With an increasing number of distributed energy resources integrated into the power system, inverters need to take on the corresponding responsibility for the security and stability of the system. ...Virtual synchronous generators (VSGs) are proposed to mimic dynamic characteristics of traditional rotational synchronous generators (RSGs) to compensate for the loss of inertia and reserve capacity. Similar to RSGs, VSGs will experience transient angle instability under certain conditions, which likely threatens the system security. In this paper, transient angle stability of a VSG is investigated by Lyapunov's direct method. The deteriorative effect of reactive power control loop on transient angle stability is first analyzed and then voltage variation is incorporated into an approximate Lyapunov's direct method. In this method, the inverter internal voltage is treated as a parameter rather than a state variable. Moreover, the influence of different parameters on transient angle stability is studied. Finally, an enhanced control strategy is presented to improve the transient angle stability by adjusting the reference power. Numerical simulation results are presented to validate the effectiveness of the proposed method and the enhanced control.
Electron-boson coupling plays a key role in superconductivity for many systems. However, in copper-based high-critical temperature (
) superconductors, its relation to superconductivity remains ...controversial despite strong spectroscopic fingerprints. In this study, we used angle-resolved photoemission spectroscopy to find a pronounced correlation between the superconducting gap and the bosonic coupling strength near the Brillouin zone boundary in Bi
Sr
CaCu
O
The bosonic coupling strength rapidly increases from the overdoped Fermi liquid regime to the optimally doped strange metal, concomitant with the quadrupled superconducting gap and the doubled gap-to-
ratio across the pseudogap boundary. This synchronized lattice and electronic response suggests that the effects of electronic interaction and the electron-phonon coupling (EPC) reinforce each other in a positive-feedback loop upon entering the strange-metal regime, which in turn drives a stronger superconductivity.
Three-dimensional topological insulators are a new state of quantum matter with a bulk gap and odd number of relativistic Dirac fermions on the surface. By investigating the surface state of Bi2Te3 ...with angle-resolved photoemission spectroscopy, we demonstrate that the surface state consists of a single nondegenerate Dirac cone. Furthermore, with appropriate hole doping, the Fermi level can be tuned to intersect only the surface states, indicating a full energy gap for the bulk states. Our results establish that Bi2Te3 is a simple model system for the three-dimensional topological insulator with a single Dirac cone on the surface. The large bulk gap of Bi2Te3 also points to promising potential for high-temperature spintronics applications.
With the development of virtual synchronous generator (VSG) techniques, parallel operations of synchronous generators (SGs) and VSGs become increasingly common in a microgrid. The differences between ...paralleled systems will affect the transient stability of the system, which probably threatens stable operation of the system, especially under fault conditions. In this article, the transient angle stability of a paralleled synchronous and virtual synchronous generators (SG-VSG) system is investigated by comparing it with that of the paralleled VSGs system. It is observed that the paralleled SG-VSG system is more prone to transient instability due to the differences between their speed governors. Then, a control method is proposed to improve the transient stability of the paralleled SG-VSG system. Furthermore, a Lyapunov method is employed to establish the nonlinear model of islanded microgrid, by which the attraction domain of paralleled system is quantified. The hardware-in-loop experiment is performed to validate the theoretical analysis.
Large transient inrush fault current in a synchronverter during a short-circuit fault can potentially damage the inverter and cause grid instability. It is therefore important to study the ...characteristics of and the way to restrain the inrush fault current. This paper investigates the characteristics of the synchronverter inrush fault current and proposes a new control method based on mode switching to protect synchronverters. First, an ideal voltage source inverter instantaneous inrush current calculation method for a grid symmetrical short circuit is discussed. Then, based on the inertia of synchronverter, the approximate calculation of the synchronverter instantaneous inrush current is described. The results show that the synchronverter's inrush current mainly consists of a gradually attenuated periodic component and a dc component when the phased voltage symmetry is lost. In our proposed new control approach, the synchronverter switches to the hysteresis controller to limit the output current quickly while staying connected to the grid, and supporting the grid voltage simultaneously. Simulation and experiment results verify the validity of the theoretical analysis.
Establishing the appropriate theoretical framework for unconventional superconductivity in the iron-based materials requires correct understanding of both the electron correlation strength and the ...role of Fermi surfaces. This fundamental issue becomes especially relevant with the discovery of the iron chalcogenide superconductors. Here, we use angle-resolved photoemission spectroscopy to measure three representative iron chalcogenides, FeTe0.56Se0.44, monolayer FeSe grown on SrTiO3 and K0.76Fe1.72Se2. We show that these superconductors are all strongly correlated, with an orbital-selective strong renormalization in the dxy bands despite having drastically different Fermi surface topologies. Furthermore, raising temperature brings all three compounds from a metallic state to a phase where the dxy orbital loses all spectral weight while other orbitals remain itinerant. These observations establish that iron chalcogenides display universal orbital-selective strong correlations that are insensitive to the Fermi surface topology, and are close to an orbital-selective Mott phase, hence placing strong constraints for theoretical understanding of iron-based superconductors.
Diversification of control schemes adopted by inverter-interfaced distributed generators (IIDGs) leads to difficulties in fault current estimation in a microgrid, which might make preexisting ...protection systems invalid and threaten the safety of power electronic devices. It is therefore important to study fault characteristics of IIDGs. This paper investigates characteristics of fault current of IIDGs caused by both symmetrical and asymmetrical faults. Two kinds of widely used control modes, current control (constant current control and PQ control) and voltage control (V/F control and droop control), are under investigation to provide an intuitive comparison on fault current. In particular, a novel algorithm is proposed to calculate fault current of droop-controlled IIDGs. It is found that different limiters have great impacts on fault response of IIDGs and detailed research works are carried out to identify the effects in this paper. Simulation results based on PSCAD/EMTDC and calculation results based on MATLAB/Simulink verify the correctness of the proposed fault models.
The interactions that lead to the emergence of superconductivity in iron-based materials remain a subject of debate. It has been suggested that electron-electron correlations enhance electron-phonon ...coupling in iron selenide (FeSe) and related pnictides, but direct experimental verification has been lacking. Here we show that the electron-phonon coupling strength in FeSe can be quantified by combining two time-domain experiments into a “coherent lock-in” measurement in the terahertz regime. X-ray diffraction tracks the light-induced femtosecond coherent lattice motion at a single phonon frequency, and photoemission monitors the subsequent coherent changes in the electronic band structure.Comparison with theory reveals a strong enhancement of the coupling strength in FeSe owing to correlation effects. Given that the electron-phonon coupling affects superconductivity exponentially, this enhancement highlights the importance of the cooperative interplay between electron-electron and electron-phonon interactions.
We develop a numerical model to study the time-dependent modulation of galactic cosmic rays in the inner heliosphere. In the model, a time-delayed modified Parker heliospheric magnetic field (HMF) ...and a new diffusion coefficient model, NLGCE-F, from Qin & Zhang, are adopted. In addition, the latitudinal dependence of magnetic turbulence magnitude is assumed to be from the observations of Ulysses, and the radial dependence is assumed to be , where we choose an expression of S as a function of the heliospheric current sheet tilt angle. We show that the analytical expression used to describe the spatial variation of HMF turbulence magnitude agrees well with the Ulysses, Voyager 1, and Voyager 2 observations. By numerically calculating the modulation code, we get the proton energy spectra as a function of time during the recent solar minimum, it is shown that the modulation results are consistent with the Payload for Antimatter-Matter Exploration and Light-nuclei Astrophysics measurements.
Interferometric synthetic aperture radar data are used to map the interseismic velocity field along the Haiyuan fault system (HFS), at the north‐eastern boundary of the Tibetan plateau. Two M ∼ 8 ...earthquakes ruptured the HFS in 1920 and 1927, but its 260 km‐long central section, known as the Tianzhu seismic gap, remains unbroken since ∼1000 years. The Envisat SAR data, spanning the 2003–2009 period, cover about 200 × 300 km2 along three descending and two ascending tracks. Interferograms are processed using an adapted version of ROI_PAC. The signal due to stratified atmospheric phase delay is empirically corrected together with orbital residuals. Mean line‐of‐sight velocity maps are computed using a constrained time series analysis after selection of interferograms with low atmospheric noise. These maps show a dominant left‐lateral motion across the HFS, and reveal a narrow, 35 km‐long zone of high velocity gradient across the fault in between the Tianzhu gap and the 1920 rupture. We model the observed velocity field using a discretized fault creeping at shallow depth and a least squares inversion. The inferred shallow slip rate distribution reveals aseismic slip in between two fully locked segments. The average creep rate is ∼5 mm yr−1, comparable in magnitude with the estimated loading rate at depth, suggesting no strain accumulation on this segment. The modeled creep rate locally exceeds the long term rate, reaching 8 mm yr−1, suggesting transient creep episodes. The present study emphasizes the need for continuous monitoring of the surface velocity in the vicinity of major seismic gaps in terms of seismic hazard assessment.
Key Points
Imaging spatial variations of interseismic strain along a strike‐slip fault
InSAR data resolves shallow slip anomaly between locked sections of a fault
Method developments improve InSAR data resolution to a subcentimeter level
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