516 research outputs found
Electrochemical synthesis of peroxomonophosphate using boron-doped diamond anodes
A new method for the synthesis of peroxomonophosphate, based on the use of boron-doped diamond electrodes, is described. The amount of oxidant electrogenerated depends on the characteristics of the supporting media (pH and solute concentration) and on the operating conditions (temperature and current density). Results show that the pH, between values of 1 and 5, does not influence either the electrosynthesis of peroxomonophosphate or the chemical stability of the oxidant generated. Conversely, low temperatures are required during the electrosynthesis process to minimize the thermal decomposition of peroxomonophosphate and to guarantee significant oxidant concentration. In addition, a marked influence of both the current density and the initial substrate is observed. This observation can be explained in terms of the contribution of hydroxyl radicals in the oxidation mechanisms that occur on diamond surfaces. In the assays carried out below the water oxidation potential, the generation of hydroxyl radicals did not take place. In these cases, peroxomonophosphate generation occurs through a direct electron transfer and, therefore, at these low current densities lower concentrations are obtained. On the other hand, at higher potentials both direct and hydroxyl radical-mediated mechanisms contribute to the oxidant generation and the process is more efficient. In the same way, the contribution of hydroxyl radicals may also help to explain the significant influence of the substrate concentration. Thus, the coexistence of both phosphate and hydroxyl radicals is required to ensure the generation of significant amounts of peroxomonophosphoric acid
Effect of static load models on hopf bifurcation point and critical modes of power systems
This paper presents the effect of different static load models on Hopf bifurcation point and critical eigenvalues of power systems. Three most commonly used static load models are investigated thoroughly under various operating conditions and with different power system controllers. Some interesting new observations hase emerged in the damping ratio of the critical mode, especially when power system controllers are introduced for control, in the system to control Hopfbifurcations. These observations would be useful in controller design for Hopf bifurcation or oscillation control
Impact of 100-MW-scale PV plants with synchronous power controllers on power system stability in northern Chile
© 2017, The Institution of Engineering and Technology. The impact that renewable energy sources interfaced by power electronics have on power systems becomes more important as their share in the generation mix increases, thus requiring detailed analyses that take into account their dynamics and controllers. In this study, the impact of photovoltaic (PV) power plants on the power system of northern Chile is analysed. The studied plants employ a controller that allows power converters to interact with the grid like virtual synchronous generators, and their model includes the dynamics of the plant and converter controllers, as well as the dc and PV system. The presented analysis, which comprises modal analysis and time-domain simulations of large disturbances, evaluates the impact of these plants with respect to PV plants based on a conventional converter controller. Tests and validations of the proposed models and controllers are carried out for an actual PV plant connected to the power system of northern Chile, and for a higher PV penetration case. The results show the ability of PV plants formed by virtually synchronous power converters to limit frequency excursions induced by large power imbalances, and to mitigate power oscillations of the synchronous machines in the system.Peer ReviewedPostprint (author's final draft
Self-consistent scattering description of transport in normal-superconductor structures
We present a scattering description of transport in several
normal-superconductor structures. We show that the related requirements of
self-consistency and current conservation introduce qualitative changes in the
transport behavior when the current in the superconductor is not negligible.
The energy thresholds for quasiparticle propagation in the superconductor are
sensitive to the existence of condensate flow (). This dependence is
responsible for a rich variety of transport regimes, including a voltage range
in which only Andreev transmission is possible at the interfaces, and a state
of gapless superconductivity which may survive up to high voltages if
temperature is low. The two main effects of current conservation are a shift
towards lower voltages of the first peak in the differential conductance and an
enhancement of current caused by the greater availability of charge
transmitting scattering channels.Comment: 31 pages, 10 PS figures, Latex file, psfig.sty file is added. To
appear in Phys. Rev. B (Jan 97
Voltage Stability Analysis of Grid-Connected Wind Farms with FACTS: Static and Dynamic Analysis
Recently, analysis of some major blackouts and failures of power system shows that voltage instability problem has been one of the main reasons of these disturbances and networks collapse. In this paper, a systematic approach to voltage stability analysis using various techniques for the IEEE 14-bus case study, is presented. Static analysis is used to analyze the voltage stability of the system under study, whilst the dynamic analysis is used to evaluate the performance of compensators. The static techniques used are Power Flow, V–P curve analysis, and Q–V modal analysis. In this study, Flexible Alternating Current Transmission system (FACTS) devices- namely, Static Synchronous Compensators (STATCOMs) and Static Var Compensators (SVCs) - are used as reactive power compensators, taking into account maintaining the violated voltage magnitudes of the weak buses within the acceptable limits defined in ANSI C84.1. Simulation results validate that both the STATCOMs and the SVCs can be effectively used to enhance the static voltage stability and increasing network loadability margin. Additionally, based on the dynamic analysis results, it has been shown that STATCOMs have superior performance, in dynamic voltage stability enhancement, compared to SVCs
Giant Conductance Oscillations In Mesoscopic Andreev Interferometers
We analyze the electrical conductance of a two-dimensional, phase
coherent structure in contact with two superconductors, which is known to be an
oscillatory function of the phase difference between the
superconductors. It is predicted that for a metallic sample, the amplitude of
oscillation is enhanced by placing a normal barrier at the interface and that,
by tuning the strength of the barrier, can be orders of magnitude greater than
values observed in recent experiments. Giant oscillations can also be obtained
without a barrier, provided a crucial sum rule is broken. This can be achieved
by disorder induced normal scattering. In the absence of zero phase
inter-channel scattering, the conductance possesses a zero phase minimum.Comment: 4 pages of Revtex, 6 figures available on reques
The predictive power of farmers’ risk attitude measures elicited by experimental methods
Aim of study: Farmers’ behavior is shaped by their individual attitudes towards risk. Consequently, an understanding of the heterogeneous risk attitudes among farmers is key to predicting their decision-making. Therefore, there is a need for reliable methods to assess individuals’ risk attitudes. The main objective of this paper was to contribute to the existing literature about the external validity of risk attitude measures obtained with diverse experimental methods.Area of study: Irrigated agriculture in a Mediterranean climate region.Material and methods: Two different experimental methods widely applied in the agricultural sector were used to elicit farmers’ risk attitudes in a sample of irrigators in southern Spain: the Eckel and Grossman lottery-choice task and a self-assessment general risk question. We evaluated the explanatory power of both measures for the farming risk borne by farmers, using an approach based on dispersion measures of farming returns.Main results: Results revealed stability across these elicitation methods, but the study yielded no evidence of statistical correlation with the farming risk actually borne by farmers, suggesting that it may not be advisable to use these methods for directly predicting farmers’ decision-making in modeling exercises.Research highlights: The most relevant innovation of this paper was the validation approach followed, based on measures assessing the overall level of farming risk borne by individual producers, and the complementary analyses controlling for key variables that could affect farmer risk-taking
Motor Dysfunction as a Prodrome of Parkinson's Disease
BACKGROUND: Recognition of motor signs in the prodromal stage could lead to best identify populations at risk for developing Parkinson's disease (PD). OBJECTIVE: This study identified motor symptoms and signs in individuals suspected of having PD but who did not have a progressive reduction in the speed and amplitude of finger tapping or other physical signs indicative of bradykinesia. METHODS: 146 patients, who had symptoms or signs suggestive of PD, were serially evaluated by a movement disorder specialist, using the Movement Disorder Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS) Part III and video recordings. If the patients 'converted' to PD during follow-up, they were categorized as cases and compared with those who did not meet PD criteria during follow-up (non-cases). RESULTS: The 82 cases were more likely to have action dystonia or postural/action/rest tremor of a limb (OR 2.8; 95% CI 1.10-7.09; p = 0.02), a reduced blink rate at rest (OR 2.32; 95% CI 1.18-4.55; p = 0.01), anxiety (OR 8.91; 95% CI 2.55-31.1; p <  0.001), depression (OR 7.03; 95% CI 2.86-17.2; p <  0.001), or a frozen shoulder (OR 3.14; 95% CI 1.58-6.21) than the 64 'non-cases'. A reduction of the fast blink rate was common in patients who met the criteria for PD (p <  0.001). CONCLUSIONS: This study emphasizes that motor dysfunction is a component of the clinical prodrome seen in some patients with PD
An Efficient Pseudospectral Method for the Computation of the Self-force on a Charged Particle: Circular Geodesics around a Schwarzschild Black Hole
The description of the inspiral of a stellar-mass compact object into a
massive black hole sitting at a galactic centre is a problem of major relevance
for the future space-based gravitational-wave observatory LISA (Laser
Interferometer Space Antenna), as the signals from these systems will be buried
in the data stream and accurate gravitational-wave templates will be needed to
extract them. The main difficulty in describing these systems lies in the
estimation of the gravitational effects of the stellar-mass compact object on
his own trajectory around the massive black hole, which can be modeled as the
action of a local force, the self-force. In this paper, we present a new
time-domain numerical method for the computation of the self-force in a
simplified model consisting of a charged scalar particle orbiting a nonrotating
black hole. We use a multi-domain framework in such a way that the particle is
located at the interface between two domains so that the presence of the
particle and its physical effects appear only through appropriate boundary
conditions. In this way we eliminate completely the presence of a small length
scale associated with the need of resolving the particle. This technique also
avoids the problems associated with the impact of a low differentiability of
the solution in the accuracy of the numerical computations. The spatial
discretization of the field equations is done by using the pseudospectral
collocation method and the time evolution, based on the method of lines, uses a
Runge-Kutta solver. We show how this special framework can provide very
efficient and accurate computations in the time domain, which makes the
technique amenable for the intensive computations required in the
astrophysically-relevant scenarios for LISA.Comment: 15 pages, 9 figures, Revtex 4. Minor changes to match published
versio
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