3,932 research outputs found
Active distribution power system with multi-terminal DC links
A fast power restoration operational scheme and relevant stabilizing control is proposed for active distribution power systems with multi-terminal DC network in replacement of the conventional normal open switches. A 9-feeder benchmark distribution power system is established with a 4-terminal medium power DC system injected. The proposed power restoration scheme is based on the coordination among distributed control among relays, load switches, voltage source converters and autonomous operation of multi-terminal DC system. A DC stabilizer is proposed with virtual impedance method to damp out potential oscillation caused by constant power load terminals. The proposed system and controls are validated by frequency domain state space model and time domain case study with Matlab/Simulink
New Bounds for Restricted Isometry Constants
In this paper we show that if the restricted isometry constant of
the compressed sensing matrix satisfies then -sparse
signals are guaranteed to be recovered exactly via minimization when
no noise is present and -sparse signals can be estimated stably in the noisy
case. It is also shown that the bound cannot be substantively improved. An
explicitly example is constructed in which ,
but it is impossible to recover certain -sparse signals
Relationship between the symmetry energy and the single-nucleon potential in isospin-asymmetric nucleonic matter
In this contribution, we review the most important physics presented
originally in our recent publications. Some new analyses, insights and
perspectives are also provided. We showed recently that the symmetry energy
and its density slope at an arbitrary density
can be expressed analytically in terms of the magnitude and momentum dependence
of the single-nucleon potentials using the Hugenholtz-Van Hove (HVH) theorem.
These relationships provide new insights about the fundamental physics
governing the density dependence of nuclear symmetry energy. Using the isospin
and momentum (k) dependent MDI interaction as an example, the contribution of
different terms in the single-nucleon potential to the and
are analyzed in detail at different densities. It is shown that the
behavior of is mainly determined by the first-order symmetry
potential of the single-nucleon potential. The density
slope depends not only on the first-order symmetry potential
but also the second-order one . Both the
and at normal density are
constrained by the isospin and momentum dependent nucleon optical potential
extracted from the available nucleon-nucleus scattering data. The
especially at high density and momentum affects
significantly the , but it is theoretically poorly understood and
currently there is almost no experimental constraints known.Comment: 9 pages, 6 figures, Review paper, Contribution to the "Topical Issue"
on "Nuclear Symmetry Energy" in European Physical Journal
Improved use of wind turbine kinetic energy for system frequency support
To ensure security operation of power systems with high wind penetration, wind turbines (WTs) are required to participate system frequency control. The amount of WT kinetic energy used for system frequency control is discussed and minimum rotor speeds according to different WT operation states are defined to avoid large drop of mechanical power during WT frequency control. The effect of different power shapes of releasing the kinetic energy on system frequency support is investigated and two methods are proposed. The first one is aimed at reducing the rate of change of frequency (ROCOF) whereas the second one aimed to reduce both the ROCOF and frequency nadir. The proposed strategies not only make full use of the available kinetic energy but also lead to a smooth transition when the rotor re-accelerates. The performance of the proposed strategies is validated by simulations using Matlab/Simulink. The results indicate significant improvement on system frequency control
Transmit Diversity Assisted Space Shift Keying for Colocated and Distributed/Cooperative MIMO Elements
Space Shift Keying (SSK) modulation is a recently proposed MIMO technique, which activates only a single transmit antenna during each time slot and uses the specific index of the activated transmit antenna to implicitly convey information. Activating a single antenna is beneficial in terms of eliminating the inter-channel interference, and mitigates the peak-to-mean power ratio, while avoiding the need for synchronisation among transmit antennas. However, this benefit is achieved at a sacrifice, since the transmit diversity gain potential of the multiple transmit antennas is not fully exploited in existing SSK assisted systems. Furthermore, a high SSK throughput requires the transmitter to employ a high number of transmit antennas, which is not always practical. Hence, we propose four algorithms, namely open-loop Space Time Space Shift Keying (ST-SSK), closed-loop feedback-aided phase rotation, feedback-aided power allocation, and cooperative ST-SSK, for the sake of achieving a diversity gain. The performance improvements of the proposed schemes are demonstrated by Monte-Carlo simulations for spatially independent Rayleigh fading channels. Their robustness against channel estimation errors is also considered. We advocate the proposed ST-SSK techniques, which are capable of achieving a transmit diversity gain of about 10 dB at a BER of 10-5, at a cost of imposing a moderate throughput loss dedicated to a modest feedback overhead. Furthermore, our proposed ST-SSK scheme lends itself to efficient communication, because the deleterious effects of deep shadow fading no longer impose spatial correlation on the signals received by the antennas, which cannot be readily avoided by co-located antenna elements
Probing isospin- and momentum-dependent nuclear effective interactions in neutron-rich matter
The single-particle potentials for nucleons and hyperons in neutron-rich
matter generally depends on the density and isospin asymmetry of the medium as
well as the momentum and isospin of the particle. It further depends on the
temperature of the matter if the latter is in thermal equilibrium. We review
here the extension of a Gogny-type isospin- and momentum-dependent interaction
in several aspects made in recent years and their applications in studying
intermediate-energy heavy ion collisions, thermal properties of asymmetric
nuclear matter and properties of neutron stars. The importance of the isospin-
and momentum-dependence of the single-particle potential, especially the
momentum dependence of the isovector potential, is clearly revealed throughout
these studies.Comment: 27 pages, 19 figures, 1 table, accepted version to appear in EPJA
special volume on Nuclear Symmetry Energ
Stability analysis of large wind farms connected to weak AC networks incorporating PLL dynamics
Voltage source converter interfaced wind turbines connected to weak grids can induce system instability. A state space model is used in this paper to study the stability of large wind farms. Dynamics of the phase locked loop is integrated to the state space model. The advantage of this model is that it allows study the stability of systems with parallel wind turbines. Studies on the dynamic responses of the system show the importance of including phase locked loop as its existence can induce system instability especially under weak network conditions. This model can be used to study the stability of converter interfaced wind farms and helps to design the converter controller to ensure wind farm system stability when connected to weak grids
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