29,764 research outputs found
A Local Control Approach to Voltage Regulation in Distribution Networks
This paper address the problem of voltage regulation in power distribution
networks with deep penetration of distributed energy resources (DERs) without
any explicit communication between the buses in the network. We cast the
problem as an optimization problem with the objective of minimizing the
distance between the bus voltage magnitudes and some reference voltage profile.
We present an iterative algorithm where each bus updates the reactive power
injection provided by their DER. The update at a bus only depends on the
voltage magnitude at that bus, and for this reason, we call the algorithm a
local control algorithm. We provide sufficient conditions that guarantee the
convergence of the algorithm and these conditions can be checked a priori for a
set of feasible power injections. We also provide necessary conditions
establishing that longer and more heavily loaded networks are inherently more
difficult to control. We illustrate the operation of the algorithm through case
studies involving 8-,34- and 123-bus test distribution systems.Comment: shorter version submitted to NAPS 201
Polyethylene and Polypropylene Nanocomposites based upon an Oligomerically-Modified Clay
Montmorillonite clay was modified with an oligomeric surfactant, which was then melt blended with polyethylene and polypropylene in a Brabender mixer. The morphology was characterized by X-ray diffraction and transmission electron microscopy, while thermal stability was evaluated from thermogravimetric analysis and the fire properties by cone calorimetry. The nanocomposites are best described as mixed immiscible/intercalated/delaminated systems and the reduction in peak heat release rate is about 40% at 5% inorganic clay loading
Fire properties of styrenic polymer–clay nanocomposites based on an oligomerically-modified clay
An oligomerically-modified clay has been used to fabricate nanocomposites with styrenic polymers, such as polystyrene, high-impacted polystyrene, poly(styrene-co-acrylonitrile) and acrylonitrile–butadiene–styrene by melt blending. The clay dispersion was evaluated by X-ray diffraction and bright field transmission electron microscopy. All of the nanocomposites have a mixed delaminated/intercalated structure. The fire properties of nanocomposites were evaluated by cone calorimetry and the mechanical properties were also evaluated
Thermal and Flame Properties of Polyethylene and Polypropylene Nanocomposites Based on an Oligomerically–Modified Clay
An oligomerically-modified clay was made using a surfactant which is the ammonium salt of an oligomer. The newly modified clay contains 37.5% inorganic clay and 62.5% oligomer. Polyethylene and polypropylene nanocomposites were made by melt blending the polymer with the oligomerically-modified clay in a Brabender mixer at various clay loadings. The structure of the nanocomposites was characterized by X-ray diffraction and transmission electron microscopy. Mechanical testing showed that the polyethylene nanocomposites had an enhanced Young\u27s modulus and slightly decreased elongation, while the changes for polypropylene nanocomposites are small compared with the virgin polymers. The thermal stability and flame properties were evaluated using thermogravimetric analysis and cone calorimetry, respectively. The plasticising effect of the oligomer was suppressed because of the increased inorganic content. The maximum reduction in peak heat release rate is about 40%
Styrenic Polymer Nanocomposites Based on an Oligomerically-Modified Clay with High Inorganic Content
Clay was modified with an oligomeric surfactant containing styrene and lauryl acrylate units along with a small amount of vinylbenzyl chloride to permit the formation of an ammonium salt so that this can be attached to a clay. The oligomerically-modified clay contains 50% inorganic clay, and styrenic polymer nanocomposites, including those of polystyrene (PS), high-impact polystyrene (HIPS), styrene–acrylonitrile copolymer (SAN) and acrylonitrile–butadiene–styrene (ABS), were prepared by melt blending. The morphologies of the nanocomposites were evaluated by X-ray diffraction and transmission electron microscopy. Mixed intercalated/delaminated nanocomposites were formed for SAN and ABS while largely immiscible nanocomposites were formed for PS and HIPS. The thermal stability and fire properties were evaluated using thermogravimetric analysis and cone calorimetry, respectively. The plasticization from the oligomeric surfactant was suppressed and the tensile strength and Young\u27s modulus were improved, compared to similar oligomerically-modified clays with higher organic content
Systematic treatment of displacements, strains and electric fields in density-functional perturbation theory
The methods of density-functional perturbation theory may be used to
calculate various physical response properties of insulating crystals including
elastic, dielectric, Born charge, and piezoelectric tensors. These and other
important tensors may be defined as second derivatives of the total energy with
respect to atomic-displacement, electric-field, or strain perturbations, or as
mixed derivatives with respect to two of these perturbations. The resulting
tensor quantities tend to be coupled in complex ways in polar crystals, giving
rise to a variety of variant definitions. For example, it is generally
necessary to distinguish between elastic tensors defined under different
electrostatic boundary conditions, and between dielectric tensors defined under
different elastic boundary conditions. Here, we describe an approach for
computing all of these various response tensors in a unified and systematic
fashion. Applications are presented for two materials, wurtzite ZnO and
rhombohedral BaTiO3, at zero temperature.Comment: 14 pages. Uses REVTEX macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/xfw_sys/index.htm
Spin Waves in Detwinned BaFeAs
Understanding magnetic interactions in the parent compounds of
high-temperature superconductors forms the basis for determining their role for
the mechanism of superconductivity. For parent compounds of iron pnictide
superconductors such as FeAs ( Ba, Ca, Sr), although spin
excitations have been mapped out throughout the entire Brillouin zone (BZ),
measurements were carried out on twinned samples and did not allow for a
conclusive determination of the spin dynamics. Here we use inelastic neutron
scattering to completely map out spin excitations of 100\% detwinned
BaFeAs. By comparing observed spectra with theoretical calculations, we
conclude that the spin excitations can be well described by an itinerant model
with important contributions from electronic correlations.Comment: 6 pages, 4 figures, with supplemental materia
The limits and basis of logical tolerance: Carnap’s combination of Russell and Wittgenstein
<p><i>Notes</i>: All data series were filtered by 40-yr Butterworth low-pass filter prior to statistical analysis. Differencing:</p>△<p>no difference,</p>α<p>1<sup>st</sup>difference. Significance (2-tailed):</p>∧<p>p<0.1,</p><p>*p<0.05,</p><p>**p<0.01,</p><p>***p<0.001.</p
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