Local Oscillation Modes Research Articles

Nonlinear pulse propagation phenomena in ion-doped dielectric crystals

We theoretically analyze pulse propagation in a medium of inhomogeneously broadened two-level quantum systems, which have a vibrational degree of freedom with respect to the center-of-mass coordinate. This system mimics local mode oscillations of rare-earth-metal-ion dopants in dielectric crystals that are coupled to electronic transitions. We show the emergence of various nonlinear optical phenomena, such as self-induced transparency or the nonlinear interaction between two pulses coupling to different electrovibrational transitions. Interaction between the pulses makes it possible to generate various Raman sidebands of the incident fields and to tune the location where they are generated. We also demonstrate controlled population transfer between electrovibrational states of the ions at specific points along the propagation axis. Similarities and differences between our results and other pulse propagation phenomena of few-level quantum systems are discussed.

Techniques for the Analysis and Optimization of Electromechanical Stability towards Local and Inter-Area Oscillation Modes

This paper investigates the problem of electromechanical stability related to the dynamic interaction between the turbine-alternators and the network. In particular, this paper refers to a software package, named ALICE and developed by CESI and Terna in MATLAB®, which makes available within an unique environment features for linearized analysis of electromechanical cycle and multivariable optimization of power system stabilizers. By using of ALICE, it is possible to make stability analysis of the linearized system, both in local and inter-area frequency interval, and, above all, to take out, thanks to different automatic optimization techniques, the most appropriate calibration of the gains of additional stabilizing feedbacks in order to achieve an adequate damping of electromechanical oscillations.

Multi-machine power system stabilizer adjustment using Simulated Annealing

In this paper, the authors are willing to find the optimal and robust design for power system stabilizers (PSSs) in a multi-machine power system. In this regard a new Meta heuristic method as Simulated Annealing (SA) is used. The PSS parameters are computed to assure maximum damping performance under different scenarios. A multi machine electric power system is used to illustrative the performance of proposed method. The efficacy of this technique in damping local and inter-area modes of oscillations in multimachine power systems is confirmed through nonlinear simulation results. Simulation results are carried out by numerical simulations on MATLAB software. Keywords: Multi Machine, Power System Stabilizer, Low Frequency Oscillations, Simulated Annealing

ROBUST DESIGN OF POWER SYSTEM STABILIZERS USING AN OPTIMAL ALGORITHM

A robust design of power system stabilizers using an optimal algorithm for multi-machine power systems is presented in this paper. Based on the proposed method the widely applied conventional power system stabilizers (CPSS) in a multi-machine system is designed in two stages. In the first stage, the time constants of CPSSs are tuned using the physical concept based phase compensation technique based on the frequency response of the GEP(s). In the second stage, the gains of the CPSSs are simultaneously tuned by converting it to an optimization problem which is solved simultaneously by solving a special form of robust pole assignment problem using a constrained optimal algorithm. The effectiveness of the proposed two-stage method in damping local and inter-area modes of oscillations over wide range operating conditions for a four-machine power system is confirmed through eigenvalue analysis and time domain simulation results.

Low-order robust power system stabilizer for single-machine systems: An LMI approach

This paper describes a linear matrix inequality (LMI) approach to the design of robust loworder power system stabilizers (PSSs), which are used to damp out local-mode oscillations of synchronous generators. The performance of a PSS is expressed as the location of the closed-loop poles, and a single fixed-gain pole-placement controller is synthesized for a wide range of operating conditions. The synthesis results in simultaneous regional pole-placement stabilization through static output feedback, and is formulated as a novel LMI feasibility problem with a rank condition. A penalty method is applied to solve the rank-constrained LMI problem. Numerical experiments with a single machine connected to an infinite bus system were performed to demonstrate the proposed LMI method, and the results were compared with those of previous work.

Collective properties of water confined in carbon nanotubes: A computer simulation study

The collective properties of water confined in the (10,10), (8,8) and (6,6) carbon nanotubes are studied by analysing the longitudinal-current autocorrelation function, calculated from computer-simulated trajectories. The corresponding spectra clearly show the presence of two excitations, but their behaviour is quite different from that observed in the case of bulk water. Instead of the strong positive dispersion of the hydrodynamic sound mode characteristic of bulk water (the fast-sound phenomenon), the sound dispersion relation of confined water is observed to flatten into a non-propagating mode, while a second excitation appears at a higher frequency. This behaviour is analysed in terms of the localized oscillation modes of the hydrogen-bond network.

Spin-torque driven magnetization dynamics in a nanocontact setup for low external fields: Numerical simulation study

We present numerical simulation studies of the steady-state magnetization dynamics driven by a spin-polarized current in a point contact geometry for the case of a relatively large contact diameter (D = 80 nm) and small external field (H = 30 Oe). We show, that under these conditions the magnetization dynamics is qualitatively different from the dynamics observed for small contacts in large external fields. In particular, the 'bullet' mode with a homogeneous mode core, which was the dominating localized mode for small contacts, is not found here. Instead, all localized oscillation modes observed in simulations correspond to different motion kinds of vortex-antivortex (V-AV) pairs. These kinds include rotational and translational motion of pairs with the V-AV distance d ~ D and creation/annihilation of much smaller (satellite) V-AV pairs. We also show that for the geometry studied here the Oersted field has a qualitative effect on the magnetization dynamics of a 'free' layer. This effect offers a possibility to control magnetization dynamics by a suitable electric contact setup, optimized to produce a desired Oersted field. Finally, we demonstrate that when the magnetization dynamics of the 'fixed' layer (induced only by the stray field interaction with the 'free' layer) is taken into account, the threshold current for the oscillation onset is drastically reduced and new types of localized modes appear. In conclusion, we show that our simulations reproduce semiquantitatively several important features of the magnetization dynamics in a point contact system for low external fields reported experimentally.

Magnetization switching driven by spin-transfer-torque in high-TMR magnetic tunnel junctions

This paper presents a numerical study of magnetization switching driven by spin-polarized current in high-TMR magnetic tunnel junctions (TMR>100%). The current density distribution throughout the free-layer is computed dynamically, by modeling the ferromagnet/insulator/ferromagnet trilayer as a series of parallel resistances. The validity of the main hypothesis, which states that the current flows perpendicular to the sample plane, has been verified by numerically solving the Poisson equation. Our results show that the nonuniform current density distribution is a source of asymmetry to the switching process. Furthermore, we observe that the reversal mechanisms are characterized by well-defined localized pre-switching oscillation modes.

Thermal conductivity and specific heat of bulk amorphous chalcogenides Ge20Te80−xSex (x=0,1,2,8)

Abstract Bulk amorphous chalcogenide samples of Ge 20 Te 80− x Se x ( x = 0, 1, 2, 8) have been prepared using a melting-quench method, and characterized by the differential scanning calorimetry, X-ray powder diffraction, high-resolution transmission electron microscopy, specific heat and thermal conductivity measurements. The low temperature specific heat measurements identified some localized low-frequency oscillation modes (Einstein modes) in conjunction with a Debye-like behavior. It was found that with increasing Se concentration the characteristic Debye temperature did not change whereas the Einstein temperature slightly decreased. The lattice thermal conductivity of all Ge 20 Te 80− x Se x samples exhibited typical amorphous heat conduction behavior, which has been discussed in connection with the phonon mean free path and in the context of a phenomenological model of heat conduction for highly disordered system.

Optimal locations and tuning of robust power system stabilizer using genetic algorithms

Optimal locations and design of robust multimachine power system stabilizers (PSSs) using genetic algorithms (GA) is presented in this paper. The PSS parameters and locations are computed to assure maximum damping performance under different operating conditions. The efficacy of this technique in damping local and inter-area modes of oscillations in multimachine power systems is confirmed through nonlinear simulation results and eigenvalues analysis.

Magnetization oscillations induced by a spin-polarized current in a point-contact geometry: Mode hopping and nonlinear damping effects

In this paper, we study magnetization excitations induced in a thin extended film by a spin-polarized dc current injected through a point contact in the current-perpendicular-to-plane geometry. Using full-scale micromagnetic simulations, we demonstrate that in addition to the oscillations of the propagating wave type, there exist also two localized oscillation modes. The first localized mode has a relatively homogeneous magnetization structure of its core and corresponds to the so-called ``bullet'' predicted analytically by Slavin and Tiberkevich [Phys. Rev. Lett. 95, 237201 (2005)]. Magnetization pattern of the second localized mode core is highly inhomogeneous, leading to a much lower power of magnetoresistance oscillations caused by this mode. We have also studied the influence of a nonlinear damping for this system and have found the following main qualitative effects: (i) the appearance of frequency jumps within the existence region of the propagating wave mode and (ii) the narrowing of the current region where the bullet mode exists, until this mode completely disappears for a sufficiently strong nonlinear damping.

Optimal location and controller design of STATCOM for power system stability improvement using PSO

The optimal location of a static synchronous compensator (STATCOM) and its coordinated design with power system stabilizers (PSSs) for power system stability improvement are presented in this paper. First, the location of STATCOM to improve transient stability is formulated as an optimization problem and particle swarm optimization (PSO) is employed to search for its optimal location. Then, coordinated design problem of STATCOM-based controller with multiple PSS is formulated as an optimization problem and optimal controller parameters are obtained using PSO. A two-area test system is used to show the effectiveness of the proposed approach for determining the optimal location and controller parameters for power system stability improvement. The nonlinear simulation results show that optimally located STATCOM improves the transient stability and coordinated design of STATCOM-based controller and PSSs improve greatly the system damping. Finally, the coordinated design problem is extended to a four-machine two-area system and the results show that the inter-area and local modes of oscillations are well damped with the proposed PSO-optimized controllers.

Band gap Green's functions and localized oscillations

We consider some typical continuous and discrete models of structures possessing band gaps, and analyse the localized oscillation modes. General considerations show that such modes can exist at any frequency within the band gap provided an admissible local mass variation is made. In particular, we show that the upper bound of the sinusoidal wave frequency exists in a non-local interaction homogeneous waveguide, and we construct a localized mode existing there at high frequencies. The localized modes are introduced via the Green's functions for the corresponding uniform systems. We construct such functions and, in particular, present asymptotic expressions of the band gap anisotropic Green's function for the two-dimensional square lattice. The emphasis is made on the notion of the depth of band gap and evaluation of the rate of localization of the vibration modes. Detailed analysis of the extremal localization is conducted. In particular, this concerns an algorithm of a ‘neutral’ perturbation where the total mass of a complex central cell is not changed

Multivariable Adaptive Control of Synchronous Machines in a Multimachine Power System

A multivariable self-tuning adaptive control scheme used to enhance power system stability in a multimachine environment is presented. The controller is implemented locally for individual generators with supplementary stabilizing signals through the AVR and governor. A discrete multivariable state space model is developed to represent the generator. The recursive subspace identification method based on the projection approximation subspace tracking approach is employed to update generator model parameters online. A generalized predictive control strategy with constraints on the control signals is used. Simulations studies carried out on a five-machine power system without infinite bus show that the proposed multivariable adaptive controller is effective in damping both local and interarea mode oscillations under small as well as large disturbances. The self-coordinating ability of the adaptive controller with the existing conventional controllers is also demonstrated.

Pulsed squeezed light: Simultaneous squeezing of multiple modes

We analyze the spectral properties of squeezed light produced by means of pulsed, single-pass degenerate parametric down-conversion. The multimode output of this process can be decomposed into characteristic modes undergoing independent squeezing evolution akin to the Schmidt decomposition of the biphoton spectrum. The main features of this decomposition can be understood using a simple analytical model developed in the perturbative regime. In the strong pumping regime, for which the perturbative approach is not valid, we present a numerical analysis, specializing to the case of one-dimensional propagation in a beta-barium borate waveguide. Characterization of the squeezing modes provides us with an insight necessary for optimizing homodyne detection of squeezing. For a weak parametric process, efficient squeezing is found in a broad range of local oscillator modes, whereas the intense generation regime places much more stringent conditions on the local oscillator. We point out that without meeting these conditions, the detected squeezing can actually diminish with the increasing pumping strength, and we expose physical reasons behind this inefficiency.

A FUZZY VARIABLE STRUCTURE CONTROLLER FOR TRANSIENT STABILITY ENHANCEMENT OF FLEXIBLE AC TRANSMISSION SYSTEM

This article presents the design of a new fuzzy variable structure controller for the unified power flow controller (UPFC) in a multimachine power system. This new design provides overall variable proportional and integral gain in damping both inter-area and local modes of oscillations of the synchronous generators in multimachine power systems. The conventional PI controllers that are used to adjust the series converter voltage and phase angle of the UPFC show inferior performance compared to the proposed controller for a variety of transient disturbances covering faults and power changes. The conventional integral control along with a variable structure fuzzy proportional control scheme based on a sliding surface and its derivative provides a superb steady-state performance as well as dynamic performance that damps the system oscillations rapidly. The sliding surface also provides robustness against any uncertainty in the system parameters and disturbance modes.

Analyzing the Influence of Induction Motor Inertia on Power System Low Frequency Oscillation

The influence of inertia constant (H D ) of induction motor on low frequency oscillation (LFO) is studied by integral manifolds theory. The oscillation frequency and the damping ratio are formulated in single-machine infinite bus (SMIB) system. The theoretical analysis shows that H D influences mainly the damping of LFO, the extent of which is related to the system structure and initial operating condition. The eigenvalue analysis of SMIB system and a simple two-area system shows the influence of H D on local and inter-area oscillation modes of LFO. The results of full-order system and reduced-order system by integral manifolds theory are compared.

Generalized Neuron-Based Adaptive PSS for Multimachine Environment

Artificial neural networks can be used as intelligent controllers to control nonlinear, dynamic systems through learning, which can easily accommodate the nonlinearities and time dependencies. Taking advantage of the characteristics of a generalized neuron (GN), that requires much smaller training data and shorter training time, a GN-based adaptive power system stabilizer (GNAPSS) is proposed. It consists of a GN as an identifier, which predicts the plant dynamics one step ahead, and a GN as a controller to damp low frequency oscillations. Results of studies with a GN-based PSS on a five-machine power system show that it can provide good damping of both local and inter-area modes of oscillations over a wide operating range and significantly improve the dynamic performance of the system.

Optimal multiobjective design of robust power system stabilizers using genetic algorithms

Optimal multiobjective design of robust multimachine power system stabilizers (PSSs) using genetic algorithms is presented in this paper. A conventional speed-based lead-lag PSS is used in this work. The multimachine power system operating at various loading conditions and system configurations is treated as a finite set of plants. The stabilizers are tuned to simultaneously shift the lightly damped and undamped electromechanical modes of all plants to a prescribed zone in the s-plane. A multiobjective problem is formulated to optimize a composite set of objective functions comprising the damping factor, and the damping ratio of the lightly damped electromechanical modes. The problem of robustly selecting the parameters of the power system stabilizers is converted to an optimization problem which is solved by a genetic algorithm with the eigenvalue-based multiobjective function. The effectiveness of the suggested technique in damping local and interarea modes of oscillations in multimachine power systems, over a wide range of loading conditions and system configurations, is confirmed through eigenvalue analysis and nonlinear simulation results.

Development of a new adaptive LQG system generator for high‐speed damping control techniques of power system oscillation

AbstractWith the advent of interconnection of large‐scale electric power systems, many new dynamics power system problems have emerged, which include low‐frequency intersystem oscillations and many others. To date, most major generators in trunk electric power systems in Japan are equipped with supplementary excitation control, commonly referred to as the conventional single and two input PSS. However, low‐frequency oscillations still occur. It is difficult for these conventional PSS to improve the additional damping of power system oscillation, because of the hardware and design of fixed PSS control constants from a one‐machine infinite‐bus model. It has therefore become necessary to develop a new adaptive LQG system generator. This paper explains the development of the new adaptive LQG system and the simulation of low‐frequency and local mode oscillation for this new adaptive LQG system. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 142(3): 30–40, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10109