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Aiming at the low-frequency oscillation (LFO) phenomenon in high-speed railways, this paper proposes a feedback linearized virtual inertia control (VIC) strategy of the traction converter based on a sliding mode observer (SMO) to improve the robustness of the system and suppress the occurrence of LFO. First, to optimize the total harmonic distortion of the traction grid-side current, the inner loop controller is designed as the feedback linearization control. Then, the voltage outer loop is designed as a combination of the VIC designed in this paper and SMO, which enhances the system inertia and reduces the impact of external interference on the system, while reducing the controller's demand for electrical power on the DC-side. Furthermore, a small-signal model is established to analyze the stability of the single-phase vehicle-grid system under the proposed control. The influence of the control parameters on stability is discussed in detail. Finally, the comparison of software simulation and hardware-in-the-loop experimental results between traditional PI control, feedback linearization control, feedback+SMO, feedback+VIC and the proposed control method shows that the proposed control has better dynamic and static characteristics, and can effectively inhibit the occurrence of LFO. [ABSTRACT FROM AUTHOR]

Following the widespread implementation of Phasor Measurement Units (PMU) across the power grid, the measurement-based methods are extensively used to track power system oscillations efficiently in real-time. This paper proposes a novel dynamic approach for rapid monitoring and identification of electromechanical oscillation modes using real-time measurement signals. The measurement-based method has been recently improved by the Variational Mode Decomposition (VMD) technique, a nonlinear, nonstationary analysis tool used to estimate low-frequency modes in the power system. However, the random selection of the initial parameter utilized in the conventional VMD process significantly affects its performance and often leads to computational complexities. Thus, a Modified Variational Mode Decomposition (MVMD) method based on Particle Swarm Optimization (PSO) has been proposed in this work. MVMD eliminates unnecessary decomposition modes involved in the conventional VMD process by optimizing parameters using PSO. The analysis of these modes is accomplished by assessing instantaneous modal parameters using the Hilbert transform and spectral analysis. The raised approach is validated using a test signal, IEEE standard 16 machine 68 bus system and real-world PMU data. Simulation results show that the dominant low-frequency oscillatory mode with high noise tolerance can be effectively determined with less computational complexity. [ABSTRACT FROM AUTHOR]

Background: Recent resting-state fMRI studies demonstrated functional dysconnectivity within the central pain matrix in migraineurs. This study aimed to investigate the spatial distribution and amplitude of low-frequency oscillations (LFOs) using fractional amplitude of low-frequency fluctuation (fALFF) analysis in migraine patients without aura, and to examine relationships between regional LFOs and clinical variables.Methods: Resting-state fMRI data were obtained and preprocessed in 44 migraine patients without aura and 31 matched controls. fALFF was computed according to the original method, z-transformed for standardization, and compared between migraineurs and controls. Correlation analysis between regional fALFF and clinical variables was performed in migraineurs as well.Results: Compared with controls, migraineurs had significant fALFF increases in bilateral ventral posteromedial (VPM) thalamus and brainstem encompassing rostral ventromedial medulla (RVM) and trigeminocervical complex (TCC). Regional fALFF values of bilateral VPM thalamus and brainstem positively correlated with disease duration, but not with migraine attack frequency or Migraine Disability Assessment Scale score.Conclusions: We have provided evidence for abnormal LFOs in the brainstem including RVM/TCC and thalamic VPM nucleus in migraine without aura, implicating trigeminothalamic network oscillations in migraine pathophysiology. Our results suggest that enhanced LFO activity may underpin the interictal trigeminothalamic dysrhythmia that could contribute to the impairments of pain transmission and modulation in migraine. Given our finding of increasing fALFF in relation to increasing disease duration, the observed trigeminothalamic dysrhythmia may indicate either an inherent pathology leading to migraine headaches or a consequence of repeated attacks on the brain. [ABSTRACT FROM AUTHOR]

Owing to the increased proportion of new energy power generation, such as wind power and photovoltaics, connected to the island grid, the system powered by the voltage source converter-based HVDC (VSC-HVDC) is prone to oscillate or even lose stability when disrupted. In this study, considering the rapid power compensation (RPC), an active power control strategy has been suggested for the receiving-end converter station of VSC-HVDC that could efficiently suppress the low-frequency oscillation of the island power system. First, the mechanism of VSC-HVDC inhibiting low-frequency oscillation of the island grid is analyzed in this study, and then, it theoretically determines that the damping capacity and inertia level of the rapid power compensation control strategy are stronger than those of conventional droop control and inertia control. Second, the receiving-end converter station switches from the RPC mode to droop control in order to allow the system to have a smooth recovery from the steady-state operation in the later stage of oscillation suppression. Moreover, detailed control logic and state-switching strategies have been designed. Finally, the simulation reflects that the proposed control strategy has a stronger oscillation suppression ability, allowing it to obtain rapid suppression of low-frequency oscillation.

The use of the conventional dual closed-loop control strategy by photovoltaic (PV) plants with grid-connected inverters may weaken the damping of a power system, which may aggravate low-frequency oscillations (LFOs). This influence will become more severe as the penetration of PV plants increases. Therefore, it is necessary to incorporate damping controls into PV plants to suppress LFOs. This paper proposed an additional compound damping control (ACDC) system that combines additional damping control (ADC) for the inverter with ADC-based dynamic power compensation control (DPCC), allowing hybrid energy storage systems (HESSs) to suppress LFOs. First, the feasibility of suppressing low-frequency oscillations in PV plants is demonstrated by the torque method and a small signal model. Then, an additional damping controller is added to the active power control link of the PV inverter to enhance the damping abilities of the system. However, given that the damping performance of PV plants with only ADC is limited by the compensated power, PV plants require devices that can rapidly compensate for the damping power. Therefore, we added the HESS to the DC bus and proposed DPCC. Finally, a three-machine nine-node system for a PV plant was modeled and simulated in the PSCAD platform. The simulation results showed that the proposed control strategy could provide effective damping for interarea oscillation.

Addressing the shortcomings of existing low-frequency oscillation research on electrified railways, which has mainly focused on single-type trains and lacks the accurate modeling of traction inverter systems, in this paper we modeled and analyzed low-frequency oscillations in an electrified railway passenger and freight mixed-operation vehicle–grid system. First, an equivalent model of the DC side of the traction inverter was established, with the inverter system being equivalent to the parallel connection of the load resistance and the current source, and the specific mathematical expression was determined and verified by impedance measurement. Secondly, based on the equivalent model of the DC side of the traction inverter, a small signal model of the vehicle–grid system under the mixed operating conditions of CRH5 and HXD2B considering the inverter system was established. The generalized Nyqusit criterion was used to study the low-frequency oscillation characteristics under mixed transportation conditions. The accuracy of the established model and the correctness of the theoretical analysis were verified based on Matlab/Simulink. Finally, using the dominant pole theory to analyze the low-frequency stability conditions, the relationship between the number of mixed trains and the minimum short-circuit ratio was obtained, and the simulation verification was carried out.

Pulse Train (PT) control is a nonlinear voltage regulation method with favorable characteristics of quick response time and simple structure. In this paper, a PT-based feedback control strategy is proposed for stabilizing the output voltage of two-stage wireless power transfer (WPT) systems. Low-frequency voltage oscillations can be observed in PT controlled front-stage power converters, which significantly degrades the output power quality of the WPT system. To solve this problem, by using feedback variables sampled from both output sides of a power converter and WPT, a capacitor current and output voltage feedback PT (CC&OV-PT) controlled two-stage WPT system is further devised in this paper. A discrete time model of the two-stage WPT system is established, and the performance of low-frequency voltage oscillations suppression for the double-side feedback CC&OV-PT controlled WPT system is analyzed. Simulation and experimental verification have been conducted, both of which show that the CC&OV-PT controlled WPT system achieves fast response with effective low-frequency oscillation suppression.

The virtual synchronous generator (VSG) technique is used to simulate the external characteristics of a synchronous generator (SG) to provide certain damping and inertia to power systems. However, it may easily cause low-frequency oscillation of the power system. We studied the small-signal stability of a grid-connected virtual synchronous generator. Firstly, the small-signal models of single-VSG and multi-VSG grid-connected systems were established. Subsequently, the system eigenvalues were obtained by solving the state matrix, and the system oscillation modes were analyzed. The eigenvalue analysis method was used to analyze the impacts of parameter changes, such as virtual moment of inertia, virtual damping coefficient, line resistance, and line inductance, on system stability. Finally, our conclusions were verified by numerous simulation models.

To better damp out the multi-mode oscillations in an uncertain environment, a novel multi-band power system stabilizer (MBPSS) is proposed. Compared with other MBPSSs, the proposed controller has a well-balanced structure, and each band is designed to address a target low-frequency oscillation (LFO) mode. A deep reinforcement learning-enabled agent is developed to effectively tune the control parameters that are adaptive to system uncertainties and different operating conditions. Comparative results with other types of PSSs on the IEEE 68-bus system demonstrate that the proposed method has better performance of damping out LFO and robustness against unseen operating conditions and faults. [ABSTRACT FROM AUTHOR]

The power transfer capability of the smart transmission gridconnected networks needs to be reduced by inter-area oscillations. Due to the fact that inter-area modes of oscillations detain andmake instability of power transmission networks. This fact is more noticeable in smart grid-connected systems. The smart grid infrastructure has more renewable energy resources installed for its operation. To overcome this problem, a deep learning widearea controller is proposed for real-time parameter control and smart power grid resilience on oscillations inter-area modes. The proposed Deep Wide Area Controller (DWAC) uses the Deep Belief Network (DBN). The network weights are updated based on real-time data from Phasor measurement units. Resilience assessment based on failure probability, financial impact, and time-series data in grid failure management determine the norm H2. To demonstrate the effectiveness of the proposed framework, a time-domain simulation case study based on the IEEE-39 bus system was performed. For a one-channel attack on the test system, the resiliency index increased to 0.962, and inter-area damping ξ was reduced to 0.005. The obtained results validate the proposed deep learning algorithm's efficiency on damping inter-area and local oscillation on the 2-channel attack as well. Results also offer robust management of power system resilience and timely control of the operating conditions. [ABSTRACT FROM AUTHOR]

When the power company is cutting and sending electricity, low frequency oscillation will occur in the power equipment at the harbor, which causes equipments to fail to function properly. This paper proposes an improved NSGA-II to coordinately control the power system, with the power system stabilizer and static reactive power compensator. The classical single machine system is simulated and analyzed in Matlab/Simulink software. The Strength Pareto Evolutionary Algorithm is selected to compare and analyze the different performance with the improved genetic algorithm which proposed in this paper. The results show that the PSS and SVC optimized by NSGA-II can improve the damping ratio of power system and restrain the low-frequency oscillation more effectively. [ABSTRACT FROM AUTHOR]

Aiming at the high-frequency resonance (HFR) and low-frequency oscillation (LFO) phenomena in high-speed railways, a comprehensive suppression method named voltage constant transient active oscillation compensation (VCT-AOC) is proposed. This method is optimized on the basis of AOC, and the adjustment time is shortened by the VCT suppression measure. When serious HFR happens, the system can be quickly restored and stabilized by completely removing the network voltage oscillation in the control part, thereby avoiding the transient overvoltage and overcurrent damage to the system. According to the characteristics of AOC and VCT, the VCT-AOC comprehensive suppression method is proposed, and the specific implementation scheme of this method is given. This paper established the train-network impedance model with AOC and VCT suppression methods. The Generalized Nyquist Analysis (GNA) method and the dominant pole analysis method are used to analyze the effect of VCT and AOC on the system. Finally, the effectiveness of suppression methods is verified by simulations and experiments.

The low-frequency oscillations (LFOs) inherent in power systems will be impacted by the increasing penetration of renewable energy sources (RESs). This paper investigates the impact of virtual synchronous machine (VSM) based RESs on the LFOs in power systems. A detailed two-machine test-bed has been developed to analyze the LFOs which exists when VSMs replace synchronous generators. The characteristics of the LFO modes, and the dominant states have been comprehensively analyzed. Furthermore, this study analyzes the LFO modes which exists in an all-VSM grid. The role of the power system stabilizers in the all-VSM grid has been comprehensively evaluated. The IEEE benchmark two-area four-machine system has been employed to corroborate the results of the small-signal analysis, and observe the transient performance. The analysis in this paper have been performed in MATLAB/SIMULINK environment. [ABSTRACT FROM AUTHOR]

Copyright of Electric Power Automation Equipment / Dianli Zidonghua Shebei is the property of Electric Power Automation Equipment Press and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Objective: it is known that the rectifier side control usually suffers from the time delay and long distance communication problems, which could make the damping controller to be invalid and decrease the system stability. Specifically, when the system is in asynchronous operation, the rectifier controllers cannot damp the oscillations in inverter side AC system. Thus, to design the HVDC damping controller at inverter side becomes a feasible solution to avoid these problems. Methods: Based on the MMC control strategy, to design the damping controllers at inverter side, the mixed H2/H∞ robust method with regional pole placement is investigated for the two MMC inverters in constant active power control based on the master‐slave control mode, where the robustness, control effort and the damping ratios can all be guaranteed simultaneously. During the design process, the simplified system models, namely the TLS‐ESPRIT identification algorithm, is also used for controller study. Finally, two robust controllers are obtained based on the proposed method and the inverter side damping controllers are added at the MMCs of hybrid HVDC. Results: The damping controllers for hybrid HVDC with cascaded multi‐infeed MMCs can be designed at inverter side, which endows the hybrid HVDC with more control functions compared with the classical HVDC systems. The damping controllers can be added at the cascaded multi‐infeed MMC inverters in constant active power control, which can enhance the connected AC systems' small stabilities. Conclusions: Based on the multi‐objective robust theory, the control effectiveness and robustness of the designed controllers can be guaranteed simultaneously, which can damp the low frequency oscillations significantly in different operating situations. [ABSTRACT FROM AUTHOR]

This article proposes a real-time power system stabilizers (PSS) parameter optimization technique employing extreme learning machine (ELM) to enhance overall system stability by damping out the low-frequency oscillations. It models two electric networks, i.e., single machine infinite bus systems where the first network's synchronous machine is equipped with a PSS only, and the second network's synchronous machine is equipped with a unified power flow controller coordinated PSS. It uses diverse loading conditions to investigate the performance of the proposed ELM model-tuned PSS technique and compares it with conventional PSS and the referenced works in terms of the eigenvalues and minimum damping ratios. Additionally, the satisfactory values of the well-known statistical performance indices including the root mean squared error (RMSE), mean absolute percentage error, RMSE-observations-to-standard deviation ratio, coefficient of determination (R2), Willmott's index of agreement, and Nash–Sutcliffe model efficiency coefficient provide confidence in the developed technique in predicting PSS parameters. Besides, comparisons of results from time-domain simulation demonstrate the ELM model tuned system's superiority over the conventional approach for both test cases. Furthermore, the ELM models require less than a cycle to predict PSS parameters for any loading condition that endorses the developed technique's real-time application. [ABSTRACT FROM AUTHOR]

The paper presents a multiaspect analysis of multivalues and the broadband nature of system oscillation. By analyzing the ambient signal caused by random small disturbances during the normal operation of interconnected power grids, many system operation characteristics can be obtained. The traditional signal processing method cannot extract the information from ambient signals effectively. Aiming at the problem of broadband oscillation mode superposition and the difficulty of extracting information from ambient signals, an iterative adaptive variational mode decomposition (IA-VMD) method is proposed based on frequency domain analysis and signal energy. Additionally, the IA-VMD method, combined with a bandpass filter and the Prony algorithm, is used to realize the modal identification of broadband oscillation and ambient signals. Simulation experiments show that the IA-VMD method has good adaptability, antinoise characteristics, and a certain significant engineering application value as well. [ABSTRACT FROM AUTHOR]

This paper proposes a fuzzy-observer-based control algorithm to improve the efficiency of traction dual rectifiers of high-speed trains and address the low-frequency oscillation issue induced by the interaction of the DC-link voltage controller with the traction dual rectifiers circuit parameters. The proposed observer checks for the errors in the system model by estimating the system premise variables, and the fuzzy controller then accurately compensates the estimated variables. The method is deduced from the Takagi-Sugeno fuzzy logic model, which is described by a class of IF-THEN fuzzy systems. Fast computation with high precision is what makes them superior to proportional-integral-derivatives controllers. Their capability to deal with nonlinearity in time-varying systems makes them a suitable choice for traction dual rectifier control. The simulation model is formed in SIMULINK, and the waveforms are checked with a conventional fuzzy controller, predictive control, and conventional proportional-integral control. The proposed method can achieve better performance, such as fast transient, more damping, and small overshoot. Furthermore, the method can maintain stability when the system is subjected to external disturbance. In the end, hardware-in-the-loop experiments are performed to validate the proposed control algorithm. [ABSTRACT FROM AUTHOR]

The wide adoption of a new type of electric trains results in multiple frequency-scale instability problems, such as low-frequency oscillation (LFO), harmonic resonance, and harmonic instability. Among them, LFO has been commonly reported at electric railway depots with multiple-energized electric trains around the whole world, thus attracting widespread research attention. LFOs cause adverse effects on the system stability and protection infrastructure of both the traction network and electric trains. This article presents a comprehensive review of LFOs that consider both academic theoretical analysis and industrial engineering requirements. A unified mathematical model of the traction network and electric train is derived with a view to revealing the mechanism and factors that induce LFOs when these two systems interact. The article also draws deep insights from results of experimental measurements and industrial mitigation schemes already implemented in Chinese railway systems. A variety of mitigation techniques and recommendations based on theoretical principles and practical constraints are presented. [ABSTRACT FROM AUTHOR]

Sogut, E. and Farhadzadeh, A., 2021. Numerical study of longshore variation in beach morphodynamics along Eastern Lake Erie shoreline due to seiche. Journal of Coastal Research, 37(1), 92–103. Coconut Creek (Florida), ISSN 0749-0208. Extreme coastal events and abrupt changes in atmospheric pressure in an enclosed or semi-enclosed basin can trigger low-frequency water surface oscillations known as seiches. This exploratory study numerically quantifies the effects of seiche on morphological changes along the eastern Lake Erie shoreline. The quantification is made by simulating the hydrodynamics of the lake using a coupled circulation and spectral wave model (ADCIRC+SWAN), providing the flow boundary conditions to the two-dimensional nearshore morphodynamic model, XBeach. The process-based XBeach model is used to simulate the nearshore morphological variations under two water-level conditions: the lake's actual water level and the synthetically generated seiche-free water level. The XBeach model is cross-calibrated using the process-based nearshore morphodynamic model, CSHORE, which was extensively validated using lab and field data. It is found that the seiching motions have a contribution of ∼1.5% to the erosion of beaches along the 2-km stretch of the shoreline in eastern Lake Erie shoreline. [ABSTRACT FROM AUTHOR]

The current stage of power system development is characterised by the transformation from local electric power systems to global interconnections. It provides economic benefits, improves the reliability of power system operation and increases the quality level. However, such large interconnected systems have some technical and economical limitations, for instance, long distance transmission, and almost have a weak tie between the electric power systems. One of the main problems of the reliability of such large interconnected systems is low-frequency oscillations, which significantly reduces network capacity and leads to dynamic instability. Therefore, the analysis of the influence of various topologies and regime modes of electric power system to the low-frequency oscillations for the development of control actions and recommendations for their effective damping and reduction is relevant. In this paper, simulation and analysis was done by using of two-area electric power systems and using «EUROSTAG» software. The factors, such as the previous steady-state condition and the structure of the electric power system, affecting the damping properties of electric power system and the impact of the Back-to-Back High-Voltage Direct Current link based on voltage source converters to the damping of low-frequency oscillations were considered. [ABSTRACT FROM AUTHOR]

For larger low-frequency oscillation of pulse train(PT) controlled switching converter under inductor current continuous conduction mode (CCM) which could degrade operation performance of switching converter, a continuous restrictive pulse train (CR-PT) control method was proposed. Operation principle of CR-PT control method and its mechanism of suppressing low frequency oscillation of switching converter were introduced in detail by taking CR-PT controlled Buck converter as an example. Switching mapping model of the CR-PT controlled Buck converter under CCM was established. Output voltage and inductor current of Buck converter were compared under PT control and CR-PT control separately through Matlab simulation and prototype experiment. Research results show that compared with PT control method, CR-PT control method can effectively suppress low-frequency oscillation of switching converter under inductor current CCM through restricting the number of continuous pulses.

The pulse train (PT) controlled switching dc/dc converters possess the benefits of simple structure, fast dynamic response, and good robustness. However, PT controlled converters also give the drawbacks of undesirable low-frequency oscillation phenomenon as operated in continuous conduction mode. In this paper, a single-input dual-output buck converter based on the coupled inductor is newly proposed with the ability to suppress the low-frequency oscillation. The inductor of the conventional buck converter is replaced by the primary side of the coupled inductor, and the secondary side of the coupled inductor connected in series with a diode is paralleled to a capacitor as an equivalent power source for the extra load. By rational allocation of inductance coupling coefficient and the equivalent power supply voltage, low-frequency oscillation of output voltage can be suppressed and the fast response speed is achieved. A hardware prototype of 48 V/16 V converter is built and tested to validate the practicability of the proposed converter.

This paper investigates the high-frequency harmonic instability (HHI) and low-frequency oscillation (LFO) phenomena in high-speed railways, and proposes an active oscillation compensation method to mitigate these HHI and LFO. Such mitigation method introduces the input voltage oscillating components of grid side to the control loop of the grid-side converter, which helps to mitigate the voltage oscillation variables and restore the steady state of the train and traction network (simplified as “train-network”) system. From the perspective of train-network impedance, this paper proposes the train impedance models with and without the mitigation process. The generalized Nyquist analysis method is used to analyze the stability of the system with and without the mitigation. Finally, the mitigation method is verified by simulated and experimental data, to be capable of suppressing both HHI and LFO effectively.

It has become a basic requirement for wind turbines (WTs) to provide frequency regulation and inertia support. The influence of WTs on the low-frequency oscillation (LFO) of the system will change after adopting inertia control methods. This paper intends to investigate and compare in detail the IC effects on LFO characteristics in two systems with different structures. First, the mechanism of inertia control of doubly fed induction generator (DFIG)-based WTs is analyzed. Then, the small-signal analysis method and modal analysis method are used to study the influence of the inertia control on the LFO characteristics based on the two-machine infinite-bus system and the four-machine two-area system, respectively. The difference in impact rules of IC on LFO is compared in detail. Finally, considering that the inertia control might worsen the LFO in some systems, an improved inertia control strategy of DFIG-based WTs is proposed to suppress the LFO. The simulation results demonstrate that, in systems with different structures, the impact rules of the inertia control parameters on LFO are different. With the improved inertia control strategy, DFIG-based WTs can suppress the LFO of the system and provide inertia support for the system.

Summary: Wide‐area damping control is able to improve the stability of inter‐connected power systems in case of low‐frequency oscillations. Nevertheless, the damping effect is closely related to the time delay of wide‐area control loops. The application potential of proportional‐integral‐derivative (PID) damper is explored in wide‐area power system damping control. First, the relative residue index based on dominant oscillation modes is employed to determine a suitable control loop. Then, the PID stability space is calculated numerically based on extended Hermite–Biehler Theorem. Finally, the damping‐robustness balance principle is discussed in the PID parameters selection. The proposed PID damper is tested by supplementing the generator excitation control in the two‐area four‐machine power system and the static var compensator control in the 16‐machine 68‐bus power system. Simulation results confirm that it is feasible to design a PID damper for the control loop with a larger relative residue index. The designed PID damper is able to enhance power system stability by inhibiting low‐frequency oscillations under considerable time delays, and it is robust to the changes of system operation state and time delay. [ABSTRACT FROM AUTHOR]

Copyright of Electric Power Automation Equipment / Dianli Zidonghua Shebei is the property of Electric Power Automation Equipment Press and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Diagnosis of cerebrovascular disease (CVD) at early stages is essential for preventing sequential complications. CVD is often associated with abnormal cerebral microvasculature, which may impact cerebral‐autoregulation (CA). A novel hybrid near‐infrared diffuse optical instrument and a finger plethysmograph were used to simultaneously detect low‐frequency oscillations (LFOs) of cerebral blood flow (CBF), oxy‐hemoglobin concentration ([HbO2]), deoxy‐hemoglobin concentration ([Hb]) and mean arterial pressure (MAP) in older adults before, during and after 70° head‐up‐tilting (HUT). The participants with valid data were divided based on Framingham risk score (FRS, 1‐30 points) into low‐risk (FRS ≤15, n = 13) and high‐risk (FRS >15, n = 11) groups for developing CVD. The LFO gains were determined by transfer function analyses with MAP as the input, and CBF, [HbO2] and [Hb] as the outputs (CA ∝ 1/Gain). At resting‐baseline, LFO gains in the high‐risk group were relatively lower compared to the low‐risk group. The lower baseline gains in the high‐risk group may attribute to compensatory mechanisms to maintain stronger steady‐state CAs. However, HUT resulted in smaller gain reductions in the high‐risk group compared to the low‐risk group, suggesting weaker dynamic CAs. LFO gains are potentially valuable biomarkers for early detection of CVD based on associations with CAs. [ABSTRACT FROM AUTHOR]

Copyright of Electric Power Automation Equipment / Dianli Zidonghua Shebei is the property of Electric Power Automation Equipment Press and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Recently, the low-frequency oscillation (LFO) frequently happens in the vehicle-grid system of high-speed railways when electric multiple units (EMUs) are put into operation. As a possible solution, a single-phase cascaded H-bridge static synchronous compensator (CHB-STATCOM) directly connected to the traction network is applied to dynamically compensate the reactive power of vehicle-grid system. This paper focuses on the impedance modeling and stability analysis in the vehicle-grid system with CHB-STATCOM. Firstly, the vehicle-grid system is simplified to a single-phase rectifier with large grid inductance. The accurate dq-frame impedance of the rectifier system with CHB-STATCOM is derived in detail. Secondly, through the generalized Nyquist stability criterion (GNSC), the influence of CHB-STATCOM and its different control parameters on the stability of rectifier system is revealed. Thirdly, the excellent performance of CHB-STATCOM in suppressing the LFO and the accuracy of theoretical analysis are verified via time-domain simulations. Finally, to further verify the effectiveness of CHB-STATCOM in real applications, a dSPACE based controller hardware-in-the-loop (HIL) simulation platform is established, which includes the physical control circuits of eight EMUs. The experimental results show that the CHB-STATCOM has a good dynamic compensation effect and can efficiently suppress the LFO. [ABSTRACT FROM AUTHOR]

Summary: The penetration of large‐scale photovoltaic farms (LPFs) is ever increasing. Given that LPFs are added to power systems or replaced by conventional power plants, they should undertake the most common tasks of synchronous generators. One of these tasks is the low‐frequency oscillation (LFO) damping using power system stabilizer. This paper proposes a general method for LFO damping using LPFs by designing an optimal proportional‐integral‐derivative (PID) controller as a power oscillation damper in the nonlinear control loop of LPF. Considering the need for a valid LPF model, therefore the second generation generic model, developed and approved by western electricity coordinating council, is used. The PID controller is optimally tuned using the particle swarm optimization algorithm in order to produce an effective LFO damping. Finally, the performance of this controller is investigated in the modified two‐area test system, showing the proper performance of the LPF for LFO damping using the proposed optimal PID controller in the various operating condition and different short circuit ratio values. [ABSTRACT FROM AUTHOR]

Considering the dynamic characteristics of phase-locked loop (PLL), the effect of interaction between doubly fed induction generator (DFIG) and power grid on system low-frequency oscillation is revealed from the perspective of dissipation energy. First, the dynamic energy model of DFIG with PLL is derived, and the component that varies nonperiodically is extracted and defined as the dissipation energy. Second, the dissipation energy is decomposed into the free dissipation energy and DFIG-grid coupled dissipation energy. On this basis, the free dissipation intensity and coupled dissipation intensity are defined, which, respectively, characterize the immanent damping of DFIG and the DFIG-grid interaction degree. Thus, the contribution degree of DFIG to electromechanical oscillation is quantified for evaluation, and the mechanism of interaction between DFIG and power grid is further revealed. Finally, hardware-in-the-loop simulation tests with real-time digital simulator of the IEEE 10-machine 39-bus system verify that when the oscillation frequency of PLL is close to electromechanical oscillation frequency, strong coupling and resonance exist between DFIG and grid, and negative dissipation effect on the oscillation of DFIG will result. Thus, DFIG, as an oscillator, continuously injects dynamic energy to the grid, causing the system to oscillate to instability. [ABSTRACT FROM AUTHOR]

Copyright of Electric Power Automation Equipment / Dianli Zidonghua Shebei is the property of Electric Power Automation Equipment Press and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Two-inputs adaptive IEEE multi-bands power system stabilizer (PSS4B) was developed for oscillations damping control in power systems. Two supplementary loops based on model reference (MR) adaptive control were added to the typical PSS4B design. The MR has the same loops' parameters of the typical PSS4B, and hence, avoiding a complex tuning process. The proposed PSS has a self-tuning gain reduction block to avoid any negative impact due to the high gains value during the disturbance time. The proposed PSS was applied on the four-machine benchmark power system. To evaluate the robustness of the proposed PSS, it was tested in comparison with the Delta W PSS, one-input multi-bands PSS4B (1iMB) and two-inputs multi-bands PSS4B (2iMB) stabilizers. The integration of the proposed PSS was demonstrating using different study cases. These cases consider the small-signal stability (SSS), large-signal stability (LSS) and the coordination test for the local and inter-area excited power modes. The proposed PSS demonstrated robust and superior responses in all cases. [ABSTRACT FROM AUTHOR]

TB6 titanium alloy is extensively applied in lightweight vehicles, biomedicine, and other domains because of its high specific strength, excellent fracture toughness, and excellent corrosion resistance. Electrochemical machining is a non-contact processing technology that has significant advantages in processing materials that are difficult to cut, such as cemented carbide, high-temperature alloys, and titanium alloys. To improve the consistency of deep narrow slots fabricated in TB6 titanium alloy via electrochemical machining, a sheet cathode design and experimental studies were carried out in this work. Based on a unidirectional fluid–structure coupling simulation, the influence of the stiffener arrangement on the cathode rigidity and flow-velocity distribution was studied. Furthermore, by modifying the geometry of the stiffener, the cathode deformation was significantly reduced, and flow-velocity uniformity at the cathode outlet was improved. The influence of a superimposed low-frequency oscillation on the gap distribution and the profile error of a deep narrow slot was investigated experimentally. The results revealed that when an applied voltage of 24 V, an oscillation frequency of 50 Hz, and an amplitude of 0.05 mm were adopted, a highly homogeneous deep narrow slot with an entrance gap of 0.24 mm and a side gap of 0.33 mm was machined into the TB6 titanium alloy. [ABSTRACT FROM AUTHOR]

The penetration of inverter-based power plants (IBPPs), such as large-scale photovoltaic (PV) power plants (LPPPs), is ever increasing considering the merits of renewable energy power plants (REPPs). Given that IBPPs are added to power systems or replaced by conventional power plants, they should undertake the most common tasks of synchronous generators. The low-frequency oscillation (LFO) damping through the power system stabilizers (PSSs) of synchronous generators is regarded as one of the common tasks in power plants. This paper proposes an optimal fractional-order proportional-integral-derivative (FOPID) controller implemented in the control loop of IBPPs for LFO damping in power systems. For this purpose, the last version of the generic dynamic model for renewable technologies (GDMRT) is used, which was released by the Western Electricity Coordinating Council (WECC) and Electric Power Research Institute (EPRI). In addition, an LPPP is studied as a case study. The FOPID controller is optimally tuned using the particle swarm optimization (PSO) algorithm in order to produce effective LFO damping. Finally, the performance of this controller is simulated and investigated in a two-area test system, showing the better performance of the LPPP for LFO damping by using the proposed optimal FOPID controller compared to the optimal lead-lag controller and optimal PID controller. [ABSTRACT FROM AUTHOR]

A reduced-order dynamic phasor model of modular multilevel converter (MMC) in long time scale, which is applied for power system low-frequency oscillation analysis, is investigated. A tenth-order dynamic phasor model is derived in consideration of the internal circulation current dynamic of MMC. The model is verified via electromagnetic transient simulation. Then, an application of the singular perturbation method to MMC model reduction is presented. Separation of eigenvalues is used as a characterization of time scales and small time constant states is eliminated in the full-order model of MMC. MMC high-voltage direct current (HVdc) models with various orders are embedded into the four-machine two-area system to compare the low-frequency oscillation analysis results. The frequency domain analysis and time domain simulations show that the second-order model has satisfactory performance in describing the low-frequency characteristics of MMC. Moreover, the influence of MMC-HVdc transmission power and outer loop controller parameters on low-frequency oscillation is investigated using the second-order model. The effectiveness of reduced-order model in supplementary controller parameter design is also validated by comparing the difference in the response of various-order models to supplementary frequency-based control. [ABSTRACT FROM AUTHOR]

To construct a new energy system during the low-carbon transition, renewable energy sources (RESs) are increasingly being installed on a large scale worldwide which require thermal power units to operate at low loads and respond quickly to compensate for frequency fluctuations of the power grid during deep peak shaving and frequent load changes. While coal-fired thermal power plants operate under the deep-peak shaving process, low-frequency oscillation (LFO) accidents from the generation side have occurred constantly. According to our investigation, the reasons are the overlook of the nonlinearity of control valves is obviously highlighted under low load, and the preset parameters of the original control device can not match the flow characteristics, which may seriously cause a recurrence of LFOs. This paper focuses on generation-side LFO caused by the prime mover and its governor under all operating conditions constructing a novel nonlinear model and designing an oscillation feature observation matrix to identify the LFO modes, and a hierarchy feature-state observation classification method preventing LFO on the generation side is proposed. The results show that the proposed method can suppress the LFOs ahead, and this approach can weaken the oscillation amplitude by 85% and reduce the oscillation duration by 30 s. • Retrofit the small frequency offset compensation and the valve flow module, simulate dynamic characteristics accuracy. • Propose an oscillation ranking method to evaluate the security level of power oscillation and verify its reliability. • Design the hierarchical prevention control strategy for LFO in stages and validate the effectiveness of thecontrol strategy. [ABSTRACT FROM AUTHOR]

Global warming, increase in environmental pollution, and high cost of electrical power generation using fossil fuels are considered the most important reasons for the application of renewable energy power plants (REPPs) around the world. In recent years, a new generation of REPPs called hybrid renewable energy power plants (HREPPs) has been implemented in order to have higher efficiency and reliability than conventional REPPs such as wind power plants and photovoltaic power plants. The HREPPs include two or more renewable energy generation units such as wind turbine generation units, and PV generation units. In case of high penetration of these types of power plants, the most common tasks of synchronous generators should be supported by them. One of these tasks is the ability to reduce the low-frequency oscillation (LFO) risk through power oscillation damper such as the power system stabilizers of synchronous generators. In this paper, a novel method is proposed for LFO damping by HREPPs, which is based on the design of an optimal power oscillation damper (OPOD) implemented in the generic HREPP controller model. The structure of the proposed OPOD is a 2nd-order single-input lead-lag controller, and its performance is investigated in a modified two-area test system. The simulation results show the proper performance of the HREPP for LFO damping using the proposed OPOD in the various loading levels and different short circuit ratio values. [ABSTRACT FROM AUTHOR]

The future Indonesian power grid will have significant REGs, especially in Java Island. Wind and solar photovoltaic (PV) will be the dominating technologies in future Indonesian grid. Although REG can provide clean and sustainable energy, they also cause undesirable effects on system stability due to the uncertainty in their output and different dynamic characteristics. Battery energy storage system (BESS) has been considered as the promising candidate to manage the intermittency and low inertia associated with wind and PV in many future power systems. In addition to the normal functionality, the resilient wide‐area power oscillation damping (POD) can be implemented in the BESS to damp the low‐frequency oscillation in the system. This research proposes a method to design a resilient wide‐area POD in BESS. A grey wolf optimiser (GWO) has been used to synthesise the resilient wide‐area POD. The representative dynamic model of Java–Indonesian power system is used to evaluate the performance of the GWO‐based resilient wide area POD. From the results, it is evident that the implementation of the proposed controller at BESS successfully damped the critical mode of the system. It is also observed that the proposed controller is robust against the failure of the input signal. [ABSTRACT FROM AUTHOR]

Using a doubly-fed induction generator (DFIG), with an additional active or reactive damping controller, is a new method of suppressing the inter-area oscillation of a power system. However, using active power modulation (APM) may decrease the damping of the shaft oscillation mode of a DFIG and the system damping target cannot be achieved through reactive power modulation (RPM) in some cases. Either single APM or RPM does not consider system damping and torsional damping simultaneously. In this paper, an active-reactive coordinated dual-channel power modulation (DCPM) damping controller is proposed for DFIGs. First, considering the electromechanical parts and control structure of the wind turbine, an electromechanical transient model and an additional damping controller model of DFIGs are established. Then, the dynamic objective function for coordinating the parameters of the additional damping controller is proposed. The ratio between the active power channel and reactive power channel modulation is derived from the parameters optimized by the particle swarm optimization algorithm. Finally, the effectiveness and practicability of the designed strategy is verified by comparing it with a traditional, simple damping controller design strategy. Standard simulation system examples are used in the comparison. Results show that the DCPM is better at maximizing the damping control capability of the rotor-side controller of a DFIG and simultaneously minimizing adverse effects on torsional damping than the traditional strategy. [ABSTRACT FROM AUTHOR]

Highlights • A method is proposed to extract the instantaneous oscillation factors. • The proposed method has a higher accuracy, compared with signal analysis methods. • A method is proposed to study the interaction of multi-eigenmodes. • Some complex phenomena can be explained by the proposed methods. • The validity and universality of the proposed method have been strictly clarified. Abstract Low-frequency oscillation is usually regarded as a small disturbance problem. Traditional methods describe oscillation through eigenvalues obtained by linearized equations at the equilibrium point. Obviously, nonlinear time-varying factors are ignored. This article introduces a trajectory section eigenvalue method that can reflect the influences of complex models and fault scenarios. Based on this theory, a trajectory reconstruction method is proposed to accomplish the decoupling of multi-eigenmodes in the time domain. On the premise that the numerical integration is accurate, the trajectory section eigenvalue and trajectory reconstruction methods can be used to study the dynamics of the oscillation and interaction of multi-eigenmodes. Simulation test results show the validity of the proposed methods. The mechanism of low-frequency oscillation is illustrated by trajectory section eigenvalues and trajectory reconstruction methods from a new perspective. These methods can be used as effective tools for theoretical research and engineering applications in power systems. [ABSTRACT FROM AUTHOR]

Simian immunodeficiency virus (SIV)-infected macaque is a widely used model to study human immunodeficiency virus. The purpose of the study is to discover the amplitude of low-frequency fluctuation (ALFF) and fractional ALFF (fALFF) changes in SIV-infected macaques. Seven rhesus macaques were involved in the longitudinal MRI scans: (1) baseline (healthy state); (2) SIV infection stage (12 weeks after SIV inoculation). ALFF and fALFF were subsequently computed and compared to ascertain the changes caused by SIV infection. Whole-brain correlation analysis was further used to explore the possible associations between ALFF/fALFF values and immune status parameters (CD4+ T cell counts, CD4/CD8 ratio and virus load). Compared with the baseline, macaques in SIV infection stage displayed strengthened ALFF values in left precuneus, postcentral gyrus, and temporal gyrus, and weakened ALFF values in orbital gyrus and inferior temporal gyrus. Meanwhile, increased fALFF values were found in left superior frontal gyrus, right precentral gyrus, and superior temporal gyrus, while decreased fALFF values existed in left hippocampus, left caudate, and right inferior frontal gyrus. Furthermore, ALFF and fALFF values in several brain regions showed significant relationships with CD4+ T cell counts, CD4/CD8 ratio, and plasma virus load. Our findings could promote the understanding of neuroAIDS caused by HIV infection, which may provide supplementary evidences for the future therapy study in SIV model. [ABSTRACT FROM AUTHOR]

Active power modulation of line-commutated converter (LCC)-based high-voltage direct current (HVDC) system is increasingly utilised, especially for the case of damping low-frequency oscillation associated with synchronous machine (SM) in electromechanical timescale (around 1 Hz). Many papers have tried to analyse the mechanism of damping effects, but almost rely on numerical studies, which cannot reveal the dynamic interaction between SM and DC-power-modulated LCC. This study proposes a small-signal model of DC-power-modulated LCC based on motion equation concept. With this model, analytical investigation of low-frequency oscillation from the scope of damping and synchronising powers is presented. Comparisons of analytical results and eigenvalues draw some general conclusions which offer insight into the dynamic behaviour. By examining the case of an SM connected to a DC-power-modulated LCC, simulations in MATLAB/Simulink are conducted to verify the analytical results. [ABSTRACT FROM AUTHOR]

Solar power being stochastic and asynchronous in nature, its integration with conventional power generation has so many challenges in damping of low‐frequency oscillations. This[AQ ID=Q2] study presents an investigation of low‐frequency oscillation and damping of stochastic solar power integrated power system by a unified power flow controller (UPFC)‐based dual optimal controller, which gains are optimised by a novel hybrid particle swarm optimisation and improved grey wolf optimiser. The[AQ ID=Q3] dual controller simultaneously controls the modulation index of series and phase angle of shunt converters of UPFC, which co‐operate with each other, optimising its efficacy and thereby implementing the advantages of both static synchronous series compensator and static synchronous compensator. A detailed Eigen value analysis has been performed with time domain simulations to study the damped oscillatory response of variable and random solar penetration with a power system along with the interaction of solar power with variable synchronous power generations. For a multi‐machine system, a multi‐input single output dual controller is proposed. A maximum sensor and time delay have been considered to design the controller providing a realistic approach. It[AQ ID=Q4] is observed that random and heavy solar penetration has a more detrimental effect on system oscillations, which can be damped heavily with the proposed controller, in contrast to particle swarm optimisation, differential evolution, grey wolf optimiser, differential evolution particle swarm optimisation optimised lead‐lag and dual controllers. [ABSTRACT FROM AUTHOR]