The Experts below are selected from a list of 270 Experts worldwide ranked by ideXlab platform
D O Gough - One of the best experts on this subject based on the ideXlab platform.
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temporal variations in the sun s Rotational Kinetic Energy
Astronomy and Astrophysics, 2008Co-Authors: H M Antia, S M Chitre, D O GoughAbstract:Aims. We study the variation of the angular momentum and the Rotational Kinetic Energy of the Sun, and associated variations in the gravitational multipole moments, on a timescale of the solar cycle. Methods. These quantities are determined by inverting helioseismic Rotational splitting data obtained by the Global Oscillation Network Group and by the Michelson Doppler Imager on the Solar and Heliospheric Observatory. Results. The temporal variation in angular momentum and Kinetic Energy at high latitudes (>π/4) through the convection zone is positively correlated with the level of solar activity, whereas at low latitudes it is anticorrelated, except in the top 10% by radius where both are correlated positively. Conclusions. The helioseismic data imply significant temporal variation in the angular momentum and the Rotational Kinetic Energy, and in the gravitational multipole moments. The properties of that variation will help constrain dynamical theories of the solar cycle.
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temporal variations in the sun s Rotational Kinetic Energy
arXiv: Astrophysics, 2007Co-Authors: H M Antia, S M Chitre, D O GoughAbstract:AIM: To study the variation of the angular momentum and the Rotational Kinetic Energy of the Sun, and associated variations in the gravitational multipole moments, on a timescale of the solar cycle. METHOD: Inverting helioseismic Rotational splitting data obtained by the Global Oscillation Network Group and by the Michelson Doppler Imager on the Solar and Heliospheric Observatory. RESULTS: The temporal variation in angular momentum and Kinetic Energy at high latitudes (>\pi/4) through the convection zone is positively correlated with solar activity, whereas at low latitudes it is anticorrelated, except for the top 10% by radius where both are correlated positively. CONCLUSION: The helioseismic data imply significant temporal variation in the angular momentum and the Rotational Kinetic Energy, and in the gravitational multipole moments. The properties of that variation will help constrain dynamical theories of the solar cycle.
H M Antia - One of the best experts on this subject based on the ideXlab platform.
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temporal variations in the sun s Rotational Kinetic Energy
Astronomy and Astrophysics, 2008Co-Authors: H M Antia, S M Chitre, D O GoughAbstract:Aims. We study the variation of the angular momentum and the Rotational Kinetic Energy of the Sun, and associated variations in the gravitational multipole moments, on a timescale of the solar cycle. Methods. These quantities are determined by inverting helioseismic Rotational splitting data obtained by the Global Oscillation Network Group and by the Michelson Doppler Imager on the Solar and Heliospheric Observatory. Results. The temporal variation in angular momentum and Kinetic Energy at high latitudes (>π/4) through the convection zone is positively correlated with the level of solar activity, whereas at low latitudes it is anticorrelated, except in the top 10% by radius where both are correlated positively. Conclusions. The helioseismic data imply significant temporal variation in the angular momentum and the Rotational Kinetic Energy, and in the gravitational multipole moments. The properties of that variation will help constrain dynamical theories of the solar cycle.
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temporal variations in the sun s Rotational Kinetic Energy
arXiv: Astrophysics, 2007Co-Authors: H M Antia, S M Chitre, D O GoughAbstract:AIM: To study the variation of the angular momentum and the Rotational Kinetic Energy of the Sun, and associated variations in the gravitational multipole moments, on a timescale of the solar cycle. METHOD: Inverting helioseismic Rotational splitting data obtained by the Global Oscillation Network Group and by the Michelson Doppler Imager on the Solar and Heliospheric Observatory. RESULTS: The temporal variation in angular momentum and Kinetic Energy at high latitudes (>\pi/4) through the convection zone is positively correlated with solar activity, whereas at low latitudes it is anticorrelated, except for the top 10% by radius where both are correlated positively. CONCLUSION: The helioseismic data imply significant temporal variation in the angular momentum and the Rotational Kinetic Energy, and in the gravitational multipole moments. The properties of that variation will help constrain dynamical theories of the solar cycle.
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The Inconstant Sun
AIP Conference Proceedings, 2007Co-Authors: H M Antia, S M Chitre, Douglas GoughAbstract:We study temporal variation in the solar angular momentum, J, Rotational Kinetic Energy, T, and the Rotational contribution to the gravitational multipole moments J2k of the Sun, inferred from helioseismic data from GONG and MDI over the last 11 years, covering most of the cycle 23. We have found that the variations in J and T at high latitudes (>45°) through the convection zone are correlated positively with solar activity, while at low latitudes they are correlated negatively, except for the top 10% of solar radius, where the correlation is positive.
Gregor Verbic - One of the best experts on this subject based on the ideXlab platform.
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Coordinated Operation Strategy of Wind Farms for Frequency Control by Exploring Wake Interaction
IEEE Transactions on Sustainable Energy, 2017Co-Authors: Ahmad Shabir Ahmadyar, Gregor VerbicAbstract:This paper proposes a coordinated wind farm (WF) operation strategy that exploits the wake interaction within a WF. In contrast to the conventional WF operation strategy where each wind turbine (WT) is optimized individually, three operation strategies are suggested: 1) maximization of the WF's power while maintaining WF's Rotational Kinetic Energy; 2) maximization of the WF's Rotational Kinetic Energy while maintaining WF's output power; and 3) a de-loaded strategy whereby the WF's Rotational Kinetic Energy is maximized for a fixed de-loading margin. The three operation strategies are formulated as nonlinear optimization problems and solved in a central WF controller. The optimal rotor speeds and pitch angles are used as reference values in the individual WTs. The optimization results for the three respective operation strategies show that: 1) up to 3% increase in the WF's output power; 2) up to 23% increase in the WF's Rotational Kinetic Energy; and 3) up to 28% increase in the WF's Rotational Kinetic Energy and 8% reserve power while operating the WF in a 5% de-loaded mode with respect to the conventional operation strategy. Time-domain simulations show that the proposed operation strategy noticeably improves the WF's performance in frequency control.
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assessment of minimum inertia requirement for system frequency stability
IEEE International Conference on Power System Technology, 2016Co-Authors: Ahmad Shabir Ahmadyar, Gregor Verbic, Shariq Riaz, Jenny Riesz, Archie C ChapmanAbstract:The reduced amount of Rotational Kinetic Energy in power systems has significant detrimental impacts on system frequency behaviour. To ensure system security, this issue must be addressed in a systematic way. Given this, we consider three different metrics, namely i) minimum power penetration from synchronous generators, ii) minimum rated power of synchronous generators, and iii) minimum synchronous Kinetic Energy, as frequency control constraints in the market dispatch model, and assess frequency behaviour of the system for these metrics. The results show that each metric has a different impact on market dispatch, and, accordingly, on the system frequency behaviour. It is shown that with high penetration of converter-based generation, we need to consider a minimum Kinetic Energy requirement as a frequency control security constraint in the market dispatch model to guarantee the frequency stability of power systems.
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optimised control approach for frequency control contribution of variable speed wind turbines
Iet Renewable Power Generation, 2012Co-Authors: A. Zertek, Gregor Verbic, Miloš PantošAbstract:This study proposes a novel control strategy for the provision of an active power reserve for variable-speed wind turbines. The proposed control strategy aims at maximising the Rotational Kinetic Energy by jointly optimising the Rotational speed and the pitch angle using differential evolution. The optimisation process takes into account the given frequency support power-reserve margin. Based on the optimisation results, a novel controller strategy is proposed and compared with other existing approaches. This comparison reveals that relative to existing methods at the same de-loaded power, the proposed method yields more Rotational Kinetic Energy released into the system in the event of frequency deviations, thus increasing the frequency stability of the power system.
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Participation of DFIG wind turbines in frequency control ancillary service by optimized Rotational Kinetic Energy
2010 7th International Conference on the European Energy Market, 2010Co-Authors: A. Zertek, Gregor Verbic, Miloš PantošAbstract:The increase in renewable and distributed Energy resources has given rise to new concerns about frequency stability of electric power systems, as renewable generation is considered an intermittent source, which by its nature does not participate in frequency control. In some countries, system operators demand wind turbines to provide some sort of ancillary services, such as reactive power control and frequency/active power control in order to increase system stability. This paper focuses on the provision of primary frequency control by wind generators and proposes a novel control strategy for active power reserve provision of doubly fed induction generator wind turbines. The proposed control strategy aims at maximizing the Rotational Kinetic Energy, using the differential evolution. The control variables in the optimization process are the rotor speed and the pitch angle, considering the given power reserve factor. The proposed control strategy is compared to the strategies proposed by other authors. The results show that, compared to the existing methods at the same de-loaded power, the proposed method yields more Rotational Kinetic Energy, that is released into the system in case of frequency deviations, thus increasing frequency stability of a power system.
S M Chitre - One of the best experts on this subject based on the ideXlab platform.
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temporal variations in the sun s Rotational Kinetic Energy
Astronomy and Astrophysics, 2008Co-Authors: H M Antia, S M Chitre, D O GoughAbstract:Aims. We study the variation of the angular momentum and the Rotational Kinetic Energy of the Sun, and associated variations in the gravitational multipole moments, on a timescale of the solar cycle. Methods. These quantities are determined by inverting helioseismic Rotational splitting data obtained by the Global Oscillation Network Group and by the Michelson Doppler Imager on the Solar and Heliospheric Observatory. Results. The temporal variation in angular momentum and Kinetic Energy at high latitudes (>π/4) through the convection zone is positively correlated with the level of solar activity, whereas at low latitudes it is anticorrelated, except in the top 10% by radius where both are correlated positively. Conclusions. The helioseismic data imply significant temporal variation in the angular momentum and the Rotational Kinetic Energy, and in the gravitational multipole moments. The properties of that variation will help constrain dynamical theories of the solar cycle.
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temporal variations in the sun s Rotational Kinetic Energy
arXiv: Astrophysics, 2007Co-Authors: H M Antia, S M Chitre, D O GoughAbstract:AIM: To study the variation of the angular momentum and the Rotational Kinetic Energy of the Sun, and associated variations in the gravitational multipole moments, on a timescale of the solar cycle. METHOD: Inverting helioseismic Rotational splitting data obtained by the Global Oscillation Network Group and by the Michelson Doppler Imager on the Solar and Heliospheric Observatory. RESULTS: The temporal variation in angular momentum and Kinetic Energy at high latitudes (>\pi/4) through the convection zone is positively correlated with solar activity, whereas at low latitudes it is anticorrelated, except for the top 10% by radius where both are correlated positively. CONCLUSION: The helioseismic data imply significant temporal variation in the angular momentum and the Rotational Kinetic Energy, and in the gravitational multipole moments. The properties of that variation will help constrain dynamical theories of the solar cycle.
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The Inconstant Sun
AIP Conference Proceedings, 2007Co-Authors: H M Antia, S M Chitre, Douglas GoughAbstract:We study temporal variation in the solar angular momentum, J, Rotational Kinetic Energy, T, and the Rotational contribution to the gravitational multipole moments J2k of the Sun, inferred from helioseismic data from GONG and MDI over the last 11 years, covering most of the cycle 23. We have found that the variations in J and T at high latitudes (>45°) through the convection zone are correlated positively with solar activity, while at low latitudes they are correlated negatively, except for the top 10% of solar radius, where the correlation is positive.
V. S. Geroyannis - One of the best experts on this subject based on the ideXlab platform.
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Third-Order Corrections and Mass-Shedding Limit of Rotating Neutron Stars Computed by a Complex-Plane Strategy
International Journal of Astronomy and Astrophysics, 2012Co-Authors: Ioannis Sfaelos, V. S. GeroyannisAbstract:We implement the so-called “complex-plane strategy” for computing general-relativistic polytropic models of uniformly rotating neutron stars. This method manages the problem by performing all numerical integrations, required within the framework of Hartle’s perturbation method, in the complex plane. We give emphasis on computing corrections up to third order in the angular velocity, and the mass-shedding limit. We also compute the angular momentum, moment of inertia, Rotational Kinetic Energy, and gravitational potential Energy of the models considered.
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Rotational Behavior of Magnetic White Dwarfs: Numerical Study of a Turnover Scenario
The Astrophysical Journal Supplement Series, 2002Co-Authors: V. S. GeroyannisAbstract:We consider a white dwarf model with differential rotation and magnetic field, assuming that (1) the symmetry axis of the toroidal magnetic field, the magnetic axis of the poloidal magnetic field, and the principal axis I3 coincide permanently with each other (this common axis is called "magnetic symmetry axis") and (2) the model declines slightly from axisymmetry, i.e., its magnetic symmetry axis is inclined at a small angle χ relative to its spin axis (this angle is called "obliquity angle" or "turnover angle"). The latter assumption turns on the "magnetic dipole radiation mechanism," which is fed by the Rotational Kinetic Energy and causes emission of weak electromagnetic power since χ is assumed small; thus, the model suffers from secular angular momentum loss. This fact leads to a gradual decrease of the moment of inertia I33 along the principal axis I3 and, in turn, to a gradual increase of the moment of inertia I11 along the principal axis I1, since the toroidal field (tending to derive prolate configurations and thus to increase I11) becomes gradually more competitive against the combined action of both rotation and poloidal field (tending to derive oblate configurations and thus to increase I33). So, a "dynamical asymmetry" is established in the sense that, after a particular time, I11 becomes greater than I33. However, a dynamically asymmetric model tends to turn over spontaneously and thus to become "oblique rotator" with its angular momentum remaining invariant. As a consequence, the turnover angle increases spontaneously up to 90° on a "turnover timescale," tTOV, since the Rotational Kinetic Energy of the model decreases from a higher level when χ 0° to a lower level when χ 90°; at this level the model becomes "perpendicular rotator" and reaches the state of least Energy consistent with its prescribed angular momentum and magnetic field. The excess Rotational Kinetic Energy due to differential rotation is totally dissipated due to the action of turbulent viscosity in the convective regions of the model. Thus, in the "turnover scenario" the casting of the roles has mainly to do with rotation, poloidal field, toroidal field, and turbulent viscosity. In the present paper, we study in detail this scenario.
