Search

Physical systems exhibiting symmetry properties may be conveniently studied using the mathematics of group theory. In structural mechanics, group theory has been successfully employed to simplify problems of the bifurcation, stability, statics, kinematics and vibration of symmetric configurations of space frames, space truss domes, double-layer and triple-layer space grids, plates and cable-net systems. Besides significantly reducing computational effort, group theory affords deeper insights on structural behaviour, and a better understanding of complex structural phenomena. The key to group-theoretic simplification is the decomposition of the space of the problem into independent subspaces that are spanned by symmetry-adapted variables obtained by applying idempotents of the symmetry group on the normal variables of the problem. However, for degenerate subspaces of a symmetry group (i.e. subspaces associated with repeating solutions), the associated idempotents do not sufficiently decompose the problem. The aim of this paper is to present, for the C 3 v symmetry group describing the symmetry of a regular 3-sided polygon, a pair of algebraic operators that fully decompose such subspaces. Compared with existing group-theoretic formulations, these operators offer an alternative approach that is simpler and more suited to practical engineering computations, and that affords clearer insights on the physical characteristics of the structural system (such as type of symmetries within the degenerate subspaces). The validity of the operators is confirmed through comparisons with results of eigenvalue vibration and stability problems drawn from the literature. • New group-theoretic operators for splitting degenerate subspaces are presented. • Subspace decomposition is illustrated by reference to a double-layer space grid. • Excellent agreement of results with those in existing literature is demonstrated. [ABSTRACT FROM AUTHOR]

Small Signal Stability Analysis (SSSA) for a hybrid renewable energy system including uncertainties requires an extensive attention to make a system more stable. This paper analyses the method involved in the stability studies of integrated hybrid power system with the help of Lyapunov's stability criteria. The stability studies can be performed for different renewable energy sources like wind generation system, solar PV system and a micro hydro system. In this paper, a study is carried out on a wind generation system which is integrated with the Western System Coordinating Council (WSCC) four machine system. The stability analysis concentrates on the different states of operation of WSCC four machine system with random values of generation and demand. The simulation studies on WSCC four machine system are carried out with the help of MATLAB/Simulink to evaluate stability. This study is validated with the aid of Monte Carlo Approach for evaluations based on iterations. This investigation highlights the significant advantage of conducting stability studies with uncertainty constraints involved, over the deterministic stability studies like probable limitations for transfer capability to cancel out poorly damped oscillations from power system operation. [ABSTRACT FROM AUTHOR]

This paper analyzes the influence of non-dispatchable energy sources on the dynamic stability of stand-alone MicroGrids (MGs) with autonomous control of frequency and voltage based on inverters. The analysis considers the use of eigenvalue techniques. Although this methodology is well known, the contribution of this paper is twofold. First, it proposes an algorithm for designing the inverter interface to meet the requirements associated with the harmonic distortion (THD) and dynamic performance of the inverter. Second, it provides an analysis of the dynamic stability of MG considering different levels of penetration of non-dispatchable energy sources, variations in the network parameters and the presence of dynamic loads (i.e., induction motors). [ABSTRACT FROM AUTHOR]

This paper explores the potential of hydrogen as an energy carrier for deep-sea applications. Finite element analysis of a type III pressurised cylinder to the intended working pressures of 300 bar internal and up to 600 bar external were carried out for different designs and safety factors. Design parameters such as helical angle, liner, helical, and hoop thicknesses were studied and optimised. A buckling analysis was carried out for the optimised designs and recommendations to increase the maximum allowable external pressure are given. Image 1 • A parametric study was carried out varying several pressure vessel parameters. • Parameters included liner, helical, and hoop thickness, among others. • The designs were optimised to endure 300 bar internal pressure. • Eigenvalue and nonlinear buckling analysis were carried out for optimised designs. • Designs without hoop reinforcement have a stronger buckling response. [ABSTRACT FROM AUTHOR]

This paper presents two modified Hager–Zhang (HZ) Conjugate Gradient methods for solving large-scale system of monotone nonlinear equations. The methods were developed by combining modified forms of the one-parameter method by Hager and Zhang (2006) and the hyperplane projection technique. Global convergence and numerical results of the methods are established. Preliminary numerical results show that the proposed methods are promising and more efficient compared to the methods presented by Mushtak and Keyvan (2018) and Sun et al. (2017). [ABSTRACT FROM AUTHOR]

• Proposing a hierarchical battery sizing approach to improve dynamic performance of MG. • Optimal sizing of BESS based on stochastic models to minimize frequency fluctuations. • Secondary BESS optimal sizing and placement to improve static and dynamic stability. • Proposing a dynamic objective function based on system eigenvalues. Integration of energy storage systems (ESSs) to microgrids (MGs) due to the growing penetration of intermittent energy resources is a great motivation to develop studies in the context of optimal sizing and placement of these systems. In this paper, a multi-objective bi-level optimization problem is addressed based on the stochastic nature and dynamic model of power system elements. As a preliminary step, the optimal size and capacity of the battery energy storage system (BESS) are determined to minimize frequency fluctuations. This level has been performed based on three approaches considering the daily intra-hour stochastic data. In the secondary level, the dynamic behaviour of all MG elements has been considered and correspondingly a dynamic objective function based on system eigenvalues and voltage sensitivity index (VSI) has been developed. According to the proposed objective function, the secondary step for sizing and placement of the battery units has been performed by adaptive particularly tuneable fuzzy particle swarm optimization (APT-FPSO) algorithm to reduce power losses, improve the voltage profile/ stability, and mitigate low-frequency fluctuations. Finally, the effectiveness of the proposed approach is evaluated using time-domain simulations, voltage sensitivity studies, and eigenvalue analysis. [ABSTRACT FROM AUTHOR]

High-frequency oscillation is one of the critical issues threatening the stability of modular multilevel converter (MMC) based high-voltage direct current (HVDC) system. A new 2 kHz high-frequency oscillation has occurred in the wind farm-side MMC station of Rudong wind farm integrated MMC-HVDC project of China during no-load charging process. The mechanism of this high-frequency oscillation is different to previous ones. Apart from the time delay of MMC, the stray capacitance of converter transformer makes a significant contribution to the high-frequency oscillation. This paper investigates the high-frequency oscillation mechanism of wind farm-side MMC station during no-load charging process. First, the harmonic state space (HSS) model of a wind farm-side MMC station is established. In addition to the leakage inductance, the stray capacitance is considered in the converter transformer model. To reveal the significance of the converter transformer stray capacitance in this new high-frequency oscillation, the root loci of the system are calculated with varying stray capacitance. Then, the state variables with high degree of participation in the high-frequency oscillation modes are determined by the participation factor analysis. A reduced-order model suitable for the high-frequency oscillation analysis is further established based on the participation factor analysis results. Next, the high-frequency oscillation mechanism is revealed by impedance analysis. Finally, the effects of system parameters are analyzed by the combination of eigenvalue analysis and impedance analysis. The analysis results are validated by electromagnetic transient simulations. • High-frequency oscillation mechanism of MMC charging with transformer is revealed. • The transformer stray capacitance must be considered in the oscillation analysis. • Participation factor analysis is carried out to determine main influencing factors. • A reduced-order model suitable for high-frequency oscillation analysis is proposed. • AC voltage control of MMC has significant effects on the high-frequency oscillation. [ABSTRACT FROM AUTHOR]

Highlights: [•] This paper presents an approach to the allocation and designs the PSSs. [•] The proposed approach has succeeded in determining the dominant generator. [•] Paper proposed a model for designing PSS regarding oscillation modes of the power. [Copyright &y& Elsevier]

• It is an attempt to develop an external SC based on current injection. The implementation mode of injecting AC current signals into current sampling loop of GSC controller has not been discussed before in the literature. • The entire CSI-ECSC is an independent modular design, which prevents redesigning the internal structure of the original controller. • The CSI-ECSC can be widely applied for diverse types of converter-based units. In other words, any DG or active load integrated to ADNs with electronic converter such as PMSG, PV, and ES, etc. can apply the CSI-ECSC, because it is made according to the specific existing control of converter controller. • This work is illustrated through comprehensive and detailed research and development process, including full eigenvalue analysis, interface circuit design, detailed simulation comparison, and effectiveness verification in hard-in-the-loop (HIL) testing based on real-time digital simulator (RTDS) system. Supplementary control (SC) technology is widely leveraged by power supply companies in active distribution networks (ADNs) to improve their stability and dynamic characteristics. Yet, the existed SCs are generally implemented from inside the converter controllers of distributed generators (DGs) or active loads, so there is a need to redesign the internal physical structure of the existing controller, resulting in the increasing work amount of assembling and workability. This paper studies the specific R & D process of a novel external coupling type SC (ECSC), which is based upon current source injection (CSI-ECSC) for improving the dynamic characteristics of ADN. The SC current signals are coupled to the current sampling loop from outside the converter controller. And the employment of the existing current sample makes it unnecessary to redesign the internal physical structure of the existing controller. As a result, the SC assembling is simplified and its workability is improved. In this paper, a detailed exemplary ADN with direct-drive permanent magnet synchronous generator (PMSG) is firstly set up in math for full eigenvalue analysis. Then, the CSI-ECSC is designed with its control loop, interface circuit, and parameter setting. Furthermore, by using PSCAD/EMTDC, groups of case studies are conducted in ADNs where photovoltaics (PVs) and energy storage (ES) are included. Finally, the real-time hardware-in-the-loop (HIL) testing validates the functionality of the realized CSI-ECSC in RTDS. [ABSTRACT FROM AUTHOR]

Abstract: In this paper a novel supplementary subsynchronous damping controller (SSDC) is proposed for the STATic synchronous COMpensator (STATCOM) which is capable of damping out subsynchronous oscillations in power system with series compensated transmission lines. An auxiliary subsynchronous damping controller (SSDC) for a STATCOM using the generator rotor speed deviation signal as the stabilizing signal has been designed to damp subsynchronous oscillations. Eigenvalue analysis and transient simulations of detailed nonlinear system are considered to investigate the performance of the controller. Robustness of the controller has been analyzed by facing the system with disturbances leading to significant changes in generator operating point. This paper deals with a cascaded multilevel converter model, which is a 48-pulse (three levels) source converter. The IEEE Second Benchmark (SBM) model is considered for the analysis and the complete digital simulation of the STATCOM within the power system is performed in the MATLAB/Simulink environment. [Copyright &y& Elsevier]

Abstract The ITER large power supplies of the superconducting magnets and heating and current drive systems, fed by the Pulsed Power Electrical Network, can absorb from the grid active and reactive power up to 500 MW and 950 Mvar respectively. In this paper, a new analytical approach based on the state space formulation is proposed to investigate the dynamic stability of such complex system, including the main power components and their control, with the aim of identifying possible instability phenomena due to interactions among the ac/dc converters, reactive power compensation systems and related controllers. This model describes the dynamics of the main components of the ITER pulsed power supply systems and it is based on small signal approach to tackle the non linearity of the ac/dc conversion and reactive power compensation systems; the discrete phenomena have been approximated by continuous transfer functions, so the linear control theory can be applied for the stability analysis, making this method very effective and fast. Moreover a modular approach is adopted; thus this analytical model can be also easily adapted to other similar electrical power systems. [ABSTRACT FROM AUTHOR]

Abstract: This paper proposes a method for PSS parameter tuning. The proposed method is based on the system identification using a Kalman filter. To eliminate the influence of inaccuracies of the mathematical model due to incomplete data of the system parameters, this paper proposes a procedure for identifying system parameters based on a Kalman filter. The identified system is the basis for determining the eigenvalue sensitivity function. PSS tuning is carried out based on the eigenvalue sensitivity analysis of changes of PSS parameters. In order to obtain the eigenvalue sensitivity function on the PSS parameter in a wider range of parameter values, it has been proposed to determine eigenvalues on the basis of the Bairstow method for solving polynomial equations. The entire procedure can be applied on-line, so the PSS structure and settings can be adaptive. Experimental tests carried out on a synchronous generator connected to an AC system demonstrate the effectiveness of the proposed method. Performance of the PSS tuned by using the proposed method was compared with the performance of the PSS tuned by using eigenvalue sensitivity. [Copyright &y& Elsevier]

Abstract: When an inter-area mode dominates a low-frequency oscillation in a stressed condition, control of the active power flow of interface lines, with compensating devices, can effectively reduce the electromechanical power oscillations. In general, interface lines in which inter-area oscillations are large are considered to be good locations for installation of compensating devices. A sensitivity analysis with respect to change in active power flow can provide an important factor in electric power system operation. This paper proposes a new methodology to calculate the damping sensitivity with respect to change in active power flow, which can be useful for accurate selection of critical lines from the viewpoint of small-signal stability. In the proposed methodology, a damping sensitivity index is used to select the critical lines to damp power system oscillations. This paper describes how to derive the damping sensitivity for the selected mode and illustrates an example applying the proposed algorithm to a simple two-area system and the New England 39-bus test system. [Copyright &y& Elsevier]

The introduction of liberalized electricity markets in many countries has resulted in more highly stressed power systems. On the other hand, operating points of a power system are acceptable in the feasible region, which is surrounded by the borders of different stabilities. Power system instability is critical for all participants of the electricity market. Determination of different stability margins can result in the optimum utilization of power system with minimum risk. This paper focuses on the small disturbance voltage stability, which is an important subset of the power system global stability. This kind of voltage stability is usually evaluated by static analysis tools such as continuation power flow, while it essentially has dynamic nature. Besides, a combination of linear and nonlinear analysis tools is required to correctly analyze it. In this paper, a hybrid evaluation method composed of static, dynamic, linear, and nonlinear analysis tools is proposed for this purpose. Effect of load scenario, generation pattern, branch and generator contingency on the small disturbance voltage stability are evaluated by the hybrid method. The test results are given for New England and IEEE68 bus test systems. [Copyright &y& Elsevier]

In a recent paper, wavelet analysis is used to perturb the coupling matrix in an array of identical chaotic systems in order to improve its synchronization. When the coupling matrix is symmetric, the synchronization criterion is determined by the second smallest eigenvalue λ2 of the coupling matrix and the problem is reduced to studying how λ2 of the coupling matrix changes with perturbation. In the aforementioned paper, a small percentage of the wavelet coefficients are modified. However, this results in a perturbed matrix where every element is modified and nonzero. The purpose of this Letter is to present some results on the change of λ2 due to perturbation. In particular, we show that as the number of systems n→∞, perturbations which only add local coupling will not change λ2. On the other hand, we show that there exists perturbations which modify an arbitrarily small percentage of matrix elements, each of which is changed by an arbitrarily small amount and yet can make λ2 arbitrarily large. These results give conditions on what the perturbation should be in order to improve the synchronizability in an array of coupled chaotic systems. This analysis allows us to justify and explain some of the synchronization phenomena observed in a recently studied network where random coupling is added to a locally connected array. We propose to classify various classes of coupling matrices such as small world networks and scale free networks according to their synchronizability in the limit. Finally, we briefly discuss the case of time-varying coupling. [Copyright &y& Elsevier]

In this paper, we apply the augmented Lagrangian (AL) approach to steady buoyancy driven flow problems. Two AL preconditioners are developed based on the variable’s order, specifically whether the leading variable is the velocity or the temperature variable. Correspondingly, two non-augmented Lagrangian (NAL) preconditioners are also considered for comparison. An eigenvalue analysis for these two pairs of preconditioners is conducted to predict the rate of convergence for the GMRES solver. The numerical results show that the AL preconditioner pair is insensitive with respect to the mesh size, Rayleigh number, and Prandtl number in terms of GMRES iterations, resulting in a significantly more robust preconditioner pair compared to the NAL pair. Accordingly, the AL pair performs much better than the NAL pair in terms of computational time. For the AL pair, the preconditioner with velocity as the leading variable gives slightly better efficiency than the one with temperature as the leading variable. [ABSTRACT FROM AUTHOR]

Presence of flaws or cracks may cause substantial decrease in the strength of a component or structure. This kind of structural damage will accumulate over time, leading to significant decrease in ultimate load carrying capacity of structural members and premature brittle failure. In this paper, a coupled analytical and numerical study is implemented in order to evaluate buckling load of cracked cylindrical shells which is often encountered in the fields of civil engineering, offshore engineering and mechanic engineering. In the first phase of the study, buckling load of cylindrical shells with circumferential crack is investigated by treating the symmetric model as a cracked rectangular beam-column on an elastic foundation. The cracked beam-column is modelled as two beam elements on elastic foundation connected by an equivalent rotational spring and governing characteristic equation is obtained. Finite element models are used to verify analytical results and assess their accuracy, based on which good agreement is observed between analytical and numerical results. Effect of different parameters such as the length of cylinder, radial stiffness of cylinder, crack location and crack severity is studied on the axial load carrying capacity of such members. A simplified equation is proposed in order to calculate buckling load of tall cylindrical shells with a circumferential crack. In the second phase, the buckling load of cylindrical shells with a partial crack is studied using the results of finite element analysis on models with varying crack length. Based on the results of numerical analysis, an empirical equation is proposed to interpolate buckling load of these members between two limiting values, namely the buckling loads of same uncracked cylinder and the cylinder with circumferential crack. [ABSTRACT FROM AUTHOR]

• A mathematical model of tube bundles subjected to two-phase cross-flow and internal flow is developed. • The eigenvalue problem of tube bundles considering the effects of two-phase cross-flow and internal flow were investigated. • The effect of internal flow on mechanism of fluidelastic instability of tube bundles was discussed. This paper investigates the fluidelastic instability of a flexible tube in a rotated triangular tube array subjected to two-phase cross-flow and internal flow. We first established a mathematical model of a flexible tube in the tube bundles conveying flow considering the effect of two-phase cross-flow. Then, the eigenvalue analysis was performed to study the influences of the internal flow on the threshold and mechanism of the fluidelastic instability of the tube system in two-phase cross-flow. The results indicate that the internal flow has a significant effect on the threshold of the fluidelastic instability of the tube system in two-phase cross-flow, and may cause different kinds of instabilities. And, the mechanism of the fluidelastic instability of the tube system subjected to two-phase cross-flow and internal flow is notably affected by the tube boundary condition. [ABSTRACT FROM AUTHOR]

Small signal stability of permanent magnet synchronous generator (PMSG)-based wind turbines connected to the power grid should be studied properly in order to facilitate damping strategy design. In this paper, unified small-signal models for different types of PMSGs are developed to study their small-signal stability. The models are composed of mechanical systems, electrical systems and control systems. A two-mass shaft model for the mechanical system is provided to analyze the dynamic and steady-state behaviors of the wind turbine and generator rotor. Meanwhile, PMSG, converter system and transmission line are separately modeled to build unified small-signal models for three PMSG-based wind turbine generator systems (WTGS). Then, based on unified small-signal models, eigenvalue analysis is conducted to determine the relation between different oscillation modes and state variables through calculating participation factors. With modal analysis, the developed small signal models are able to find out all types of oscillation modes for PMSGs connected to the power grid, which are subsynchronous oscillation (SSO), subsynchronous control interaction (SSCI) and low-frequency oscillation, including frequency and damping of each oscillation mode. Different initial values of the small signal models can influence both frequencies and damping ratios of oscillation modes, which lay basis for further damping strategy study. [ABSTRACT FROM AUTHOR]

• The inverse Lax-Wendroff boundary treatment method is developed for a third-order weighted nonlinear scheme. • The linear stability of the scheme is proved by both the method of eigenvalue analysis and the Gustafsson-Kreiss-Sundströom (GKS) theory. • The validity of the scheme is demonstrated by numerical examples in terms of accuracy and stability. Cartesian grids are often used in applications due to the simplicity of grid generation and the efficiency of discretization algorithms. However, for problems with curved boundaries, grid points are often away from the boundaries, leading to the issue of imposing boundary conditions. Recently, it was shown that the inverse Lax-Wendroff (ILW) boundary treatment is very useful for addressing this issue. While most of the investigations focus on the WENO schemes for hyperbolic conservation laws, we present in this paper a new third-order weighted nonlinear scheme, which is based on the framework of weighted compact nonlinear schemes. We show that the scheme is applicable directly on Cartesian grids by using the ILW boundary treatment. To demonstrate the linear stability of the scheme, both the method of eigenvalue analysis and the Gustafsson-Kreiss-Sundström (GKS) theory are employed and analyzed in details. Some numerical tests are also performed to show the accuracy and validity of the proposed method. [ABSTRACT FROM AUTHOR]

The ever-increasing penetration of wind power integration into a power system can produce significant impacts on the operation of an interconnected power system. As the major energy conversion technology for large wind turbines, the doubly fed induction generator (DFIG) will play an important role in future power systems. Hence, the impacts of the DFIG on the low-frequency oscillations of interconnected power systems have become an important issue with extensive concerns. This paper examines the impacts of several factors, including the DFIG transmission distance, tie-line power of the interconnected system, DFIG capacity, with/without a power system stabilizer (PSS), on the low frequency oscillation characteristic of an interconnected power system using both eigenvalue analysis and dynamic simulations. To investigate the effects of these factors on the interarea oscillation mode, case studies are carried out on two two-area interconnected power systems, and some conclusions are obtained. [ABSTRACT FROM AUTHOR]

In this paper, we consider linear stability issues for one-dimensional hyperbolic conservation laws using a class of conservative high order upwind-biased finite difference schemes, which is a prototype for the weighted essentially non-oscillatory (WENO) schemes, for initial–boundary value problems (IBVP). The inflow boundary is treated by the so-called inverse Lax–Wendroff (ILW) or simplified inverse Lax–Wendroff (SILW) procedure, and the outflow boundary is treated by the classical high order extrapolation. A third order total variation diminishing (TVD) Runge–Kutta time discretization is used in the fully discrete case. Both GKS (Gustafsson, Kreiss and Sundström) and eigenvalue analyses are performed for both semi-discrete and fully discrete schemes. The two different analysis techniques yield consistent results. Numerical tests are performed to demonstrate the stability results predicted by the analysis. [ABSTRACT FROM AUTHOR]

An experimental double multi-modal flutter instability was recorded during wind tunnel tests on a full bridge aero elastic model. The analysis of this instability is presented in this paper, and it is studied with a multi-modal eigenvalue approach. It is shown that two different coupling mechanisms, one involving the symmetric modes and the other involving the anti-symmetric modes, lead to two different instabilities that occur at the same wind speed. The numerical analysis shows that a multi-modal framework and a complete set of aerodynamic coefficients are necessary to correctly estimate the flutter onset. The effect on flutter of structural and aerodynamic characteristics is discussed thoroughly, with a focus on structural modes and on the dependence of the aerodynamic coefficients upon the reduced velocity and upon the angle of attack. [ABSTRACT FROM AUTHOR]

This paper illustrates a formulation of a continuous state space model in dq frame of the Thyristor Controlled Reactor (TCR), which can be used for stability studies by eigenvalue analysis. Differently from the previous approaches, this model is just addressed to reproduce the operation of the non-linear part of the TCR system, without considering shunted capacitor or control system, thus obtaining a characterization of the TCR independent from the application. A typical problem of the continuous models in dq frame of the TCR system is the presence of an un-damped homogenous current component in the solution, which is inconsistent with the real operation; the proposed model solves this problem improving the accuracy. The model performance has been also evaluated in the frequency domain comparing the obtained results with a numerical simulation of the TCR implemented by PSIM program. [ABSTRACT FROM AUTHOR]

• 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 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]

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]

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]

Crevasse patterns are the writings in a glacier''s history book—the movement, strain and deformation frozen in ice. Therefore by analysis of crevasse patterns we can learn about the ice-dynamic processes which the glacier has experienced. Direct measurement of ice movement and deformation is time-consuming and costly, in particular for large glaciers; typically, observations are lacking when sudden changes occur. Analysis of crevasse patterns provides a means to reconstruct past and ongoing deformation processes mathematically. This is especially important for fast-moving ice.Ice movement and deformation are commonly described and analyzed using continuum mechanics and measurements of ice velocities or strain rates. Here, we present a different approach to the study of ice deformation based on principles of structural geology. Fast ice movement manifests itself in the occurrence of crevasses. Because crevasses remain after the deformation event and may be transported, overprinted or closed, their analysis based on aerial videography and photography or satellite data gives information on past deformation events and resulting strain states. In our treatment, we distinguish (A) continuously fast-moving glaciers and ice streams, and (B) surge-type glaciers, based on observations of two prototypes, Jakobshavns Isbræ, Greenland, for (A), and Bering Glacier, Alaska, during the 1993–1995 surge, for (B).Classes of ice-deformation types are derived from aerial images of ice surfaces using structural geology, i.e. structural glaciology. For each type, the deformation gradient matrix is formed. Relationships between invariants used in structural geology and continuum mechanics and the singular value decomposition are established and applied to ice-surface classification.Deformation during a surge is mostly one of the extensional deformation types. Continuously, or infinitesimally repeated, deformation acting in continuously fast-moving ice causes different typical crevasse patterns. The structural-geology approach also includes a way to treat the problem of shear, as observed in the margins of fast-moving ice streams within slow-moving surrounding ice. In this paper we provide the first link between a physical analysis of ice-surface deformation and a connectionist-geostatistical analysis of the same problem. [Copyright &y& Elsevier]

A research programme into both the static and dynamic performance of ground anchorage systems, with an emphasis on resin bonded rock bolts, started at the University of Aberdeen in the early 1980s. The work involved measurements on active construction sites which was underpinned by laboratory and computer modelling and led to the development of a new method for the non-destructive testing of anchorages. Part of the research programme was focused on assessing how changes in the load influence the dynamic responses of an anchorage. This in turn produced a new testing method able to determine the variation in frequency response of the anchorage with changes in load. The development of a lumped parameter model able to simulate the response of anchorages to changes in static load and an applied impulse load was a further step forward in the research programme. The achievements of the lumped parameter model revealed the head of the anchorage to be the most influential component of the anchorage system in determining dynamic response. The results obtained from the numerical model when laboratory anchorages were simulated are shown to be in agreement with the results obtained from the actual laboratory tests. Based on the successful results obtained to date regarding laboratory rock bolt anchorages, a further step was made in order to employ the numerical model for the first time to observe the influence of load on the frequency response of rock bolt anchorages installed in the field. This paper describes the application of the model to such field anchorages and highlights some of the detailed modelling aspects required to replicate real anchorage behaviour. [Copyright &y& Elsevier]

Increasing amounts of wind turbines are connected to electrical power systems. This affects many aspects of their operation and behaviour. In this paper, the impact of large scale wind power generation on power system oscillations is treated. The three main types of power system oscillations, namely oscillations of a group of generators against a strong system and intra- and inter-area oscillations are studied. To this end, test systems are used. The effect of wind power on the oscillations is investigated by gradually replacing the power generated by the synchronous generators in the system by power from either constant or variable speed wind turbines, while observing the movement of the eigenvalues through the complex plane. From the results, it is concluded that the effect of wind power on power system oscillations depends on the wind turbine concept (constant or variable speed) and on the wind power penetration. In most cases, an increase in frequency and damping of power system oscillations was observed. This observation is caused by a relative reduction in size of the synchronous generators that engage in power system oscillations, which is equivalent to strengthening their mutual coupling. Further, it was shown that constant speed wind turbines damp power system oscillations more than variable speed turbines. This is caused by the damping effect of the squirrel cage induction generator used in constant speed wind turbines. [Copyright &y& Elsevier]