22,455 research outputs found
A Policy Switching Approach to Consolidating Load Shedding and Islanding Protection Schemes
In recent years there have been many improvements in the reliability of
critical infrastructure systems. Despite these improvements, the power systems
industry has seen relatively small advances in this regard. For instance, power
quality deficiencies, a high number of localized contingencies, and large
cascading outages are still too widespread. Though progress has been made in
improving generation, transmission, and distribution infrastructure, remedial
action schemes (RAS) remain non-standardized and are often not uniformly
implemented across different utilities, ISOs, and RTOs. Traditionally, load
shedding and islanding have been successful protection measures in restraining
propagation of contingencies and large cascading outages. This paper proposes a
novel, algorithmic approach to selecting RAS policies to optimize the operation
of the power network during and after a contingency. Specifically, we use
policy-switching to consolidate traditional load shedding and islanding
schemes. In order to model and simulate the functionality of the proposed power
systems protection algorithm, we conduct Monte-Carlo, time-domain simulations
using Siemens PSS/E. The algorithm is tested via experiments on the IEEE-39
topology to demonstrate that the proposed approach achieves optimal power
system performance during emergency situations, given a specific set of RAS
policies.Comment: Full Paper Accepted to PSCC 2014 - IEEE Co-Sponsored Conference. 7
Pages, 2 Figures, 2 Table
Dynamics of a nanomechanical resonator coupled to a superconducting single-electron transistor
We present an analysis of the dynamics of a nanomechanical resonator coupled
to a superconducting single electron transistor (SSET) in the vicinity of the
Josephson quasiparticle (JQP) and double Josephson quasiparticle (DJQP)
resonances. For weak coupling and wide separation of dynamical timescales, we
find that for either superconducting resonance the dynamics of the resonator is
given by a Fokker-Planck equation, i.e., the SSET behaves effectively as an
equilibrium heat bath, characterised by an effective temperature, which also
damps the resonator and renormalizes its frequency. Depending on the gate and
drain-source voltage bias points with respect to the superconducting resonance,
the SSET can also give rise to an instability in the mechanical resonator
marked by negative damping and temperature within the appropriate Fokker-Planck
equation. Furthermore, sufficiently close to a resonance, we find that the
Fokker-Planck description breaks down. We also point out that there is a close
analogy between coupling a nanomechanical resonator to a SSET in the vicinity
of the JQP resonance and Doppler cooling of atoms by means of lasers
A Model Approximation Scheme for Planning in Partially Observable Stochastic Domains
Partially observable Markov decision processes (POMDPs) are a natural model
for planning problems where effects of actions are nondeterministic and the
state of the world is not completely observable. It is difficult to solve
POMDPs exactly. This paper proposes a new approximation scheme. The basic idea
is to transform a POMDP into another one where additional information is
provided by an oracle. The oracle informs the planning agent that the current
state of the world is in a certain region. The transformed POMDP is
consequently said to be region observable. It is easier to solve than the
original POMDP. We propose to solve the transformed POMDP and use its optimal
policy to construct an approximate policy for the original POMDP. By
controlling the amount of additional information that the oracle provides, it
is possible to find a proper tradeoff between computational time and
approximation quality. In terms of algorithmic contributions, we study in
details how to exploit region observability in solving the transformed POMDP.
To facilitate the study, we also propose a new exact algorithm for general
POMDPs. The algorithm is conceptually simple and yet is significantly more
efficient than all previous exact algorithms.Comment: See http://www.jair.org/ for any accompanying file
Oscillating epidemics in a dynamic network model: stochastic and mean-field analysis
An adaptive network model using SIS epidemic propagation with link-type-dependent link activation and deletion is considered. Bifurcation analysis of the pairwise ODE approximation and the network-based stochastic simulation is carried out, showing that three typical behaviours may occur; namely, oscillations can be observed besides disease-free or endemic steady states. The oscillatory behaviour in the stochastic simulations is studied using Fourier analysis, as well as through analysing the exact master equations of the stochastic model. By going beyond simply comparing simulation results to mean-field models, our approach yields deeper insights into the observed phenomena and help better understand and map out the limitations of mean-field models
Second-layer nucleation in coherent Stranski-Krastanov growth of quantum dots
We have studied the monolayer-bilayer transformation in the case of the
coherent Stranski-Krastanov growth. We have found that the energy of formation
of a second layer nucleus is largest at the center of the first-layer island
and smallest on its corners. Thus nucleation is expected to take place at the
corners (or the edges) rather than at the center of the islands as in the case
of homoepitaxy. The critical nuclei have one atom in addition to a compact
shape, which is either a square of i*i or a rectangle of i*(i-1) atoms, with
i>1 an integer. When the edge of the initial monolayer island is much larger
than the critical nucleus size, the latter is always a rectangle plus an
additional atom, adsorbed at the longer edge, which gives rise to a new atomic
row in order to transform the rectangle into the equilibrium square shape.Comment: 6 pages, 4 figures. Accepted version, minor change
Low temperature shape relaxation of 2-d islands by edge diffusion
We present a precise microscopic description of the limiting step for low
temperature shape relaxation of two dimensional islands in which activated
diffusion of particles along the boundary is the only mechanism of transport
allowed. In particular, we are able to explain why the system is driven
irreversibly towards equilibrium. Based on this description, we present a
scheme for calculating the duration of the limiting step at each stage of the
relaxation process. Finally, we calculate numerically the total relaxation time
as predicted by our results and compare it with simulations of the relaxation
process.Comment: 11 pages, 5 figures, to appear in Phys. Rev.
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