2,590 research outputs found
Session 12: \u3cem\u3eAnalysis of State and Parameter Estimation Techniques using Dynamic Perturbation Signals\u3c/em\u3e
The trend in electric power systems is the displacement of traditional synchronous generation (e.g., coal, natural gas) with renewable energy resources (e.g., wind, solar photovoltaic) and battery energy storage. These energy resources require power electronic converters (PECs) to interconnect to the grid and have different response characteristics and dynamic stability issues compared to conventional synchronous generators. As a result, there is a need for validated models to study and mitigate PEC-based stability issues, especially for converter dominated power systems (e.g., island power systems, remote microgrids).
This presentation will introduce methods related to dynamic state and parameter estimation via the design of active perturbation signals for converter dominated power systems
The R-Process Alliance: A Comprehensive Abundance Analysis of HD 222925, a Metal-Poor Star with an Extreme R-Process Enhancement of [Eu/H] = -0.14
We present a detailed abundance analysis of the bright (V = 9.02), metal-poor
([Fe/H] = -1.47 +/- 0.08) field red horizontal-branch star HD 222925, which was
observed as part of an ongoing survey by the R-Process Alliance. We calculate
stellar parameters and derive abundances for 46 elements based on 901 lines
examined in a high-resolution optical spectrum obtained using the Magellan
Inamori Kyocera Echelle spectrograph. We detect 28 elements with 38 <= Z <= 90;
their abundance pattern is a close match to the Solar r-process component. The
distinguishing characteristic of HD 222925 is an extreme enhancement of
r-process elements ([Eu/Fe] = +1.33 +/- 0.08, [Ba/Eu] = -0.78 +/- 0.10) in a
moderately metal-poor star, so the abundance of r-process elements is the
highest ([Eu/H] = -0.14 +/- 0.09) in any known r-process-enhanced star. The
abundance ratios among lighter (Z <= 30) elements are typical for metal-poor
stars, indicating that production of these elements was dominated by normal
Type II supernovae, with no discernible contributions from Type Ia supernovae
or asymptotic giant branch stars. The chemical and kinematic properties of HD
222925 suggest it formed in a low-mass dwarf galaxy, which was enriched by a
high-yield r-process event before being disrupted by interaction with the Milky
Way.Comment: Accepted for publication in the Astrophysical Journal (17 pages, 4
figures, 3 tables
Optimizing Grid Resilience: A Capacity Reserve Market for High Impact Low Probability Events
This paper addresses the challenges of high-impact low-probability (HILP)
events by proposing a novel capacity reserve event market for mobile generation
assets, aimed at supporting the transmission network during such incidents.
Despite the usefulness of portable generators and mobile energy units in
restoring power, there are drawbacks such as environmental impact, finite
operation, and complex cost recovery. The proposed market integrates these
resources into a dispatch framework based on pre-established contracts,
ensuring fair compensation and considering factors like capacity, pricing, and
travel distance. Resource owners receive advanced notifications for potential
events, allowing them to adjust their bids for cost recovery. Simulations on an
IEEE 30-bus case have been conducted to demonstrate the model effectiveness in
increasing grid resiliency
Functional plasticity in the type IV secretion system of Helicobacter pylori.
Helicobacter pylori causes clinical disease primarily in those individuals infected with a strain that carries the cytotoxin associated gene pathogenicity island (cagPAI). The cagPAI encodes a type IV secretion system (T4SS) that injects the CagA oncoprotein into epithelial cells and is required for induction of the pro-inflammatory cytokine, interleukin-8 (IL-8). CagY is an essential component of the H. pylori T4SS that has an unusual sequence structure, in which an extraordinary number of direct DNA repeats is predicted to cause rearrangements that invariably yield in-frame insertions or deletions. Here we demonstrate in murine and non-human primate models that immune-driven host selection of rearrangements in CagY is sufficient to cause gain or loss of function in the H. pylori T4SS. We propose that CagY functions as a sort of molecular switch or perhaps a rheostat that alters the function of the T4SS and "tunes" the host inflammatory response so as to maximize persistent infection
Observational Constraints on First-Star Nucleosynthesis. I. Evidence for Multiple Progenitors of CEMP-no Stars
We investigate anew the distribution of absolute carbon abundance, (C) (C), for carbon-enhanced metal-poor (CEMP) stars in the halo of
the Milky Way, based on high-resolution spectroscopic data for a total sample
of 305 CEMP stars. The sample includes 147 CEMP- (and CEMP-r/s) stars, 127
CEMP-no stars, and 31 CEMP stars that are unclassified, based on the currently
employed [Ba/Fe] criterion. We confirm previous claims that the distribution of
(C) for CEMP stars is (at least) bimodal, with newly determined peaks
centered on (C) (the high-C region) and (C) (the low-C
region). A very high fraction of CEMP- (and CEMP-r/s) stars belong to the
high-C region, while the great majority of CEMP-no stars reside in the low-C
region. However, there exists complexity in the morphology of the (C)-[Fe/H]
space for the CEMP-no stars, a first indication that more than one class of
first-generation stellar progenitors may be required to account for their
observed abundances. The two groups of CEMP-no stars we identify exhibit
clearly different locations in the (Na)-(C) and (Mg)-(C) spaces,
also suggesting multiple progenitors. The clear distinction in (C) between
the CEMP- (and CEMP-) stars and the CEMP-no stars appears to be $as\
successfullikely\ more\ astrophysically\ fundamental$, for the
separation of these sub-classes as the previously recommended criterion based
on [Ba/Fe] (and [Ba/Eu]) abundance ratios. This result opens the window for its
application to present and future large-scale low- and medium-resolution
spectroscopic surveys.Comment: 26pages, 7 figures, and 3 Tables ; Accepted for publication in ApJ;
added more data and corrected minor inconsistencies existed in the compiled
data of the previous studie
The Nuclear Ionized Gas in the Radio Galaxy M84 (NGC 4374)
We present optical images of the nucleus of the nearby radio galaxy M84 (NGC
4374 = 3C272.1) obtained with the Wide Field/Planetary Camera 2 (WFPC2) aboard
the Hubble Space Telescope (HST). Our three images cover the H + [N II]
emission lines as well as the V and I continuum bands. Analysis of these images
confirms that the H + [N II] emission in the central 5'' (410 pc) is
elongated along position angle (P.A.) \approx 72\arcdeg, which is roughly
parallel to two nuclear dust lanes.Our high-resolution images reveal that the
H + [N II] emission has three components, namely a nuclear gas disk,an
`ionization cone', and outer filaments. The nuclear disk of ionized gas has
diameter pc and major axis P.A. \approx 58\arcdeg \pm
6\arcdeg. On an angular scale of 0\farcs5, the major axis of this nuclear
gas disk is consistent with that of the dust. However, the minor axis of the
gas disk (P.A. \approx 148\arcdeg) is tilted with respect to that of the
filamentary H + [N II] emission at distances > 2'' from the nucleus;
the minor axis of this larger scale gas is roughly aligned with the axis of the
kpc-scale radio jets (P.A. \approx 170\arcdeg). The ionization cone (whose
apex is offset by \approx 0\farcs3 south of the nucleus) extends 2'' from the
nucleus along the axis of the southern radio jet. This feature is similar to
the ionization cones seen in some Seyfert nuclei, which are also aligned with
the radio axes.Comment: 11 pages plus 4 figure
Virtual Inertia: Current Trends and Future Directions
The modern power system is progressing from a synchronous machine-based system towards an inverter-dominated system, with large-scale penetration of renewable energy sources (RESs) like wind and photovoltaics. RES units today represent a major share of the generation, and the traditional approach of integrating them as grid following units can lead to frequency instability. Many researchers have pointed towards using inverters with virtual inertia control algorithms so that they appear as synchronous generators to the grid, maintaining and enhancing system stability. This paper presents a literature review of the current state-of-the-art of virtual inertia implementation techniques, and explores potential research directions and challenges. The major virtual inertia topologies are compared and classified. Through literature review and simulations of some selected topologies it has been shown that similar inertial response can be achieved by relating the parameters of these topologies through time constants and inertia constants, although the exact frequency dynamics may vary slightly. The suitability of a topology depends on system control architecture and desired level of detail in replication of the dynamics of synchronous generators. A discussion on the challenges and research directions points out several research needs, especially for systems level integration of virtual inertia systems
A modeling case for high atmospheric oxygen concentrations during the Mesozoic and Cenozoic
Changes in atmospheric oxygen concentration over Earth history are commonly related to the evolution of animals and plants. But there is no direct geochemical proxy for O2 levels, meaning that estimations rely heavily on modeling approaches. The results of such studies differ greatly, to the extent that today's atmospheric mixing ratio of 21% might be either the highest or lowest level during the past 200 m.y. Long-term oxygen sources, such as the burial in sediments of reduced carbon and sulfur species, are calculated in models by representation of nutrient cycling and estimation of productivity, or by isotope mass balance (IMB)—a technique in which burial rates are inferred in order to match known isotope records. Studies utilizing these different techniques produce conflicting estimates for paleoatmospheric O2, with nutrient-weathering models estimating concentrations close to, or above, that of the present day, and IMB models estimating low O2, especially during the Mesozoic. Here we re-assess the IMB technique using the COPSE biogeochemical model. IMB modelling is confirmed to be highly sensitive to assumed carbonate δ13C, and when this input is defined following recent compilations, predicted O2 is significantly higher and in reasonable agreement with that of non-IMB techniques. We conclude that there is no model-based support for low atmospheric oxygen concentrations during the past 200 m.y. High Mesozoic O2 is consistent with wildfire records and the development of plant fire adaptions, but links between O2 and mammal evolution appear more tenuous
The R-Process Alliance: Chemical Abundances for a Trio of R-Process-Enhanced Stars -- One Strong, One Moderate, One Mild
We present detailed chemical abundances of three new bright (V ~ 11),
extremely metal-poor ([Fe/H] ~ -3.0), r-process-enhanced halo red giants based
on high-resolution, high-S/N Magellan/MIKE spectra. We measured abundances for
20-25 neutron-capture elements in each of our stars. J1432-4125 is among the
most r-process rich r-II stars, with [Eu/Fe]= +1.44+-0.11. J2005-3057 is an r-I
star with [Eu/Fe] = +0.94+-0.07. J0858-0809 has [Eu/Fe] = +0.23+-0.05 and
exhibits a carbon abundance corrected for evolutionary status of [C/Fe]_corr =
+0.76, thus adding to the small number of known carbon-enhanced r-process
stars. All three stars show remarkable agreement with the scaled solar
r-process pattern for elements above Ba, consistent with enrichment of the
birth gas cloud by a neutron star merger. The abundances for Sr, Y, and Zr,
however, deviate from the scaled solar pattern. This indicates that more than
one distinct r-process site might be responsible for the observed
neutron-capture element abundance pattern. Thorium was detected in J1432-4125
and J2005-3057. Age estimates for J1432-4125 and J2005-3057 were adopted from
one of two sets of initial production ratios each by assuming the stars are
old. This yielded individual ages of 12+-6 Gyr and 10+-6 Gyr, respectively.Comment: 30 pages, includes a long table, 5 figure
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