42,559 research outputs found
Hybrid HVDC for supply of power to offshore oil platforms
A HVDC hybrid system, comprising a line commutated thyristor HVDC converter and a STATCOM, is proposed in this paper for supplying power to offshore oil platforms that do not have their own generation. The proposed system combines the robust performance, low capital cost and low power loss of a line commutated HVDC converter, with the fast dynamic performance of an equivalent VSC Transmission system. The paper describes the principles and control strategies of the proposed system. PSCAD/EMTDC simulations are presented to demonstrate the robust performance of the system using case studies of various operating conditions such as black-start, load perturbations, AC fault conditions and disturbance caused by the starting of large local induction machines
Pion and Kaon Condensation at Finite Temperature and Density
In this paper, we study O(2N)-symmetric -theory at finite temperature
and density using the 2PI-1/N expansion. As specific examples, we consider pion
condensation at finite isospin chemical potential and kaon condensation at
finite chemical potential for hyper charge and isospin charge. We calculate the
phase diagrams and the quasiparticle masses for pions and kaons in the large-N
limit. It is shown that the effective potential and the gap equation can be
renormalized by using local counterterms for the coupling constant and mass
parameter, which are independent of temperature and chemical potentials.Comment: 10 pages. 7 Figures. v2: Better plots and figs. Added significant
number of refs v3: Accepted for publication in PRD. Added a figure and
improved part on renormalization as well as presentatio
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Changing the way we learn: towards agile learning and co-operation
This paper addresses the need for learning and competence development in industrial organizations. The people that enter professional organizations today are part of a gamer generation that have some or much experience with on-line games. Therefore they are more open to e-learning and in general more open to access anything on-line. At the same time industrial organizations experience a pressure on their ability to train employees faster due to the increase in complexity. We argue that games are not yet mature enough to support this training challenge as stand alone efforts. But games can support the training and competence development in a synchronized setup with other means
Mass Expansions of Screened Perturbation Theory
The thermodynamics of massless phi^4-theory is studied within screened
perturbation theory (SPT). In this method the perturbative expansion is
reorganized by adding and subtracting a mass term in the Lagrangian. We
analytically calculate the pressure and entropy to three-loop order and the
screening mass to two-loop order, expanding in powers of m/T. The truncated
m/T-expansion results are compared with numerical SPT results for the pressure,
entropy and screening mass which are accurate to all orders in m/T. It is shown
that the m/T-expansion converges quickly and provides an accurate description
of the thermodynamic functions for large values of the coupling constant.Comment: 22 pages, 10 figure
Band structure and atomic sum rules for x-ray dichroism
Corrections to the atomic orbital sum rule for circular magnetic x-ray
dichroism in solids are derived using orthonormal LMTOs as a single-particle
basis for electron band states.Comment: 7 pages, no figure
Statistical Analysis of Project Pyro Liquid Propellant Explosion Data
Statistical regression analysis of Project Pyro cryogenic propellant explosion test dat
"Nonlinear" covariance matrix and portfolio theory for non-Gaussian multivariate distributions
This paper offers a precise analytical characterization of the distribution
of returns for a portfolio constituted of assets whose returns are described by
an arbitrary joint multivariate distribution. In this goal, we introduce a
non-linear transformation that maps the returns onto gaussian variables whose
covariance matrix provides a new measure of dependence between the non-normal
returns, generalizing the covariance matrix into a non-linear fractional
covariance matrix. This nonlinear covariance matrix is chiseled to the specific
fat tail structure of the underlying marginal distributions, thus ensuring
stability and good-conditionning. The portfolio distribution is obtained as the
solution of a mapping to a so-called phi-q field theory in particle physics, of
which we offer an extensive treatment using Feynman diagrammatic techniques and
large deviation theory, that we illustrate in details for multivariate Weibull
distributions. The main result of our theory is that minimizing the portfolio
variance (i.e. the relatively ``small'' risks) may often increase the large
risks, as measured by higher normalized cumulants. Extensive empirical tests
are presented on the foreign exchange market that validate satisfactorily the
theory. For ``fat tail'' distributions, we show that an adequete prediction of
the risks of a portfolio relies much more on the correct description of the
tail structure rather than on their correlations.Comment: Latex, 76 page
Disorder-induced freezing of dynamical spin fluctuations in underdoped cuprates
We study the dynamical spin susceptibility of a correlated d-wave
superconductor (dSC) in the presence of disorder, using an unrestricted
Hartree-Fock approach. This model provides a concrete realization of the notion
that disorder slows down spin fluctuations, which eventually "freeze out". The
evolution of disorder-induced spectral weight transfer agrees qualitatively
with experimental observations on underdoped cuprate superconductors. For
sufficiently large disorder concentrations, static spin density wave (SDW)
order is created when droplets of magnetism nucleated by impurities overlap. We
also study the disordered stripe state coexisting with a dSC and compare its
magnetic fluctuation spectrum to that of the disorder-generated SDW phase.Comment: 5 pages, 4 figure
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