An inrush current is a transient current with high amplitude that may occur when a transformer is energized under no load or lightly loaded conditions. The magnitude of inrush current may be as high as ten times or more times of transformer rated current. This could result in huge mechanical and thermal stresses on transformer in addition to inadvertent operation of the protective relay systems. This paper represents the effects of some factors on the inrush current of transformers. For this purpose, a one-phase transformer is simulated in MATLAB and the effects of switching angle variation, the energizing circuit impedance and the remanent flux on the characteristics of inrush current are investigated. The results show that increasing circuit resistance or switching angle will decrease inrush current amplitude. Also, it is concluded that for reducing inrush current, appropriate switching angle with respect to the remanent flux must be selected. The results can be used for a better understanding of the inrush current characteristics and proper actions of the protective system. Ill. 7, bibl. 13, tabl. 1 (in English; abstracts in English and Lithuanian).http://dx.doi.org/10.5755/j01.eee.109.3.162
Geopolymer concrete as a green concrete has attracted great attention among scientists to reduce cement consumption and emissions. This study examines the use of recycled aggregates in geopolymer concrete as a substitute for coarse aggregates. To assess the mechanical performance, compressive strength, tensile strength, and flexural strength were measured for 20 mixing designs and their relationships with suitable convergence were obtained. The scanning electron microscope (SEM) images taken from the samples made with the highest and lowest resistance showed that the formation of hardened geopolymer gel depends on the concentration of chemical solutions. The results of the freezing and thawing cycle and mass loss of samples showed that the compressive strength of the geopolymer concrete had not decreased significantly and had a good durability in these conditions.
The phenomenon of magnetizing inrush is a transient condition, which occurs primarily when a transformer is energized. The magnitude of inrush current may be as high as ten times or more times of transformer rated current that causes malfunction of protection system. So, for safe running of a transformer, it is necessary to distinguish inrush current from fault currents .In this paper, a wavelet-based technique has been used to discriminate inrush current from faults. For this purpose, a wavelet transform (WT) concept is presented and a criterion based on different behaviors of the differential currents under fault and inrush conditions have been derived. For checking the validity of the wavelet-based technique in discriminating of inrush currents from faults a three-phase 20/63 kV transformer has been simulated in Maxwell software that is based on finite element method (FEM). Then, obtained data from different situations have been used in MATALB software for implementation of the WT technique.
This paper is dedicated to investigate the nonlinear free vibration analysis of prestressed orthotropic membranes. The fundamental frequency of the system is obtained asymptotically by employing a modern analytical approach. The effects of different parameters such as vibration amplitude, initial conditions and geometric parameters on the frequency ratio and nonlinear frequency of membrane are investigated. Comparison between the obtained results in the present study, the reported results in the literature, and Runge-Kutta fourth-order numerical method verify the strength of the analytical procedure.
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