Secondary Dips Research Articles

A consecutive power dispatch in wind farms to mitigate secondary frequency dips

With the rapid increase of wind energy integrated into power systems, wind turbine generators (WTGs) are required to provide frequency support to maintain the system frequency stability. However, the frequency regulation is achieved by employing temporary energy reserves from WTGs at the initial stage of a disturbance. Therefore, a second frequency dip (SFD) may occur, if no other energy reserve is available to compensate the power deficiency as WTGs have to recover their operating points and rotor speeds back to the initial operating points. To deal with this problem, this paper proposes a consecutive power dispatch scheme to reduce the SFD and prevent WTGs from over-deceleration. All WTGs are divided into two groups with in a wind farm: Group 1 (G1) WTGs operating at maximum power point tracking (MPPT), Group 2 (G2) WTGs operating at deloading power. If a frequency contingency occurs, the proposed scheme aims to release an amount of kinetic energy (KE) stored in the rotating masses of G1 WTGs to improve the frequency nadir (FN). Following this, energy reserves are released from G2 WTGs to compensate the power shortage during the period when G1 WTGs rotor speeds have to be recovered. The simulation results show that the scheme causes a small SFD while improving the first FN and preventing the rotor from over-decelerations in various wind conditions, contingency sizes, and wind penetration levels.

Fast Frequency Control Strategy for Direct Drive Wind Turbines Considering the Suppression of Secondary Frequency Drop in the Power Grid

Inertia and damping in the power system are important parameters in frequency regulation and oscillation suppression. Inertia will reduce the rate and amplitude of frequency changes after active power sudden changes. Damping in the system can effectively suppress low-frequency oscillations after grid disturbances. As the penetration rate of wind turbines increases, it will significantly reduce the effective inertia in the power grid and reduce the friendliness of wind power grid connection. To provide better frequency support for the power grid in the event of large disturbances in the power system, this paper proposes a fast frequency control strategy for wind farms that considers the suppression of secondary frequency dips in the power grid. The strategy combines frequency droop control with grid frequency bias control and evaluates the kinetic energy reserve online based on wind power prediction and hourly speed, thus balancing the frequency regulation and suppression of secondary frequency dips in the power grid. A 2.5 MW direct drive wind turbine simulation model was built based on the PSCAD simulation platform, verifying the effectiveness of the proposed comprehensive control strategy.

Variable-Speed Wind Turbine Control Designed for Coordinated Fast Frequency Reserves

Modern power systems present low levels of inertia due to the growing shares of converter-interfaced generation. Consequently, renewable energy sources are increasingly requested to provide frequency support. In addition, due to the inertia loss, the requirements regarding frequency containment reserves (FCR) are becoming tough to meet with traditional units such as hydro, whose non-minimum phase (NMP) characteristic reduces the closed-loop stability margins. The shortcomings of traditional synchronous generation motivates new protocols for fast frequency reserves (FFR). In this work, we design a wind turbine (WT) model useful for FFR. It is shown that the dynamical shortcomings of the WT, in providing steady-power or slow FCR support, are suitably described by a first-order transfer function with a slow NMP zero. The WT model is tested in a 5-machine representation of the Nordic synchronous grid. It is shown that the NMP model is useful for designing a controller that coordinates FFR from wind with slow FCR from hydro turbines. By simulating the disconnection of a 1400 MW importing dc link in a detailed nonlinear model, it is shown that the wind--hydro combination not only satisfies the latest regulations, but also presents a smooth response avoiding overshoot and secondary frequency dips during frequency recovery.

Analytic modeling of recurrent Forbush decreases caused by corotating interaction regions

Context.On scales of days, the galactic cosmic ray (GCR) flux is affected by coronal mass ejections and corotating interaction regions (CIRs), causing so-called Forbush decreases and recurrent Forbush decreases (RFDs), respectively.Aims.We explain the properties and behavior of RFDs recorded at about 1 au that are caused by CIRs generated by solar wind high-speed streams (HSSs) that emanate from coronal holes.Methods.We employed a convection-diffusion GCR propagation model based on the Fokker-Planck equation and applied it to solar wind and interplanetary magnetic field properties at 1 au.Results.Our analysis shows that the only two effects that are relevant for a plausible overall explanation of the observations are the enhanced convection effect caused by the increased velocity of the HSS and the reduced diffusion effect caused by the enhanced magnetic field and its fluctuations within the CIR and HSS structure. These two effects that we considered in the model are sufficient to explain not only the main signatures of RFDs, but also the sometimes observed “over-recovery” and secondary dips in RFD profiles. The explanation in terms of the convection-diffusion GCR propagation hypothesis is tested by applying our model to the observations of a long-lived CIR that recurred over 27 rotations in 2007–2008.Conclusions.Our analysis demonstrates a very good match of the model results and observations.

Advanced adaptive frequency support scheme for DFIG under cyber uncertainty

Abstract In recent years, the development of information and communication technology has promoted the transformation of traditional electrical power grid into a deeply-intertwined cyber physical power system (CPPS). However, it also introduces cyber uncertainties that have a significant impact on the economic operation and real-time control of CPPS. This paper aims to solve the control problem of frequency support of wind farms in a cyber uncertainty environment, and proposes an advanced adaptive frequency support scheme for doubly-fed induction generator (DFIG). In the proposed scheme, both the minimum rotor speed and the deloaded operating level of wind turbines are required to be determined at first. Then, an advanced adaptive droop control method is proposed with consideration of cyber uncertainty to improve the response speed of wind turbines for frequency support. Finally, the stability of the proposed scheme is theoretically proved. The proposed scheme combines deloading control and inertial control, so that the controller can not only make up for the impact of cyber uncertainty on frequency regulation, but also can adaptively determine the participation factor of each wind turbine. The effectiveness of this control scheme is highlighted on a simulation platform built on Matlab/Simulink. The obtained results demonstrate that the proposed control scheme is capable for improving the frequency nadir and preventing secondary frequency dips.

The odderon: myths and reality

In the framework of a Regge-pole model, we discussthe forward phase of the proton-proton elastic scattering amplitude (ρ(13 TeV) = 0.1 ± 0.01), recently measured at the LHC. Our model includesa dipole pomeron, odderon and secondary reggeon exchanges. It accommodetes forthe low- |t| «break» (departure from the exponential behavior of the diffraction cone), the accelerating rise with energy of the forward slope B(s, t = 0) and absence of secondary dips and bumps. Contrary to the majority of Regge-pole models, our approach is based on he use of a dipole (double pole) pomeron (DP). It has a number of advantages over the conventional simple pole pomeron: DP generates rising cross sections even at unit pomeron intercept and, what is even more important, it produces a single diffraction minimum and maximum in the differential cross section, in accord with the experimental data. Relative contributions from different components to the scattering amplitude are evaluated from the fitted model.

New physics from TOTEM’s recent measurements of elastic and total cross sections

We analyze the recently discovered phenomena in elastic proton–proton scattering at the LHC, challenging the standard Regge-pole theory: the low- ‘break’ (departure from the exponential behavior of the diffraction cone), the accelerating rise with energy of the forward slope B(s, t = 0), the absence of secondary dips and bumps on the cone and the role of the odderon in the forward phase of the amplitude, ρ(13 TeV) = 0.1 ± 0.01, and especially its contribution at the dip region, measured recently by TOTEM. Relative contributions from different components to the scattering amplitude are evaluated from the fitted model.

Rotation Speed Recovery Strategy Based On Particle Swarm Optimization for Variable Power Curve Optimization

With the increase of wind power capacity incorporated into the grid, the permeability of wind power increases continuously, which leads to the reduction of the equivalent inertia of the system. Rotor speed recovery will cause the secondary frequency dips when DFIG uses rotor inertial control to participate in system frequency modulation. In order to optimize the secondary frequency dips, this paper uses PSO (Particle Swarm Optimization) to optimize the Parameters of Variable Power Functions on the basis of the previously proposed variable power curve strategy. This article simulates this strategy through setting up a system by Simulink. Simulate the results of optimization, compared with the traditional recovery strategy, validity of the proposed strategy is demonstrated through simulation and analysis.

Rotation Speed Recovery Strategy Based On Variable Power Curve of Inertia Control from DFIG Wind Turbine

With the increase of wind power capacity incorporated into the grid, the permeability of wind power increases continuously, which leads to the reduction of the equivalent inertia of the system. A new strategy of recovering rotor speed that increases the output power by modifying the variable power curve is proposed in this paper, which decreases the drop of the active power. So as to reduce the secondary frequency dips caused by the sudden drop of the active power when the speed restores. The paper optimizes the secondary frequency dips by modifying the variable power curve continuously. This article simulates this strategy through setting up a system by Simulink. Compared with the traditional recovery strategy, validity of the proposed strategy is demonstrated through simulation and analysis.

Frequency Support From Wind Turbine Generators With a Time-Variable Droop Characteristic

The changing inertia profile caused by the increased penetration of inertialess renewable energy sources has raised concerns about power system frequency control. The kinetic energy of the turbines in wind turbine generators (WTGs) can be utilized to support power system frequency during contingencies. In this paper, we investigate the frequency support capability of WTGs operating at the maximum power point (MPP). The requirements to prevent secondary frequency dips, provoked from switching between normal operating mode and the frequency support mode, are formulated. A time-variable droop characteristic is proposed for frequency support from WTGs, which is quite effective in preventing large frequency excursions and facilitates smooth recovery of the kinetic energy of WTGs. The performance of the proposed method is examined in different operating conditions of WTGs in a single bus model of power systems, as well as a regional 14-generator model of the Australian National Electricity Market. The results show that with the time-variable droop characteristic, the frequency nadir following a contingency can be largely improved and simultaneously, the WTGs can smoothly regain their kinetic energy and continue operating at the MPP.

Secondary dips and the asymptotics

We point out how to detect experimentally the energy region where asymptotics start to manifest themselves by relating the appearance of the secondary dips in the differential cross-section of elastic scattering [Formula: see text] with the beginning of the asymptotic energy region. The consideration relies on differential characteristics. The impact parameter picture of proton–proton interactions is used.

Recent LHC/TOTEM data challenging the standard Regge pole theory

We analyse the recently discovered phenomena in elastic protonproton scattering at the LHC: the low-|t| “break” (departure from the exponential behaviour of the diffraction cone), the accelerating rise with energy of the forward slope B(s), the absence of secondary dips and bumps on the cone and the role of the odderon in the forward phase of the amplitude, ρ(13 TeV) = 0:09 ± 0:01, especially its contribution at the dip region, measured recently by TOTEM. The dip at 13 TeV seems to become more shallow with respect to lower energies, and we attribute this important new phenomena to the odderon contribution. These findings call for a revision of the standeard Regge-pole theory.

Topological Departures from Translational Invariance along a Filament Observed by THEMIS

We study the topology of the 3D magnetic field in a filament channel to address the following questions: Is a filament always formed in a single flux tube? How does the photospheric magnetic field lead to filament interruptions and to feet formation? What is the relation between feet-related field lines and the parasitic polarities? What can topological analyses teach us about EUV filament channels? To do so, we consider a filament observed on 6 October 2004 with THEMIS/MTR, in Hα with the full line profile simultaneously and cospatially with its photospheric vector magnetic field. The coronal magnetic field was calculated from a “linear magnetohydrostatic” extrapolation of a composite THEMIS-MDI magnetogram. Its free parameters were adjusted to get the best match possible between the distribution of modeled plasma-supporting dips and the Hα filament morphology. The model results in moderate plasma β≤1 at low altitudes in the filament, in conjunction with non-negligible departures from force-freeness measured by various metrics. The filament here is formed by a split flux tube. One part of the flux tube is rooted in the photosphere aside an observed interruption in the filament. This splitted topology is due to strong network polarities on the edge of the filament channel, not to flux concentrations closer to the filament. We focus our study to the northwest portion of the filament. The related flux tube is highly fragmented at low altitudes. This fragmentation is due to small flux concentrations of two types. First, some locally distort the tube, leading to noticeable thickness variations along the filament body. Second, parasitic polarities, associated with filament feet, result in secondary dips above the related local inversion line. These dips belong to long field lines that pass below the flux tube. Many of these field lines are not rooted near the related foot. Finally, the present model shows that the coronal void interpretation cannot be ruled out to interpret the wideness of EUV filament channels.

A Parameterization Study of the Properties of the X‐Ray Dips in the Low‐Mass X‐Ray Binary X1916−053

The ultracompact low-mass X-ray binary X1916–053, which is composed of a neutron star and a semidegenerate white dwarf, exhibits periodic X-ray dips of variable width and depth. We have developed a new method to parameterize this dipping behavior to systematically study its variations. This can help in further understanding binary and accretion disk behaviors. Observations by the Rossi X-Ray Timing Explorer from 1998 clearly show a 4.87 day periodic variation in the dip width. This is probably due to nodal precession of the accretion disk, although an epoch-folding search finds no significant sidebands in the spectrum. From the negative-superhump model of Larwood et al., the mass ratio can be estimated as q = 0.045. Combining more than 24 years of historical data, we find an orbital period derivative orb/Porb = (1.62 ± 0.34) × 10−7 yr−1 and establish a quadratic ephemeris for the X-ray dips. This period derivative seems inconsistent with the prediction from the standard model of binary orbital evolution proposed by Rappaport et al. On the other hand, Tavani's radiation-driven model can properly account for the period derivative, although the large mass outflow predicted by this model has never been observed in this system. With the best ephemeris, we find that the standard deviation of the primary dips is smaller than that of the secondary dips. This means that the primary dips are more stable than the secondary dips. Thus, we conclude that the primary dips of X1916–053 occur from the bulge where the accretion stream encounters the rim of the disk, rather than the inner disk ring as previously proposed by Frank et al.

ASCAObservation of the Dipping X‐Ray Source XB 1916−053

We present the results of timing and spectral studies of the dipping X-ray source XB 1916-053, observed by ASCA during its performance verification phase. The detected dipping activity is consistent with previous observations, with a period of 3008 s and an intermittent secondary dip observed roughly 0.4 out of phase with the primary dip. The energy spectra of different intensity states are fitted with a power law with partial covering fraction absorption and interstellar absorption. The increase in the hardness ratio during the primary and secondary dips, and the increase in the covering fraction and column density with decreasing X-ray intensity, all imply that the dipping is caused by the photoabsorbing materials that have been suggested to be where the accreted flow hits the outer edge of the disk materials. The spectra at all intensity levels show no apparent evidence for Fe or Ne emission lines. This may be due to the low metal abundance in the accretion flow. Alternatively, the X-ray luminosity of the central source may be too weak to excite emission lines, which are assumed to be produced by X-ray photoionization of the disk materials.

Azimuthally averaged backscattering yield near the 〈100〉 axis in Si

An azimuthally averaged backscattering yield near the 〈100〉 axis in Si is studied experimentally and by Monte Carlo simulation, for 1.5 MeV 4He ions and tilt angles θ up to 16°. A complete compensation of the 〈100〉 dip is obtained within 0° ⩽ θ⩽ 6°, in agreement with the rule of angular compensation. For larger tilts a fine structure in the yield, with a number of distinct secondary dips, is observed. The results of two simulation programs (CXX and FLUX) are found to be fully consistent with each other, and in excellent agreement with the experimental data. The extension of the simulation methods to include trajectories that substantially deviate from the string direction is presented.

Random spectrum for the channeling-backscattering technique: A rotating axial-dip study

The 〈100〉 axial channeling dip for a Si single crystal, obtained by averaging over azimuthal angles, is studied experimentally and by Monte Carlo simulation for tilt angles θ up to 16° using a 1.5 MeV energy 4He + ion beam. Both in the experiment and the simulation, the dip shape is found to be in a quantitative agreement with the rule of angular compensation; the influence of the 〈100〉 axis extends to θ ≈ 6°. Surprisingly strong fine structure in the yield, with a number of distinct secondary dips, is observed for θ > 6°. It is shown that this structure is related to some high-index crystallographic planes being nearly tangent to the azimuthal scans at certain tilts. Good agreement between the experiment and simulation is obtained at all tilt angles investigated. Consequences for the random yield determination in channeling studies are discussed.

Compaction of Lime Mud as a Cause of Secondary Structure

ABSTRACT Calculations Based on confined compression tests carried out with lime mud indicate that compaction of this material may produce secondary dips of 60° or more in overlying beds if the material surrounds a relatively incompressible core whose slopes are as steep as 75°. The secondary dips are of course gentler if the slopes of the core are less steep. A comparison of the data for the lime mud with similar data for a lean and for a fat clay, and with the observed void ratios of deeply buried shale, indicates that the compaction of this lime mud may be at least as great as that of a lean clay, and possibly as great as that of a medium clay.