Multiple Oscillation Modes Research Articles

Small signal stability analysis of a wind power system integrated with a multi-band supplementary damping controller

Abstract With the integration of wind turbines represented by doubly-fed induction generators (DFIG) into the power system, the inertia level of the system decreases sharply, and small disturbances can easily cause low-frequency oscillation (LFO) in the system. In this paper, we propose a multi-band supplementary damping controller (MBSDC) directly applied to the active power control link of the rotor-side converter in the wind power generation system. We establish a linearized model for DFIG integrated with the power system, calculate eigenvalues, and screen out multiple electromechanical oscillation modes to be suppressed. Based on the calculation results, an MBSDC is designed to suppress LFO, and we propose a parameter-setting algorithm to enhance the suppression effect. Taking the New England 10-generator 39-bus system containing DFIG as an example for simulation verification, the results show that the MBSDC and parameter setting algorithm designed in this paper can effectively suppress LFO.

Refined composite hierarchical bidirectional diversity entropy and its application in fault type diagnosis and fault degree diagnosis of diesel engine fuel injectors

The performance of a diesel engine is closely related to the injector, and the fuel pressure shock signal for monitoring the state of the injector always contains multiple natural oscillation modes at different levels. Therefore, it is very necessary to detect the dynamic changes of the injector signal from multiple levels. Multi-scale Diversity Entropy (MDE) is a recent commonly used method of information entropy. However, due to its inherent defects, it limits the wide application of MDE in fault feature extraction. Therefore, this paper proposes a new nonlinear dynamics method, called the Refined Composite Hierarchical Bidirectional Diversity Entropy (RCHBDE). The ability of RCHBDE to extract signal complexity changes was verified through simulation signals, which is superior to MDE and multi-scale bidirectional diversity entropy (MBDE). On this basis, this article proposes a diagnostic framework for fuel injector fault type and degree based on RCHBDE and Seagull optimized Support Vector Machine. We apply the proposed method to the injector failure test data. The results show that RCHBDE can better reflect the dynamic change process of the injector fault signal than MDE and MBDE. Moreover, compared with existing methods, the proposed method in this paper improves the accuracy and efficiency of fault diagnosis.

A grid-side multi-modal adaptive damping control of super-/sub-synchronous oscillations in type-4 wind farms connected to weak AC grid

• A multi-modal damping control (MADC) is proposed to damp multiple oscillation modes independently. • A general design framework for damping multiple oscillation modes is given. • MADC is validated to damp sub-synchronous and super-synchronous oscillations in Type-4 wind farms. Type-4 wind farms connected to weak AC grids have the risk of sub-synchronous oscillation/interaction (SSO/SSI) with multiple oscillation modes, which can lead to unplanned outages and system destabilization. This paper first presents the magnitude and frequency characteristics of the oscillations followed by the control design requirements. Then, it gives a general design of grid-side multi-modal adaptive damping control (MADC) to stabilize multiple oscillation modes. The MADC utilizes an FFT-based inter-harmonic phasor measurement algorithm, which can detect and track multiple oscillation modes simultaneously. The MADC uses bus voltages and line currents as inputs to extract and control each oscillation mode without considering the coupling of the modes due to the frequency coupling effect. The unstable modes are stabilized by injecting phase-shifted currents with multiple frequencies such that the impedance response of the whole system around the concerned frequencies is reshaped. Notable features of the proposed damping control include automatic and independent detection, tracking and suppression of multiple oscillation modes. The proposed MADC's damping performance is validated through extensive electromagnetic transient (EMT) simulations of type-4 wind farms connected to a weak ac grid causing super-/sub-synchronous oscillations. The system model and the proposed damping control are designed and tested in commercial EMT software.

Using Empirical Modal Decomposition to Improve the Daily Milk Yield Prediction of Cows

In this study, the daily lactation data of Holstein dairy cows in one lactation period (305 days) were used as lactation time series data. Empirical mode decomposition (EMD) was used to decompose milk yield series. The nonstationary milk yield series with multiple oscillation modes was decomposed into several components. After eliminating the interference components, the interference components were superimposed. Remaining component reconstruction was used to get the denoising milk yield series. The denoising milk yield series retained the basic characteristics of the original milk yield series and corrected the errors of the original data. The back propagation neural network (BPNN) was used to predict and compare the original milk yield series and the denoising milk yield series. The results showed that it was feasible to use EMD to smooth the original daily milk production data. The noise reduction milk production series was beneficial to the learning of prediction model and could improve the accuracy of prediction of daily milk production of dairy cows.

Oscillation Characteristics of an Artificial Cochlear Sensory Epithelium Optimized for a Micrometer-Scale Curved Structure.

Based on the modern microelectromechanical systems technology, we present a revolutionary miniaturized artificial cochlear sensory epithelium for future implantation tests on guinea pigs. The device was curved to fit the spiral structure of the cochlea and miniaturized to a maximum dimension of <1 mm to be implanted in the cochlea. First, the effect of the curved configuration on the oscillation characteristics of a trapezoidal membrane was evaluated using the relatively larger devices, which had a trapezoidal and a comparable curved shape designed for high-precision in vitro measurements. Both experimental and numerical analyses were used to determine the resonance frequencies and positions, and multiple oscillation modes were clearly observed. Because the maximum oscillation amplitude positions, i.e., the resonance positions, differed depending on the resonance frequencies in both trapezoidal and curved membrane devices, the sound frequency was determined based on the resonance position, thus reproducing the frequency selectivity of the basilar membrane in the organ of Corti. Furthermore, the resonance frequencies and positions of these two devices with different configurations were determined to be quantitatively consistent and similar in terms of mechanical dynamics. This result shows that despite a curved angle of 50–60°, the effect of the curved shape on oscillation characteristics was negligible. Second, the nanometer-scale oscillation of the miniaturized device was successfully measured, and the local resonance frequency in air was varied from 157 to 277 kHz using an experimental system that could measure the amplitude distribution in a two-dimensional (2D) plane with a high accuracy and reproducibility at a high speed. The miniaturized device developed in this study was shown to have frequency selectivity, and when the device was implanted in the cochlea, it was expected to discriminate frequencies in the same manner as the basilar membrane in the biological system. This study established methods for fabricating and evaluating the miniaturized device, and the proposed miniaturized device in a curved shape demonstrated the feasibility of next-generation cochlear implants.

Impact of Random Grain Structure on Spin-Hall Nano-Oscillator Modal Stability

Spin-Hall nano-oscillators are a promising class of microwave spintronic devices with potential applications in RF/microwave communication and neuromorphic computing. The nano-constriction spin-Hall nano-oscillators (NC-SHNO) have relatively high power, narrow linewidth, and low drive current. Several synchronization schemes e.g. arrays of spin-wave coupled oscillators have been proposed for more stable operation and higher output power. For such arrays, it is crucial to have good oscillator stability and small device-to-device variability. Here, a micromagnetic simulation technique is proposed that includes realistic material properties and hence enables variability and modal stability to be investigated. It is demonstrated, using both measurements and simulation, that the presence of physical grains in the free magnetic layer can induce multiple oscillation modes or frequency sidebands. Our investigation could help in the development of more stable NC-SHNOs that would enable oscillator arrays with stronger synchronization.

AI-Based Damping of Electromechanical Oscillations by Using Grid-Connected Converter

The proliferation of grid-connected converter interfaced energy sources in Smart Grids, enhance sustainability and efficiency as well as minimizing power losses and costs. However, concerns arise regarding the stability and reliability of future smart grids due to this wide integration of power electronic devices, which are recognized to affect the dynamic response of the system, especially during disturbances. For instance, apart from the lower damping of existing electromechanical modes, new low-frequency oscillations begin to appear. Yet, the ability of grid-connected converters to provide grid support functionalities can alleviate the aforementioned challenges. Relevant studies show that these functionalities can be enhanced even further, if information regarding the oscillation characteristics are available. Traditional methods for extracting modal information are very well suited for monitoring purposes, however, they pose certain limitations when considered for control applications. Therefore, this paper proposes a multi-band intelligent power oscillation damper (MiPOD) that exploits 1) the inherent characteristics of grid-connected converters to damp multiple power oscillations and 2) the modeling capabilities of Artificial Intelligence (AI) for predicting the frequency of electromechanical oscillations in the system, as operating conditions change. Essentially, the MiPOD integrates the AI model in the control loop of the converter to attenuate multiple modes of oscillation. The proposed controller is validated for different disturbances and randomly generated operating points in the two area system. Specifically, in this case the AI model is a Random Forest ensemble regressor that is developed for tracking two electromechanical modes. As it is shown, the MiPOD can improve the overall performance of the system under various contingency scenarios with only 6% of the corresponding total nominal capacity of synchronous generators. In addition, the monitoring and damping abilities of the MiPOD are demonstrated for a vast range of operating points just by tuning two parameters; the predicted oscillation frequencies of the local and inter-area mode.

Frequency weighted [formula omitted] optimization of multi-mode input shaper

An optimization-based design of a robust multi-mode input shaper is presented with an embedded weighting of the frequency response. By prefixing the delay length in the shaper, the optimization design task is formulated as a standard quadratic programming problem. The shaper robustness in terms of zeroed or upper-bounded residual vibration function for the modes to be precompensated is imposed via the equality and inequality constraints. As the main result, an additional degree of freedom in the shaper design is introduced via parametrization of the quadratic form cost matrix and utilized to modulate the amplitude frequency response over a given frequency range. In particular, this allows to embed the high-frequency roll-off to the shaper and improve its insensitivity to modelling errors by adjusting its stop-band response. The proposed shaper design is demonstrated and experimentally validated on a problem of positioning a flexible manipulator arm with multiple oscillatory modes.

Nonlinear dynamics of a conical dielectric elastomer oscillator with switchable mono to bi-stability

Dielectric elastomer actuators (DEAs) are an emerging type of soft actuator that show many advantages including large actuation strains, high energy density and high theoretical efficiency. Due to the inherent elasticity, such actuators can also be used as soft oscillators and, when at resonance, the dielectric elastomer oscillators (DEOs) can exhibit a peak oscillation amplitude and power output with an improved energy efficiency in comparison to non-resonant behaviour. However, most existing DEOs have a fixed pre-defined morphology and demonstrate a single stable equilibrium, which limits their versatility. In this work, a conical DEO is proposed which may exhibit either monostability (i.e. one stable equilibrium point) or bistability (two equilibria). The system demonstrates a transition between two regimes using a voltage control. Such a feature allows the DEO system to have multiple oscillation modes with different equilibrium points and the transition between equilibria is controlled by an effective control strategy proposed in this work. A mathematical model based on the Euler-Lagrange method is developed to investigate the stability of this system and its complex nonlinear dynamic response in unforced and parametrically forced cases. This design has potential in more advanced and versatile DEO applications such as active vibrational controllers/ shakers, active morphing structures, smart energy harvesting and highly programmable robotic locomotion.

Lava lake sloshing modes during the 2018 Kīlauea Volcano eruption probe magma reservoir storativity

Basaltic volcanoes like Kīlauea, Hawai'i, pose hazards through lava flows and other eruptive activity. Quantifying hazard requires constraining the volume of magma stored in shallow reservoirs, as well as the capacity of those reservoirs to accumulate and discharge magma in response to pressure changes (i.e., storativity). Shallow reservoirs are often connected through magmatic conduits to lava lakes, which are frequently perturbed by rockfalls, convection, and outgassing. Here we present a novel method to quantify reservoir storativity at Kilauea that exploits the seismically observable oscillatory response of the lake-conduit-reservoir system to perturbations during the May 2018 eruption. Seismic events recorded by the summit broadband seismometer network during the eruption feature multiple oscillatory modes in the very long period (VLP) band. The longest period mode (∼30-40 s, here termed the conduit-reservoir mode) persisted during drainage of the lava lake, indicating its primary association with the underlying conduit-reservoir system. Additional modes, with periods of 10-20 s, vanished during lake drainage and are attributed to lava lake sloshing. The surface displacements of the two longest period sloshing modes are explained as the superimposed response to a near-surface horizontal force along the axes of the crater (from the pressure imbalance during sloshing on the crater walls) and a volume change in a reservoir located at the conduit-reservoir mode source centroid. Both sloshing modes are in the “deep-water” surface gravity wave limit where disturbances are largest near the lake surface, though pressure changes on the top of the conduit at the lake bottom provide the observed excitation of the underlying conduit-reservoir system. Combining magma sloshing dynamics in the crater and a conduit-reservoir oscillation model, we bound the storativity of the reservoir to be greater than 0.4 m3/Pa. This implies a storage volume of a few km3 for an approximately equidimensional (e.g., spherical) reservoir. A smaller volume is possible but requires the presence of other shapes of magma bodies, such as sills or dikes, and/or significant finite source effect. Our work demonstrates the signature of lava lake sloshing modes in the seismic data and the potential of these modes in constraining the structure of the shallow magmatic system.

Research on Positioning Method of Low Frequency Oscillating Source in DFIG-Integrated System With Virtual Inertia Control

Concerning the difficulty in locating the oscillating source of DFIG-integrated system with virtual inertia control, a method to track the low frequency oscillating source based on energy function is proposed in this article. First, the dynamic energy function of DFIG containing PLL and virtual inertia control modules is constructed. On this basis, considering the effects of PLL and virtual inertia, the energy spectrum of synchronous generator and analogical energy spectrum of DFIG are established, and the characteristics of energy spectrums of oscillating-source and non-oscillating-source generators in the dominant oscillation mode are revealed. And then, according to the characteristic of energy spectrum, the similarity coefficients between energy spectrums of different generators in the frequency band in the vicinity of the dominant oscillation mode are calculated. Thus by comparing the similarity coefficients, system oscillating source in the dominant oscillation mode can be located. Finally, hardware-in-the-loop tests are conducted in real time digital simulator (RTDS) based on the IEEE 10-machine 39-bus system. Simulation results verify that, in a disturbed system with multiple oscillation modes, the proposed method can eliminate the effect of nondominant oscillation mode on oscillating source locating and can realize accurate identification of oscillating-source generators even when the system is in complex operating conditions.

Application of wavelet analysis on transient reflectivity in ultra-thin films.

Applications of wavelet analysis in ultra-thin film transient reflectivity (TR) measurements have been investigated. Advantages of utilizing different localized wavelet bases, in position and time, have been addressed on the residual TR signals. Morse wavelets have been used to obtain information from the abrupt oscillatory modes in the signal, which are not distinguishable with conventional methods such as Fourier transforms. These abrupt oscillatory modes are caused by the surface, interface, or any short-lived oscillatory modes which are suppressed in the TR signal in ultra-thin films. It is demonstrated that by choosing different Morse wavelets, information regarding different oscillatory modes in the TR signal of a heterostructure thin film is achievable. Moreover, by performing wavelet analysis on multiferroic heterostructures, oscillatory modes with very close energy ranges are easily distinguishable. For illustration, residuals of the TR signals have been obtained by a pump-probe setup in reflectivity mode on La0.7Sr0.3MnO3/SrTiO3 and BaTiO3/La0.7Sr0.3MnO3/SrTiO3 samples, where sufficient signal to noise ratios have been achieved by taking multiple scans. The residual signals have been analyzed with Morse wavelets, and multiple oscillatory modes with close energy ranges have been observed and distinguished. This approach can isolate the location of various oscillatory modes at the surface, interface and in the bulk of the heterostructure sample.

High power folded waveguide traveling wave tube based on variable-width technology

Variable-width technology applied to the folded waveguide traveling wave tube (FW-TWT) is proposed in this paper to suppress the lower band-edge oscillation and to expand the operating bandwidth when the operating frequency shifts toward the lower band for obtaining larger output power. Changes in the width of the slow wave structure (SWS) determine the variation of the lower-cutoff-frequency (LCF), which makes the LCF's distribution a frequency range rather than a specific frequency-point. The lower band-edge oscillation will not be focused on a special position but distributed in a frequency-band range in which multiple oscillation modes compete with each other near the LCF. The lower band-edge oscillation can be suppressed by mode competition to some extent. In addition, the width variation of SWS also results in the phase-velocity variation, and thus, the operating bandwidth of FW-TWT can be further improved by the optimized design of variable-width-SWS parameters. Beam-wave interaction simulations predicted that power and bandwidth improvements have been achieved without the lower band-edge oscillation. Meanwhile, four solutions are given for further certifying the effectiveness of the variable-width method in lower band-edge oscillation suppression and bandwidth widening. An output power level of over 350 W in a frequency range of 85–90 GHz (5 GHz) is obtained, with a maximum power of ∼648 W and an electron efficiency of ∼12.1% at 88 GHz with a beam current of 260 mA and a beam voltage of 20.5 kV.

The application of modal analysis to the design of multi-mode point absorber wave energy converters

Point absorbers are one of the most common wave energy converters, which are typically designed to extract power primarily from heave motion. Pure heave motion of an axisymmetric wave energy converter results in a relative capture width limited to a third of the maximum possible. Over the past few decades, an increasing amount of attention has been given to the design of point absorbing wave energy converters operating in multiple oscillation modes, in an attempt to more efficiently extract power from waves. However, it is not a trivial task as wave energy converters operating in multiple modes demonstrate complex coupled vibrational characteristics across several degrees of freedom. This paper addresses this challenge using modal analysis, a modern approach developed for determining, improving and optimising dynamic characteristics of complex engineering systems. Case studies are conducted on three multi-mode submerged point absorber designs, each with distinct modal behaviour to show the generality and efficacy of the approach. Results show that in combination with knowledge of wave power absorption in the frequency-domain, modal analysis can be used as an effective analytical tool to evaluate the vibrational characteristics and the power absorption potential of the multi-mode system, as well as to explore the corresponding working principles and the physical limits of the design.

Improved oscillation detection via noise-assisted data analysis

Oscillation detection is usually a precursor to more advanced performance monitoring steps such as plant wide oscillation detection and root cause detection. Therefore any false or missed detection can have serious implications. Oscillation detection is a challenging problem due to the presence of noise and multiple modes in the plant data. This paper presents an improved and robust automatic oscillation detection algorithm based on noise-assisted data analysis that can handle multiple oscillatory modes in the presence of both coloured and white noise along with non-stationary effects. The dyadic filter bank property of multivariate empirical mode decomposition has been used to accurately detect the oscillations and to calculate the associated characteristics. This work improves upon the existing auto covariance function based methods. The robustness and reliability of the proposed scheme is demonstrated via simulation and industrial case studies.

Gap-junction coupling can prolong beta-cell burst period by an order of magnitude via phantom bursting.

Pancreatic β-cells show multiple intrinsic modes of oscillation with bursting electrical activity playing a crucial role. Bursting is seen both in experimentally isolated β-cells as well as in electrically coupled cells in the pancreatic islets, but the burst period is typically an order of magnitude greater in coupled cells. This difference has previously been attributed to noisier dynamics, or perturbed electrophysiological properties, in isolated β-cells. Here, we show that diffusive coupling alone can extend the period more than ten-fold in bursting oscillators modeled with a so-called phantom burster model and analyze this result with slow-fast bifurcation analysis of an electrically coupled pair of cells. Our results should be applicable to other scenarios where coupling of bursting units, e.g., neurons, may increase the oscillation period drastically.

Robustness, flexibility, and sensitivity in a multifunctional motor control model

Motor systems must adapt to perturbations and changing conditions both within and outside the body. We refer to the ability of a system to maintain performance despite perturbations as “robustness,” and the ability of a system to deploy alternative strategies that improve fitness as “flexibility.” Different classes of pattern-generating circuits yield dynamics with differential sensitivities to perturbations and parameter variation. Depending on the task and the type of perturbation, high sensitivity can either facilitate or hinder robustness and flexibility. Here we explore the role of multiple coexisting oscillatory modes and sensory feedback in allowing multiphasic motor pattern generation to be both robust and flexible. As a concrete example, we focus on a nominal neuromechanical model of triphasic motor patterns in the feeding apparatus of the marine mollusk Aplysia californica. We find that the model can operate within two distinct oscillatory modes and that the system exhibits bistability between the two. In the “heteroclinic mode,” higher sensitivity makes the system more robust to changing mechanical loads, but less robust to internal parameter variations. In the “limit cycle mode,” lower sensitivity makes the system more robust to changes in internal parameter values, but less robust to changes in mechanical load. Finally, we show that overall performance on a variable feeding task is improved when the system can flexibly transition between oscillatory modes in response to the changing demands of the task. Thus, our results suggest that the interplay of sensory feedback and multiple oscillatory modes can allow motor systems to be both robust and flexible in a variable environment.

Tuned LC-Resonator Dual-Band VCO

By exploiting the intrinsic multiple oscillation modes of LC resonator with cross-coupled transistor pair, a differential VCO is designed for multi-standard RF transceivers. The series resistance introduced by transistor switches has been omitted to improve the quality factor of resonator. It operates at the high-frequency series-resonant and low-frequency parallel-resonant mode via varactor switching bias. Both frequencies are not harmonically related. Accumulation-mode MOS varactor tuning changes the net impedance of LC-tank to shift the oscillations from one resonance mode to another.

Coordinated Design of Multiple Robust FACTS Damping Controllers: A BMI-Based Sequential Approach With Multi-Model Systems

This paper presents the coordinated design of multiple FACTS supplementary damping controllers to improve system small-signal stability. A BMI-based multi-objective multi-model system approach is formulated to solve the robust damping control problem. Two-step method is proposed to determine controller variables. Regional pole placement and control effort optimization are set as the control objectives. An SVC and a TCSC damping controller are designed sequentially with minimized decoupling effort. Selection of the feedback signals for the FACTS damping controllers is realized by evaluating the modal residues of each feedback signal to the system input. To cover multiple inter-area oscillation modes, wide-area decentralized control methodology is proposed and remote feedback signals are selected. Real-time simulation is carried out on real-time digital simulator (RTDS) to test the performance of the proposed controller. The results show that the FACTS damping controllers are feasible and able to increase system damping to a satisfactory level under different system operating points.

Dual-band rotary standing-wave voltage-controlled oscillator

In this article, a dual-band rotary standing-wave oscillator (RSWO) is introduced that generates sinusoidal signals by the formation of a standing wave on a ring (closed-loop)-distributed composite right/left-handed (CRL) Inductor-Capacitor (LC) transmission line network. The LC network consists of four unit cells of CRL LC resonator stacked in series, and two pairs of cross-coupled transistors are used to compensate for the loss of LC resonator. Varactors are used as the control to switch on/off the high- or low-frequency bands. In the fundamental mode, the RSWO operates at the high-frequency band. In the harmonic mode, the oscillator provides low-frequency band outputs. The dual-band function exploits the multiple oscillation modes of the CRL RSWO. The proposed RSWO has been implemented with the Taiwan Semiconductor Manufacturing Company, Limited (TSMC) 0.18-μm SiGe BiCMOS technology. It can generate differential signals in the high-band frequency range of 6.73–8.60 GHz and in the low-band frequency range of 3.68–3.73 GHz. The die area of the RSWO is 1.123 × 1.123 mm2. © 2014 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:536–543, 2014.