10,454 research outputs found
The Impact of Local Power Balance and Link Reliability on Blackout Risk in Heterogeneous Power Transmission Grids
Many critical infrastructures such as the power transmission grid are heterogeneous both in their basic structure and in some of their underlying characteristics, This heterogeneity can be good for system robustness if it reduces the spread of failures or bad if it adds risk or vulnerability to the system. In this paper we investigate the effect of heterogeneity in the strength of the links between parts of the system network structures, as well as the balance of local generation and demand, on the robustness of the power transmission grid using the OPA complex system model of the power transmission system. It is found that increasing or decreasing the reliability of the links between parts of the grid changes the likelihood of different size failures with neither being optimal for all sizes. Furthermore, imbalances between load and generation in the local regions further degrades the system reliability
Towards operational measures of computer security
Ideally, a measure of the security of a system should capture quantitatively the intuitive notion of ‘the ability of the system to resist attack’. That is, it should be operational, reflecting the degree to which the system can be expected to remain free of security breaches under particular conditions of operation (including attack). Instead, current security levels at best merely reflect the extensiveness of safeguards introduced during the design and development of a system. Whilst we might expect a system developed to a higher level than another to exhibit ‘more secure behaviour’ in operation, this cannot be guaranteed; more particularly, we cannot infer what the actual security behaviour will be from knowledge of such a level. In the paper we discuss similarities between reliability and security with the intention of working towards measures of ‘operational security’ similar to those that we have for reliability of systems. Very informally, these measures could involve expressions such as the rate of occurrence of security breaches (cf rate of occurrence of failures in reliability), or the probability that a specified ‘mission’ can be accomplished without a security breach (cf reliability function). This new approach is based on the analogy between system failure and security breach. A number of other analogies to support this view are introduced. We examine this duality critically, and have identified a number of important open questions that need to be answered before this quantitative approach can be taken further. The work described here is therefore somewhat tentative, and one of our major intentions is to invite discussion about the plausibility and feasibility of this new approach
Enhanced dispersion interaction between quasi-one dimensional conducting collinear structures
Recent investigations have highlighted the failure of a sum of terms
to represent the dispersion interaction in parallel metallic, anisotropic,
linear or planar nanostructures [J. F. Dobson, A. White, and A. Rubio, Phys.
Rev. Lett. 96, 073201 (2006) and references therein]. By applying a simple
coupled plasmon approach and using electron hydrodynamics, we numerically
evaluate the dispersion (non-contact van der Waals) interaction between two
conducting wires in a collinear pointing configuration. This case is compared
to that of two insulating wires in an identical geometry, where the dispersion
interaction is modelled both within a pairwise summation framework, and by
adding a pinning potential to our theory leading to a standard oscillator-type
model of insulating dielectric behavior. Our results provide a further example
of enhanced dispersion interaction between two conducting nanosystems compared
to the case of two insulating ones. Unlike our previous work, this calculation
explores a region of relatively close coupling where, although the electronic
clouds do not overlap, we are still far from the asymptotic region where a
single power law describes the dispersion energy. We find that strong
differences in dispersion attraction between metallic and semiconducting /
insulating cases persist into this non-asymptotic region. While our theory will
need to be supplemented with additional short-ranged terms when the electronic
clouds overlap, it does not suffer from the short-distance divergence exhibited
by purely asymptotic theories, and gives a natural saturation of the dispersion
energy as the wires come into contact.Comment: 10 pages, 5 figures. Added new extended numerical calculations, new
figures, extra references and heavily revised tex
Local Analysis of Inverse Problems: H\"{o}lder Stability and Iterative Reconstruction
We consider a class of inverse problems defined by a nonlinear map from
parameter or model functions to the data. We assume that solutions exist. The
space of model functions is a Banach space which is smooth and uniformly
convex; however, the data space can be an arbitrary Banach space. We study
sequences of parameter functions generated by a nonlinear Landweber iteration
and conditions under which these strongly converge, locally, to the solutions
within an appropriate distance. We express the conditions for convergence in
terms of H\"{o}lder stability of the inverse maps, which ties naturally to the
analysis of inverse problems
Characteristics and Risk of Microgrid Outages from a Complex Systems Point of View
Cordova is a town of approximately 2,000 people located on the southern coast of Alaska. A power grid for a town this size, with a large seasonal fishing economy, is considered a moderate to large sized microgrid in terms of power produced. Understanding the vulnerabilities and risks of failures in such a grid is important for planning and operations. Investigating these characteristics in the context of complex system dynamics is a novel approach. The analysis of Cordova’s microgrid is a case study relevant to a large class of microgrid communities. We analyze the outage data based on size, cause characteristics and load demand on the system and find long time correlations and power laws in the failure size distributions. Finally we apply a risk metric to give a single numerical value to the risk of an outage occurring during certain time periods and under certain conditions
Does size matter?
Failures of the complex infrastructures society depends on having enormous human and economic cost that poses the question: Are there ways to optimize these systems to reduce the risks of failure? A dynamic model of one such system, the power transmission grid, is used to investigate the risk from failure as a function of the system size. It is found that there appears to be optimal sizes for such networks where the risk of failure is balanced by the benefit given by the size
Revenue divergence and competitive balance in a divisional sports league
The North American model of resource allocation in professional sports leagues is adapted for English (association) football. The theoretical relationship between revenue and competitive balance is shown to be robust with respect to changes in teams’ objectives and labour market conditions. Empirical revenue functions are reported for 1926-1999. These indicate a shift in the composition of demand favouring big-city teams and an increase in the sensitivity of revenue to performance. An analysis of match results in the FA Cup competition suggests an increase in competitive imbalance between teams at different levels of the league’s divisional hierarchy, as the theory suggests
Time-dependent density functional theory beyond the adiabatic local density approximation
In the current density functional theory of linear and nonlinear
time-dependent phenomena, the treatment of exchange and correlation beyond the
level of the adiabatic local density approximation is shown to lead to the
appearance of viscoelastic stresses in the electron fluid. Complex and
frequency-dependent viscosity/elasticity coefficients are microscopically
derived and expressed in terms of properties of the homogeneous electron gas.
As a first consequence of this formalism, we provide an explicit formula for
the linewidths of collective excitations in electronic systems.Comment: RevTeX, 4 page
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