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Computation Scheme (computation + scheme)
Selected AbstractsA perturbation analysis of harmonic generation from saturated elements in power systemsELECTRICAL ENGINEERING IN JAPAN, Issue 4 2010Teruhisa Kumano Abstract Nonlinear phenomena such as saturation of magnetic flux have considerable effects in power systems analysis. It is reported that a failure in a real 500-kV system triggered islanding operation, where resultant even harmonics caused malfunctions in protective relays. It is also reported that the major origin of this wave distortion is nothing but unidirectional magnetization of the transformer iron core. Time simulation is widely used today to analyze phenomena of this type, but it has basically two shortcomings. One is that the time simulation takes too much computing time in the vicinity of inflection points in the saturation characteristic curve because certain iterative procedures such as N-R (Newton,Raphson) must be used and such methods tend to be caught in an ill-conditioned numerical hunting. The other is that such simulation methods sometimes do not aid an intuitive understanding of the studied phenomenon because all of the nonlinear equations are treated in matrix form and are not properly divided into understandable parts, as is done in linear systems. This paper proposes a new computation scheme that is based on the so-called perturbation method. Magnetic saturation of iron cores in a generator and a transformer are taken into account. The proposed method has a special feature to deal with the first shortcoming of the N-R-based time simulation method stated above. The proposed method does not use an iterative process to reduce the equation residue, but uses perturbation series, so that it is free of the ill-conditioning problem. The user need only calculate the perturbation terms one by one until the necessary accuracy is attained. In a numerical example treated in the present paper, first-order perturbation can achieve reasonably high accuracy, which means very fast computing time. In a numerical study, three nonlinear elements are considered. The calculation results are almost identical to the conventional N-R-based time simulation, which shows the validity of the method. The proposed method can be effectively used in screening where many case studies are needed. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 170(4): 35,42, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20895 [source] A simple, scalable and provably stable explicit rate computation scheme for flow control in communication networksINTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 6 2001James Aweya Abstract This paper describes fast rate computation (FASTRAC), an explicit rate flow control algorithm for available bit rate (ABR) traffic. Using digital control theory, we develop a simple rate controller for the ABR flow control process. We prove that the controller is stable, fair to all participating sources and configurable with respect to responsiveness. The analysis presented shows that stability of the flow control process depends primarily on two factors, the control update rate and the feedback delay. The implementation of the proposed algorithm is much simpler than other fair rate allocation algorithms. The proposed algorithm demonstrates the ability to scale with speed, distance, different feedback delays, number of users, and number of nodes while remaining robust, efficient, and fair under stressing and dynamic traffic conditions. Copyright © 2001 John Wiley & Sons, Ltd. [source] Optimizing the formula of rare earth-bearing materials: A computational chemistry investigationINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2007Marjorie Bertolus Abstract We present a computational investigation into the nature of bonds formed by rare earth elements (REE) in materials. This study focuses on the incorporation of neodymium in minerals called apatites, which are derived from fluorapatite: Ca10(PO4)6F2. These minerals, which allow many substitutions on all three Ca, P, and F sites, are considered as potential host phases for radioactive elements separated from nuclear waste. Nd and trivalent actinides have very similar physical and chemical properties, and Nd is not radioactive and much more easily handled. It is therefore very often used as a surrogate for actinides with oxidation degree three in experimental studies. Several formulas can be considered to substitute Nd3+ to Ca2+ and maintain charge balance of the apatite. Existing experimental and theoretical studies, however, mostly concern the Ca9Nd(PO4)5SiO4F2 formula, where the Nd incorporation is compensated by the replacement of one PO by a SiO group. Moreover, only the cation position has been studied, whereas the silicate position and its influence on stability are unknown. We present a more general investigation of possible charge compensations on the one hand, and of the various resulting configurations on the other. All possible configurations of the two formulas Ca9Nd(PO4)5 SiO4F2 and Ca8NdNa(PO4)6F2 have been considered. Calculations have been performed within the framework of density functional theory (DFT). A computation scheme that permits good accuracy in these systems within reasonable computation times is determined. The results obtained for cohesion energies, geometries, and electronic densities are discussed. As for the formulation, it is shown that the Ca8NdNa(PO4)6F2 formula is less stable than the fluorapatite, while Ca9Nd(PO4)5 SiO4F2 is more stable. For the structures, it is found that Nd substitutes preferably in the second cationic site. Moreover, the most stable structures exhibit the shortest Na,Nd or Nd,Si distances. Local charge balance therefore seems favorable. Then, it is shown that Nd forms covalent bonds both in apatite and in britholite, while Na forms ionic bonds. Finally, a first correlation between the material stability and the covalent character of the bonds formed is established. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Robust hyperplane synthesis for sliding mode control systems via sensitivity minimizationOPTIMAL CONTROL APPLICATIONS AND METHODS, Issue 3 2002Hei Ka Tam Abstract A robust hyperplane computation scheme for sliding mode control systems is proposed in this paper. A novel sensitivity index for the sliding eigenvalues with respect to perturbations in the system matrix, the input matrix and the hyperplane matrix is derived in the first instance. The robust hyperplane design problem is then formulated as an optimization task in which the proposed sensitivity index is accordingly minimized. Gradient information of the objective function is established which permits optimization to be proceeded effectively. A numerical example with statistical testing is employed to illustrate the design technique. Copyright © 2002 John Wiley & Sons, Ltd. [source] Diagnosis and tuning of observational error in a quasi-operational data assimilation settingTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 615 2006Bernard Chapnik Abstract Desroziers and Ivanov proposed a method to tune error variances used for data assimilation. The implementation of this algorithm implies the computation of the trace of certain matrices which are not explicitly known. A method proposed by Girard, allowing an approximate estimation of the traces without explicit knowledge of the matrices, was then used. This paper proposes a new implementation of the Desroziers and Ivanov algorithm, including a new computation scheme for the required traces. This method is compared to Girard's in two aspects: its use in the implementation of the tuning algorithm, and the computation of a quantification of the observation impacts on the analysis known as Degrees of Freedom for Signal. Those results are illustrated by studies utilizing the French data assimilation/numerical weather-prediction system ARPEGE. The impact of a first quasi-operational tuning of variances on forecasts is shown and discussed. Copyright © 2006 Royal Meteorological Society [source] | ||||||||||