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The Gorkha earthquake in 2015 was a recent large-scale earthquake that caused severe damage to many historic masonry buildings in the Kathmandu Valley, Nepal. The authors conducted a visual inspection survey of seismically damaged buildings after the earthquake in the historic town district of Bhaktapur in the Kathmandu Valley. The first part of this paper reports the distribution of the historic masonry buildings for each damage level in the surveyed area. A concentrating zone of severely-damaged buildings was explicitly found in the damage-level distribution map. Almost half of all the surveyed buildings were severely damaged during the earthquake, and most of these were historic masonry townhouse buildings. In the second part, the ambient vibration characteristics of the conventional historic masonry buildings in Bhaktapur are investigated. Typical dwelling houses in a historic town district in Nepal, which generally formed terraced houses built around a courtyard, were targeted for the measurement. Thus, the influence of adjacent buildings, which makes it difficult to identify the predominant natural frequencies from building vibration measurements, was also observed. Microtremor measurements using two accelerometers were conducted at 11 historic masonry buildings to investigate the discrimination degree for identifying the predominant natural frequency of conventional townhouse buildings. The estimated primary natural frequencies of these buildings were compared with the results of the screening model analysis. The advantages of using the proposed screening model analysis to improve the uncertainty of the first natural frequency identification by the microtremor measurement are discussed. Additional measurements of the microtremors at 4 of the 11 measured buildings were conducted using a different location combination of the two acceleration sensors. The sensors were placed between two different floors, and the transfer functions of each floor were investigated to observe the predominant vibration components on the floors in the entire building. Moreover, the coherence values observing the in-plane correlation of the floor responses were also analyzed for the dataset measured at two points separated on the same floor. Finally, an effective method for measuring the microtremors is discussed to improve the assessment of the vibration characteristics of conventional historic masonry buildings in Nepal.

• A mathematical model of tube bundles subjected to cross-flow was presented; • An experimental study was performed to study the streamwise fluidelastic instability of the tube bundles; • The mechanism of streamwise fluidelastic instability of the tube bundle in cross-flow was revealed; • Effects of boundary constraint condition and void fraction of two-phase flow on streamwise fluidelastic instability were discussed. Fluidelastic instability is an important security issue for the steam generator tube bundles subjected to cross-flow. Lots of studies have been conducted on the fluidelastic instability of the tube bundles in the transverse direction. Recently, the streamwise fluidelastic instability of the tube bundles in cross-flow has been observed in some experimental studies. However, the mechanism of the fluidelastic instability in the parallel is not clear. Thus, in this paper, experimental and theoretical studies were performed to investigate the fluidelastic instability of a rotated triangular tube array subjected to cross-flow in the parallel direction. The influences of the boundary constraint condition and the void fraction of two-phase flow on the threshold of the streamwise fluidelastic instability of the tube bundles were discussed. [ABSTRACT FROM AUTHOR]

• Eigenmodes of bulk-reactive lined duct with shear flow obtained. • Calculation procedure with considerable practical interest provided. • Convergence rate of S e 4 realized, with S e being the mesh size. • Eigenmode characteristics analyzed, focusing on shear flow and perforated panel effect. In this paper, the eigenmodes of bulk-reactive lined duct with shear flow are calculated and analyzed by a wave and finite element (WFE) method. Using this method, the eigenmodes can be calculated based on commercially available finite element (FE) matrices, which is of considerable practical interest when dealing with complicated cases. The proposed method is validated by comparing results with Chebyshev collocation method. It is shown that the axial wavenumbers obey the convergence rate of S e 4 for all kinds of modes, where S e is the mesh size. The calculated eigenmodes can be divided into air-borne modes, lining-borne modes and surface modes, where air-borne modes mainly propagate in air domain; lining-borne modes propagate in both air and lining domains; surface modes propagate near the interface between air and lining domains. It is found that, for air-borne and lining-borne modes, boundary layer thickness has small effect on lower order modes, but has relatively large effect on mode shapes of higher order modes. For the surface modes, boundary layer thickness can greatly change their mode properties. In the calculated example, with perforated panel considered, two coupled modes emerge. With perforation ratio evolving from large to small, one coupled mode evolves from a lower-order air-borne mode to a higher-order air-borne mode, and the other evolves from a lining-borne mode to an air-borne mode. Generally, the perforation ratio has small effect on air-borne and lining-borne modes; while it has large effect on surface modes and the coupled mode, especially in the range of [3%, 13%]. [ABSTRACT FROM AUTHOR]

In this study, a Monte Carlo simulation (MCs)-based isogeometric stochastic Finite Element Method (FEM) is proposed for uncertainty quantification in the vibration analysis of piezoelectric materials. In this method, deterministic solutions (natural frequencies) of the coupled eigenvalue problem are obtained via isogeometric analysis (IGA). Moreover, MCs is employed to solve various uncertainty parameters, including separate elastic and piezoelectric constants and their combined cases. [ABSTRACT FROM AUTHOR]

This paper presents the small-signal stability performance of a dc microgrid (MG) and investigates the interactions between the converter controllers by studying the critical modes. A comprehensive small-signal analysis using both the impedance-based method and eigenvalue-based analysis method has been presented thoroughly to investigate the high-frequency oscillation in the dc MG. The results show that there are inherent weak modes within the dc MG which could destabilize the MG in the presence of disturbances. Moreover, the controller gains and dc-link capacitance are found to be the most sensitive to substantial disturbance and have a significant influence on the weak modes leading to oscillation and resonance. The source side converter is found to be more influential than the load side converter in regard to various disturbances.

This paper is dedicated to presenting control-tuning methodologies for the rotor-side converter (RSC) and grid-side converter (GSC) of series-compensated DFIG-based wind farms (WF) and to determining the origin of subsynchronous resonance (SSR). Unlike conventional approaches to re-tuning controller gains to achieve desirable performance, these methodologies only consider the mathematical models of the states variables to be controlled. A PI cascade topology is used to control the RSC and GSC. An inner loop is required for current control and an outer loop for voltage control. Special consideration is taken for tuning the RSC because this converter is coupled with the mechanical variables as mechanical rotor speed of the DFIG. Compared with standard tuning, a better system behavior during a resonance condition is achieved with the proposed tuning methodology. Regarding the analysis of the SSR origin, the modal impedance (MI) technique is used as a tool to study SSR issues; its advantages compared to the driving point impedance technique are highlighted. In this context, the small-signal-stability (SSS) analysis is performed to evaluate the overall system. With the combination of MI and SSS, the phenomena of induction generator effect, torque interaction, and torque amplification are analyzed. As a result, the modes involved in the SSR phenomenon are identified and a discrimination procedure is described to determine the origin of the SSR.

For real time monitoring of the wing state, in this paper, the inverse Finite Element Method (iFEM) is applied, which describes the displacement field of beam according to the Timoshenko theory, to sense the wing frame deformation. In order to maintain the accuracy and stability of frame deformation sensing with iFEM, an optimal placement model of strain sensors based on eigenvalue analysis is constructed. Through the model solution with the Particle Swarm Optimization (PSO) algorithm, two different optimal placement schemes of sensors are obtained. Finally, a simulation is performed on a simple cantilever beam and a static load experiment is conducted on an aluminum alloy wing frame. The results demonstrate that the iFEM is able to accurately sense the deformation of the wing frame, when the two optimal placement schemes of sensors are used. [ABSTRACT FROM AUTHOR]

[Display omitted] • Improvement of Galerkin-based solutions for curved beam vibrations by introducing more reasonable assumed modes. • Formulation of forced vibrations for curved beams by utilizing the mode orthogonality derived based on the reciprocal theorem of work. • Comparative analysis on rails simulated based on straight and curved beam theories. • New insights into train-track interactions considering rail actual curvature and a nonlinear wheel-curved rail spatial contact model. This paper presents a semi-analytical solution to free and forced vibrations of a pinned–pinned Euler-Bernoulli curved beam, and extends it into train-track spatial interactions. The in-plane and out-of-plane modes of the curved beam are approximated by the partial sums of sinusoidal Fourier series, and the forced vibration equations in a generalized coordinate are formulated by utilizing the Galerkin method and mode orthogonality derived based on the reciprocal theorem of work. Comparisons of natural frequencies and dynamic responses with published results and finite element method (FEM) illustrate the reliability and advancement of current approach, which provides more accurate results for higher frequency analysis and possesses better adaptability to different boundary conditions by supplementing polynomials. Moreover, the influence of radius and subtended angle on the natural frequencies of rails simulated by the straight and curved beam models are also investigated. Finally, the established model is further extended to simulate the curved rails implemented in a train-track coupled dynamics system with consideration of a nonlinear wheel-curved rail contact model, since they used to be idealized as straight beams instead of taking the actual curvature into account. Some new recognitions are gained by examining the effect of rail modelling options and additional creepages on a curved track, which is conducive to more accurate evaluations on curve negotiation effects induced by train-track spatial interactions. [ABSTRACT FROM AUTHOR]

In this paper, we study the stability of a numerical boundary treatment of high order compact finite difference methods for parabolic equations. The compact finite difference schemes could achieve very high order accuracy with relatively small stencils. To match the convergence order of the compact schemes in the interior domain, we take the simplified inverse Lax–Wendroff (SILW) procedure (Tan et al., 2012; Li et al., 2017) as our numerical boundary treatment. The third order total variation diminishing (TVD) Runge–Kutta method (Shu and Osher, 1988) is taken as our time-stepping method in the fully-discrete case. Two analysis techniques are adopted to check the algorithm's stability, one is based on the Godunov–Ryabenkii theory, and the other is the eigenvalue spectrum visualization method (Vilar and Shu, 2015). Both the semi-discrete and fully-discrete cases are investigated, and these two different analysis techniques yield consistent results. Several numerical experimental results are shown to validate the theoretical results. [ABSTRACT FROM AUTHOR]

This paper provides an H∞ robust control strategy for a permanent magnet synchronous generator (PMSG)-based wind farm to realize subsynchronous resonance suppression (SSR) subject to uncertain system distortions and parameter perturbation. Firstly, an eighth-order state space mathematical model of a PMSG-based wind farm is established, including the grid-side converter (GSC), GSC controller, and phase-locked loop (PLL) model. Secondly, the SSR characteristics of a PMSG-based wind farm are analyzed through eigenvalue analysis. Thirdly, a robust subsynchronous damping controller is designed based on eigenvalue analysis of SSR. Finally, the designed robust subsynchronous damping controller is validated with case studies of wind farms. The results show that the controller can increase the stability of PMSG-based wind farm systems and restrain SSR.

Abstract: Elastic liners are used for in situ repair and retrofitting of pipes as a cost effective alternative to the replacement of damaged parts and sections. In this paper, we studied the role of an elastic liner on the buckling behavior of a cracked cylindrical shell using finite element method. A special meshing scheme that could mimic the stress singularity at the crack tip was employed to model the cracked shells. Linear eigenvalue analysis was carried out to study the effect of crack geometry (length and orientation) as well as the material properties and thickness of the elastic liner on the buckling load and buckling shape of the cylindrical shell. We considered a combination of axial compression and internal pressure which is a typical loading for pipelines and pressurized liquid-retaining structures. Our results show that cracked shell's strength and mode of buckling for different crack length and orientations can be largely influenced by thickness and relative stiffness of the liner layer. In particular we report a gradual transition from local to global instability due to these size and orientation effects. Finally, the role of internal pressure on structural stability and local buckling of cracked shells, which strongly depends on the crack orientation and liner thickness, is discussed. [Copyright &y& Elsevier]

Abstract: This paper is motivated to analyze the transient behavior of a single-phase self-excited induction generator (SEIG) using a three-phase machine due to switching of single-phase dynamic load like induction motors. The generator consists of a three-phase star connected induction machine excited with three-capacitors and a single-phase induction motor (IM) load. The developed dynamic models of the SEIG and the motor are based on stationary reference frame d–q axes theory incorporating the effect of cross-saturation in the magnetic circuit of the machine and the equations of excitation capacitors are described by three-phase abc model. The system suffers from heavy transients during switching of induction motor and becomes unstable. These problems may be due to resonance caused by series capacitors and the inductive motor load. The use of damping resistors across one series capacitor is proposed to damp out the starting transients for the stable operation. The motor can be started up successfully using the damping resistor. The variation of the damping resistance with the increase in load on the motor after successful starting to maintain constant terminal voltage has been presented. The eigenvalue technique is also employed to examine the transient conditions in the studied SEIG-IM system. The simulated and experimental results are presented for both the unsuccessful and successful starting of the motor. These results are in close agreement with each other, which show the effectiveness of the approach. [Copyright &y& Elsevier]

Abstract: To ensure the small-signal stability of a power system, power system stabilizers (PSSs) are extensively applied for damping low frequency power oscillations through modulating the excitation supplied to synchronous machines, and increasing interest has been focused on developing different PSS schemes to tackle the threat of damping oscillations to power system stability. This paper examines four different PSS models and investigates their performances on damping power system dynamics using both small-signal eigenvalue analysis and large-signal dynamic simulations. The four kinds of PSSs examined include the Conventional PSS (CPSS), Single Neuron based PSS (SNPSS), Adaptive PSS (APSS) and Multi-band PSS (MBPSS). A steep descent parameter optimization algorithm is employed to seek the optimal PSS design parameters. To evaluate the effects of these PSSs on improving power system dynamic behaviors, case studies are carried out on an 8-unit 24-bus power system through both small-signal eigenvalue analysis and large-signal time-domain simulations. [Copyright &y& Elsevier]

Abstract: The majority of wind turbines currently in operation have the conventional concept design. That is a single-rotor wind turbine (SRWT) which is connected through spur gearbox to a generator. Recently, dual-rotor wind turbine (DRWT) has been introduced to the market. It has been proven that the steady state performance of the DRWT system for extracting energy is better than the SRWT. But, a comparison of fault-ride-through capability of these two types of turbines requires further research. In this paper, the fault-ride-through capability of DRWT and SRWT are evaluated and compared when generating units are operating at constant pitch angle and constant speed modes. Constant pitch angle mode is simulated to investigate the natural damping of DRWT and SRWT. To verify the time domain simulation results, damping characteristics of DRWT and SRWT are also compared through eigenvalue analysis and speed droop characteristics of the control system. The accuracy of the aerodynamic model of the DRWT is enhanced by including the stream tube effect in the simulation. It was uncovered that DRWT introduces higher damping torque to the network in both constant speed and constant pitch angle modes. This advantage improves the transient performance of DRWT-based wind farms. [Copyright &y& Elsevier]

Abstract: One of the most important aims of potential field geophysicists is delineate the edges of subsurface structures. There are many methods based on horizontal and vertical derivative of potential field data for edge detection and enhancement. The structure tensor technique one of the image processing techniques is used to edge detection studies in many scientific areas. In this paper, the technique was applied to potential field data and detected edges of the subsurface lineaments using its eigenvalue analysis. Based on noise-free and noisy synthetic data sets, the technique was tested and satisfactory results were obtained. The proposed method was applied on two real potential field data which are gravity data of Konya region and magnetic data of Eastern Anatolia Region in Turkey. These examples demonstrate that the technique provides beneficial information to geoscientists for determining the horizontal location of subsurface structures such as contacts, faults or various source bodies. [Copyright &y& Elsevier]

Abstract: This study is intended to develop an analytical approach to simulate the coupled vibration of bridges and running vehicles, and to investigate the dynamic characteristics of rationalized curved bridges. In this paper, a curved twin Igirder bridge with general cross-section is adopted for the analysis. As a preliminary step, a general large dump truck with one axis at front and two axes at rear is modeled as a sprung-mass system with 2-degree-of-freedom for the vehicle-bridge interaction analysis. The coupled vibration of the interaction system is formulized and a computer program is developed. Eigenvalue analyses are carried out for different types of bridges to find acceptable torsional stiffness. The coupled vibration of the running vehicle and the bridge is simulated using the developed analytical approach, based on which the dynamic responses of the curved bridge are evaluated. [Copyright &y& Elsevier]

Abstract: This paper proposes a topology optimization for a linear elasticity design problem subjected to an uncertain load. The design problem is formulated to minimize a robust compliance that is defined as the maximum compliance induced by the worst load case of an uncertain load set. Since the robust compliance can be formulated as the scalar product of the uncertain input load and output displacement vectors, the idea of “aggregation” used in the field of control is introduced to assess the value of the robust compliance. The aggregation solution technique provides the direct relationship between the uncertain input load and output displacement, as a small linear system composed of these vectors and the reduced size of a symmetric matrix, in the context of a discretized linear elasticity problem, using the finite element method. Introducing the constraint that the Euclidean norm of the uncertain load set is fixed, the robust compliance minimization problem is formulated as the minimization of the maximum eigenvalue of the aggregated symmetric matrix according to the Rayleigh–Ritz theorem for symmetric matrices. Moreover, the worst load case is easily established as the eigenvector corresponding to the maximum eigenvalue of the matrix. The proposed structural optimization method is implemented using topology optimization and the method of moving asymptotes (MMA). The numerical examples provided illustrate mechanically reasonable structures and establish the worst load cases corresponding to these optimal structures. [Copyright &y& Elsevier]

Abstract: In this paper, by using the center manifold reduction method, together with the eigenvalue analysis, we made bifurcation analysis for the Kuramoto–Sivashinsky equation, and proved that the Kuramoto–Sivashinsky equation with constraint condition bifurcates an attractor as crossed the first critical value under the two cases. Our analysis was based on a new and mature attractor bifurcation theory developed by Ma and Wang (2005) . [Copyright &y& Elsevier]

Abstract: This paper deals with the analysis of stability of a hinged-hinged viscoelastic column subjected to a non-zero mean stochastic axial force. The randomly variable part of this is described by a stationary Gaussian white noise process. The viscosity affects the curvature of the column, for which the classic Euler–Bernoulli''s model is adopted. The viscosity is described by the linear Kelvin–Voigt''s model. A dynamic stability analysis is performed. Normal modes are introduced in the integro-differential equation of motion so that uncoupled modal equations are retrieved. With reference to the first mode, by using an additional state variable, three Itô’s ODE are obtained, from which the differential equations ruling the response statistical moment evolution are written by means of Itô’s differential rule. The zero solution, that is undeformed straight column, corresponds to zero moments. If the column is perturbed, it is stable when the response moments tend to zero. A necessary and sufficient condition of stability in the moments of order r is that the matrix A r of the coefficients of the ODE system ruling them has negative real eigenvalues and complex eigenvalues with negative real parts. Because of the linearity of the system the stability of the first two moments is the strongest condition of stability. If the mean axial force μ P or the white noise intensity are increased, there exist critical values μ Pcr , for which almost an eigenvalue is positive. The critical mean axial force is found to be inversely proportional to the parameter φ ∞ , which measures the amount of viscous deformation. The search for the critical values of is made numerically, and several graphs are presented for a representative column. [Copyright &y& Elsevier]

Abstract: In this paper, on the basis of matrix splitting, two preconditioners are proposed and analyzed, for nonsymmetric saddle point problems. The spectral property of the preconditioned matrix is studied in detail. When the iteration parameter becomes small enough, the eigenvalues of the preconditioned matrices will gather into two clusters—one is near and the other is near —for the PPSS preconditioner no matter whether is Hermitian or non-Hermitian and for the PHSS preconditioner when is a Hermitian or real normal matrix. Numerical experiments are given, to illustrate the performances of the two preconditioners. [Copyright &y& Elsevier]

Abstract: The engineering response of metamaterials has a dramatic impact on the physics, optics and engineering communities, because they offer electromagnetic properties that are difficult or impossible to achieve with conventional materials. In this paper, an asymptotic analysis of the electromagnetic fields at multi-material wedges composed of metamaterials is proposed. This is made possible by removing the assumption of positive electric permittivities and magnetic permeabilities, an hypothesis which usually applies to conventional materials. Exploring the whole range of variability of these electromagnetic properties, it is shown that, in addition to the classical real eigenvalues 0⩽ λ <1 leading to power-law singularities of the type O(r λ−1) as r →0, it is also possible to find imaginary eigenvalues leading to hypersingular solutions, as well as nonsingular configurations for a suitable choice of the negative electric permittivities and magnetic permeabilities of the media. Moreover, to fully characterize the asymptotic fields, the analysis is not only restricted to the determination of the lowest real and complex eigenvalues, but is also extended to the evaluation of the higher-order nonsingular ones. The obtained analytical results collected in synthetic diagrams are expected to have impact on the design of micro- and nano-electro-mechanical systems. [Copyright &y& Elsevier]

Abstract: In the present paper, the process of the formulation of the equations of dynamic equilibrium and of the respective equations of natural vibration of Timoshenko beams on three-parameter elastic foundation within the framework of the second order theory is presented. The corresponding mass matrices are also formulated. In order to define the values of the parameters of the three-parameter model, an analytical method is applied. With the aid of two numerical examples, it is proved that the three-parameter model is capable of yielding natural period values that sufficiently approximate those derived by the solution using 2D finite element soil models. [Copyright &y& Elsevier]

Abstract: Use of power electronic equipment has increased and introduced new dynamical phenomena in power systems. For example, new electric rail vehicles (locomotives) equipped with modern power electronic traction chains have caused situations of low-frequency power oscillations and instability in single-phase railway power supply systems. This paper presents the development and implementation of an instantaneous value model and simplified fundamental frequency (RMS) models of such an advanced electric rail vehicle in order to investigate their representation of low-frequency dynamics. The dynamical behaviour is studied by use of both time-domain simulations and linear analysis (eigenvalues) and the degree of simplifications regarding controller dynamics and power system dynamics are presented and discussed. An enhanced RMS model is tested in order to account for the impact of fast current dynamics on the low-frequency behaviour. The results show that this enhanced model is corresponding more closely to the instantaneous value model than what can be obtained by the traditional RMS simplifications and indicate that current dynamics should be included in stability studies involving power electronic inverters. [Copyright &y& Elsevier]

A periodic steady state is a familiar phenomenon in many areas of theoretical biology and provides a satisfying explanation for those animal communities in which populations are observed to oscillate in a reproducible periodic manner. In this paper we explore models of three competing species described by symmetric and asymmetric May–Leonard models, and specifically investigate criteria for the existence of periodic steady states for an adapted May–Leonard model: Using the Routh–Hurwitz conditions, six inequalities that ensure the stability of the system are identified. These inequalities are solved simultaneously, using numerical methods in order to generate three-dimensional phase portraits to illustrate the steady states. Then the “stability boundary” is defined as the almost linear boundary between stability and instability. All the mathematics discussed is suitable for advanced undergraduate mathematics or applied mathematics students, offering them the opportunity to incorporate a computer algebra system such as Mathematica, DERIVE or Matlab in their investigations. The adapted May–Leonard model provides a practical application of steady states, stability and possible limit cycles of a nonlinear system. [Copyright &y& Elsevier]

The stability and controllability of asymmetric complex dynamical networks are investigated in detail based on eigenvalue analysis. Pinning control is suggested to stabilize the homogenous stationary state of the whole coupled network. The complicated coupled problem is reduced to two independent problems: clarifying the stable regions of the coupled network and specifying the eigenvalue distribution of the asymmetric coupling and control matrices. The dependence of the controllability on both pinning density and pinning strength is studied. [ABSTRACT FROM AUTHOR]

Abstract: The tuning of the PSS parameters for a multi-machine power system is usually formulated as an objective function with constraints consisting of the damping factor and damping ratio. A novel mixed-integer ant direction hybrid differential evolution algorithm, called MIADHDE, is proposed to solve this kind of problem. The MIADHDE is improved from ADHDE by the addition of accelerated phase and real variables. The performances of three different objective functions are compared to the MIADHDE in this paper. Both local and remote feedback signals of machine speed deviation measurements can be selected as input signals to the PSS controllers in the proposed objective function. The New England 10-unit 39-bus standard power system, under various system configurations and loading conditions, is employed to illustrate the performance of the proposed method with the three different objective functions. Eigenvalue analysis and nonlinear time domain simulation results demonstrate the effectiveness of the proposed algorithm and the objective function with a remote signal. [Copyright &y& Elsevier]

Due to the rapid development of three-dimensional laser technique, the registration algorithm attracts more attention in industrial application. In this paper, a rapid registration method of 3D point clouds is proposed to improve the efficiency of reverse engineering. The method is based on weighted principal analysis method and re-weighted iterative closest point algorithm. The pipeline consists three stages: coarse alignment, data simplification, and fine alignment. During the first phase, a weighted principal component analysis method is used to calculate original transformation matrices between model data and scene data. In the second part, principal component eigenvalue analysis scheme is implemented to filter the redundant points of datasets. This procedure could reduce the input number of fine registration and refine outliers. Finally, the fine registration is obtained by a weighted iterative closest point algorithm based on Cauchy's and Welsch's function using the partial points obtained in the second phase. The experimental results and comparison experiments are utilized to demonstrate the validity and effectiveness of the proposed matching algorithm. [ABSTRACT FROM AUTHOR]

Abstract: In Argentinean Patagonia, there exists a growing interest in understanding local problems associated with the increasing of wind energy penetration in the grid. Recent papers show that some of these power quality problems can be reduced by the proper control of the wind farms. In this way, this work deals with the impact and potential contribution of wind generation on the damping of the electromechanical oscillations called inter- and intra-area oscillations. For gaining qualitative insights and understandings on this complex subject, a test system that allows to isolate the oscillations modes is considered. The analysis shows that new control concepts for wind farm can efficiently contribute to the power system damping. [Copyright &y& Elsevier]

Abstract: As it is well known, different kinds of wind farms can contribute to frequency and voltage regulation by means of active and reactive powers, respectively. This paper analyzes how the wind farm reactive power loop, which contribute to voltage regulation, modifies the location of the eigenvalues of a selected power system when the wind power penetration is increased. Previous to this analysis, aspects of the modeling of double fed induction generators in wind energy generation and power systems are discussed. Finally, simulations, which verify the theoretical results are presented. [Copyright &y& Elsevier]

Abstract: This paper analyzes the frequency dynamic behavior in a power system with a high wind power penetration. To this end, wind farms equipped with squirrel cage and doubly fed induction generators are compared. Aspects of the modeling of the different kinds of wind generation and power systems are cited. Then, it is shown, through an eigenvalue analysis, that wind farms equipped by doubly fed induction machines, adequately controlled, can contribute to improve the frequency dynamics. Simulations are presented which verify the theoretical results. [Copyright &y& Elsevier]

In this paper a static synchronous series compensator (SSSC) along with a fixed capacitor is used to avoid torsional mode instability in a series compensated transmission system. A 48-step harmonic neutralized inverter is used for the realization of the SSSC. The system under consideration is the IEEE first benchmark model on SSR analysis. The system stability is studied both through eigenvalue analysis and EMTDC/PSCAD simulation studies. It is shown that the combination of the SSSC and the fixed capacitor improves the synchronizing power coefflcient. The presence of the fixed capacitor ensures increased damping of small signal oscillations. At higher levels of fixed capacitor compensation, a damping controller is required to stabilize the torsional modes of SSR. [ABSTRACT FROM PUBLISHER]

This paper presents a method for finding the eigenvalues of some equations, or the zeros of analytic functions. There are two steps in the method. In the first step, integration along the edges of rectangle for an analytic function is performed. From the result of integration, one can know whether the zero exists in the rectangle or not. If the zero of an analytic function exists in the rectangle, we can perform the second step. In the second step, the zero is obtained by iteration. Therefore, the method is called a region searching method. Particular advantage of the suggested method is that the process for finding zero can be visualized. For example, one can clearly indicate the rectangles, which contain the zeros of an analytic function. Three numerical examples are presented. The obtained results are satisfactory even for a complicated case, for example, for finding eigenvalues of a composed wedge of dissimilar materials. Copyright © 2006 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: This paper deals with the asymptotic stability analysis of a discrete dynamical inclusion whose right-hand side is a convex process. We provide necessary and sufficient conditions for weak asymptotic stability, and obtain sharp estimates for the asymptotic null-controllability set. These estimates involve not only standard, but also higher-order spectral information on the convex process and its adjoint. [Copyright &y& Elsevier]

Abstract: This paper deals with both linear and nonlinear analyses of sheet flutter in a narrow passage caused by fluid–structure interaction. The sheet is considered as a combination of massless beam elements, springs and discrete mass particles, in which the mass of each particle and spring coefficients are calculated based on the beam model. In the linear analysis, equations of motion of the system are obtained by formulating fluid dynamic forces and moments acting on each sheet element, yielding an eigenvalue problem. To show the validity of the proposed method, calculated results were compared with previously reported experimental ones. In the nonlinear analysis, nonlinear fluid dynamic forces are introduced to simulate the behavior of the sheet, showing the appearance of limit-cycle vibration in high flow speed. [Copyright &y& Elsevier]