900 research outputs found
Smoothing Output Fluctuations of Wind Turbines and Enhancing Power System Frequency Using Coefficient Diagram Method
Due to recent expansion of renewable energy applications, Wind Energy System (WES) is receiving much interest all over the world. However, output fluctuations of wind generators can cause network frequency variations in power systems, which can consequently decrease the power quality. This problem of output fluctuations needs to be solved for further expanding wind energy conversion into power system. On the other hand, area load change and abnormal conditions can lead to mismatches in frequency and these mismatches have to be corrected by the load frequency system. This paper therefore proposes a new load frequency control (LFC) design using Coefficient Diagram Method (CDM) in the presence of wind turbines (WT), for improving network frequency quality. The CDM technique reduces the effect of uncertainty due to governor and turbine parameters variations and load disturbance. Digital simulation performed on a single-area power system with wind turbines validates the effectiveness of the proposed scheme. Results show that, with the proposed CDM technique, the overall closed loop system performance demonstrated robustness. Performance comparisons between the proposed controller, a classical integral control and Model predictive control is carried out confirming the superiority of the proposed technique in presence of doubly fed induction generator (DFIG) WT
Critical points and supersymmetric vacua, III: String/M models
A fundamental problem in contemporary string/M theory is to count the number
of inequivalent vacua satisfying constraints in a string theory model. This
article contains the first rigorous results on the number and distribution of
supersymmetric vacua of type IIb string theories compactified on a Calabi-Yau
3-fold with flux. In particular, complete proofs of the counting formulas
in Ashok-Douglas and Denef-Douglas are given, together with van der Corput
style remainder estimates. We also give evidence that the number of vacua
satisfying the tadpole constraint in regions of bounded curvature in moduli
space is of exponential growth in .Comment: Final revision for publication in Commun. Math. Phys. Minor
corrections and editorial change
Phylogenetic lineages in the Botryosphaeriaceae
Botryosphaeria is a species-rich genus with a cosmopolitan
distribution, commonly associated with dieback and cankers of woody plants. As
many as 18 anamorph genera have been associated with Botryosphaeria,
most of which have been reduced to synonymy under Diplodia (conidia
mostly ovoid, pigmented, thick-walled), or Fusicoccum (conidia mostly
fusoid, hyaline, thin-walled). However, there are numerous conidial anamorphs
having morphological characteristics intermediate between Diplodia
and Fusicoccum, and there are several records of species outside the
Botryosphaeriaceae that have anamorphs apparently typical of
Botryosphaeria s.str. Recent studies have also linked
Botryosphaeria to species with pigmented, septate ascospores, and
Dothiorella anamorphs, or Fusicoccum anamorphs with
Dichomera synanamorphs. The aim of this study was to employ DNA
sequence data of the 28S rDNA to resolve apparent lineages within the
Botryosphaeriaceae. From these data, 12 clades are recognised. Two of
these lineages clustered outside the Botryosphaeriaceae, namely
Diplodia-like anamorphs occurring on maize, which are best
accommodated in Stenocarpella (Diaporthales), as well as an
unresolved clade including species of
Camarosporium/Microdiplodia. We recognise 10 lineages within
the Botryosphaeriaceae, including an unresolved clade
(Diplodia/Lasiodiplodia/Tiarosporella),
Botryosphaeria s.str. (Fusicoccum anamorphs),
Macrophomina, Neoscytalidium gen. nov.,
Dothidotthia (Dothiorella anamorphs), Neofusicoccum
gen. nov. (Botryosphaeria-like teleomorphs, Dichomera-like
synanamorphs), Pseudofusicoccum gen. nov., Saccharata
(Fusicoccum- and Diplodia-like synanamorphs),
“Botryosphaeria” quercuum
(Diplodia-like anamorph), and Guignardia
(Phyllosticta anamorphs). Separate teleomorph and anamorph names are
not provided for newly introduced genera, even where both morphs are known.
The taxonomy of some clades and isolates (e.g. B. mamane) remains
unresolved due to the absence of ex-type cultures
How many species of fungi are there at the tip of Africa?
Several recent studies have reviewed the extent of fungal biodiversity, and
have used these data as basis for revised estimates of species numbers based
on known numbers of plants and insects. None of these studies, however, have
focused on fungal biodiversity in South Africa. Coinciding with the 100th
anniversary of the National Collection of Fungi (PREM) in South Africa in
2005, it is thus timely to reflect on the taxonomic research that has been
conducted in South Africa over the past Century. Information is presented on
the extent of fungal collections preserved at PREM, and the associated
research publications that have largely resulted from this resource. These
data are placed in context of the known plant and insect biodiversity, and
used as basis to estimate the potential number of fungi that could be expected
in South Africa. The conservative estimate is of approximately 200 000 species
without taking into account those associated with a substantial insect
biodiversity
Effects of non-perturbatively improved dynamical fermions in QCD at fixed lattice spacing
We present results for the static inter-quark potential, lightest glueballs,
light hadron spectrum and topological susceptibility using a non-perturbatively
improved action on a lattice at a set of values of the bare
gauge coupling and bare dynamical quark mass chosen to keep the lattice size
fixed in physical units ( fm). By comparing these measurements with a
matched quenched ensemble, we study the effects due to two degenerate flavours
of dynamical quarks. With the greater control over residual lattice spacing
effects which these methods afford, we find some evidence of charge screening
and some minor effects on the light hadron spectrum over the range of quark
masses studied (). More substantial differences between
quenched and unquenched simulations are observed in measurements of topological
quantities.Comment: 53 pages, LaTeX/RevTeX, 16 eps figures; corrected clover action
expression and various typos, no results change
Antifungal actinomycetes associated with the pine bark beetle, Orthotomicus erosus, in South Africa
Actinomycete bacteria are often associated with insects that have a mutualistic association with fungi.
These bacteria are believed to be important to this insect–fungus association as they produce antibiotics
that exclude other saprophytic fungi from the immediate environment. The aim of this study was to
investigate the presence of potentially protective actinomycetes associated with Orthotomicus erosus, an
alien invasive pine bark beetle, in South Africa. This bark beetle and its relatives have an association with
Ophiostomatales species which are often the only fungi found in the bark beetle galleries. We hypothesised
that antibiotic-producing actinomycetes could be responsible for the paucity of other fungi in the galleries
by producing compounds to which the Ophiostoma spp. are tolerant. Several actinomycetes in the genus
Streptomyces and one Gordonia sp. were isolated from the beetle. Interestingly, most isolates were from
the same species as actinomycetes associated with other pine-infesting insects from other parts of the
world, including bark beetles and the woodwasp Sirex noctilio. Most actinomycetes isolated had strong
antifungal properties against the selected test fungi, including Ophiostoma ips, which is the most common
fungal symbiont of Orthotomicus erosus. Although the actinomycetes did not benefit Ophiostoma ips and
the hypothesis was not supported, their sporadic association with Orthotomicus erosus suggests that
they could have some impact on the composition of the fungal communities present in the bark beetle
galleries, which is at present poorly understood.
SIGNIFICANCE :
• Discovery of four putative undescribed Streptomyces spp. with antibiotic potential
• First record of the introduction of actinomycete bacteria with pine-infesting insects into South Africa
• Actinomycetes from South Africa group with undescribed Streptomyces spp. from pine-infesting insects
of North AmericaDST-NRF Centre of Excellence
in Tree Health Biotechnology, the National Research Foundation
(South Africa) and the University
of Pretoria.http://www.sajs.co.zaam2017Forestry and Agricultural Biotechnology Institute (FABI)GeneticsMicrobiology and Plant Patholog
Matrix elements of unstable states
Using the language of non-relativistic effective Lagrangians, we formulate a
systematic framework for the calculation of resonance matrix elements in
lattice QCD. The generalization of the L\"uscher-Lellouch formula for these
matrix elements is derived. We further discuss in detail the procedure of the
analytic continuation of the resonance matrix elements into the complex energy
plane and investigate the infinite-volume limit
Baryon Washout, Electroweak Phase Transition, and Perturbation Theory
We analyze the conventional perturbative treatment of sphaleron-induced
baryon number washout relevant for electroweak baryogenesis and show that it is
not gauge-independent due to the failure of consistently implementing the
Nielsen identities order-by-order in perturbation theory. We provide a
gauge-independent criterion for baryon number preservation in place of the
conventional (gauge-dependent) criterion needed for successful electroweak
baryogenesis. We also review the arguments leading to the preservation
criterion and analyze several sources of theoretical uncertainties in obtaining
a numerical bound. In various beyond the standard model scenarios, a realistic
perturbative treatment will likely require knowledge of the complete two-loop
finite temperature effective potential and the one-loop sphaleron rate.Comment: 25 pages, 9 figures; v2 minor typos correcte
WIMP-nucleus scattering in chiral effective theory
We discuss long-distance QCD corrections to the WIMP-nucleon(s) interactions
in the framework of chiral effective theory. For scalar-mediated WIMP-quark
interactions, we calculate all the next-to-leading-order corrections to the
WIMP-nucleus elastic cross-section, including two-nucleon amplitudes and
recoil-energy dependent shifts to the single-nucleon scalar form factors. As a
consequence, the scalar-mediated WIMP-nucleus cross-section cannot be
parameterized in terms of just two quantities, namely the neutron and proton
scalar form factors at zero momentum transfer, but additional parameters
appear, depending on the short-distance WIMP-quark interaction. Moreover,
multiplicative factorization of the cross-section into particle, nuclear and
astro-particle parts is violated. In practice, while the new effects are of the
natural size expected by chiral power counting, they become very important in
those regions of parameter space where the leading order WIMP-nucleus amplitude
is suppressed, including the so-called "isospin-violating dark matter" regime.
In these regions of parameter space we find order-of-magnitude corrections to
the total scattering rates and qualitative changes to the shape of recoil
spectra.Comment: 23 pages, 6 figures, 1 tabl
Hadron Spectroscopy: Theory and Experiment
Many new results on hadron spectra have been appearing in the past few years
thanks to improved experimental techniques and searches in new channels. New
theoretical techniques including refined methods of lattice QCD have kept pace
with these developments. Much has been learned about states made of both light
(u, d, and s) and heavy (c, b) quarks. The present review treats light-quark
mesons, glueballs, hybrids, particles with a single c or b quark, charmonium,
and bottomonium states. Some prospects for further study are noted.Comment: 29 pages, 9 figures, to be published in Journal of Physics G. Further
updating of reference
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