of strength of alloy.     

Numerical integration scheme to reduce the errors in the satisfaction of constrained dynamic equation

Assuming that the existence of the constraints yields the change of the inertia force, this study derives the time-varying mass matrix for describing the constrained dynamic equation. It is displayed that the results corresponds with the ones by Udwadia and Kalaba. The numerical results obtained by integrating the constrained dynamic equation of second-order differential equations yield the errors in the satisfaction of the constraints. Modifying the derived dynamic equation this study presents a numerical algorithm to reduce the errors and to compute more precise motion. It is illustrated that the proposed method can be more precisely utilized in constrained mechanical systems through two applications of constrained nonlinear robotic systems.     

Neuro-Fuzzy Modeling for the Prediction of Below-Bubble-Point Viscosity

Abstract

Accurate prediction of reservoir fluid is one of the important factors that needs to be determined due to it usefulness in fluid characterizations, material balance calculations, and general management of reserves. Below-bubble-point viscosity is one of the important variables that has been determined either experimentally or empirically.

This work focuses on the use of neuro-fuzzy techniques to develop a below-bubble-point viscosity model using 1,693 data obtained from different oil fields in the Niger Delta, Nigeria. The data set was randomly divided into three parts with 56.3% used for training, 18.7% for validation, and 25% for testing. The accuracy of the developed model in this study was compared with some published correlations. The statistical analysis results show that the developed model outperformed existing published correlations.     

Failure Analysis of Laser Weld Joint of X2NiCoMo18-8-5 Steel

Laser welded T-shaped steel joints were received in debris form. The debris were recovered from the crashed component. Different metallurgical analysis was made on these debris to reach the root cause of the failure. In this regard, optical microscope and scanning electron microscope were utilized for the confirmation of different observations. Huge deposits of oxidation products were observed on the thick part (i.e., plate) of the T-shaped joint. For the confirmation of these products a set of simulated experiments were also performed.     

In situ preparation of CdS/PVA nanocomposites using gamma radiation

Films of poly(vinyl alcohol)/cadmium sulphide (PVA/CdS) nanocomposite containing various concentrations of Cd²⁺ ions were prepared using gamma radiation at different doses from 50 up to 200 kGy. The UV/VIS spectra revealed that the CdS/PVA nanocomposites showed blue shift for the absorption peak as compared with bulk CdS. As the irradiation dose increased, a gradual red shift in the wavelength accompanying with broadening of the absorption peak was observed. The estimated optical band gap energies and the calculated CdS particle sizes of (PVA/CdS) showed correlation between their values and the variable parameters (irradiation dose and Cd⁺²:S^?2 molar ratio). Transmission electron microscopy images showed that the CdS/PVA nanocomposites were dispersed as spherical CdS nanoparticles with homogeneity at either lower concentration of CdCl₂ or irradiation dose. The nano‐rod structures of CdS was accompanied with small agglomeration at either higher CdCl₂ concentration or irradiation dose. A cubic phase and mixture of cubic and hexagonal phases of the prepared CdS nanoparticles were formed at lower and higher CdCl₂ concentrations, respectively. Fourier Transform Infrared spectra confirmed the coordination of CdS nanoparticles with the hydroxyl groups of PVA matrix. POLYM. ENG. SCI., 55:2583–2590, 2015. © 2015 Society of Plastics Engineers