Design of an undervoltage load shedding scheme for the Hydro-Québec system

This paper deals with the control logic of an undervoltage, closed-loop load shedding scheme aimed at protecting the Hydro-Quebec system against long-term voltage instability. This scheme relies on a set of "if-then" rules whose parameters are determined through combinatorial optimisation, relying on the system dynamic response over a set of scenarios. Preliminary results of the above optimisation technique are given, besides a brief description of the foreseen implementation

An advanced tool for Preventive Voltage Security Assessment

peer reviewedThis paper deals with methods for the preventive assessment of voltage security with respect to contingencies. We describe a computing tool for the determination of secure operation limits, together with methods for contingency filtering. Examples from two very different real-life systems are provided. We outline extensions in the field of preventive control

Combinatorial optimization approaches to the design of load shedding schemes against voltage instability

peer reviewedThis paper proposes a methodology for the design of automatic load shedding against long-term voltage instability. In a first step, a set of training scenarios is set up, corresponding to various operating conditions and disturbances. Each scenario is analyzed to determine the minimal load shedding which stabilizes the system, with due consideration for the shedding location and delay. In a second step, the parameters of a closed-loop undervoltage load shedding scheme are determined so as to: (i) approach as closely as possible the optimal sheddings computed in the first step, over the whole set of scenarios; (ii) stabilize the system in all the unstable scenarios and (iii) shed no load in the stable ones. The corresponding optimization problem is solved using three methods : genetic algorithms, branch and bound approach and the so-called “sequential design”. A detailed example is given on the Hydro-Qu´ebec system in which load shedding is presently planned

An educational simulation tool for power system control and stability

This paper presents a Simulink-based educational tool developed for the purpose of illustrating power system control and stability notions as well as introducing students to realistic, though tractable in size, design problems. Relevant courses are taught to last-year undergraduate as well as graduate students at the University of, Liege, Belgium, and the National Technical University of Athens, Greece.. After a brief description of the corresponding curricula, the paper describes the simulation tool and gives examples of problems and assignments given to the students

Determination of optimal load shedding against voltage instability

peer reviewe

Imaging and chemically probing catalytic processes using field emission techniques: a study of NO hydrogenation on Pd and Pd–Au catalysts

Nitric oxide hydrogenation is investigated on palladium and gold–palladium alloy crystallites, i.e. the extremity of sharp tip samples aimed at modelling a single catalytic grain. Field ion microscopy and field emission microscopy are used to monitor adsorption and reaction in real time. One-dimensional atom probe and atom probe tomography are used on the same samples to unravel the surface composition of the adsorbed layers and the composition of the very first atomic layers of the Pd–Au surface. At constant NO pressure and at 450 K, the surface composition of the adsorbed layer on Pd samples shows a strong hysteresis behavior when H2 gas is varied. Under oxidizing conditions, N2O is formed via the occurrence of surface (NO)2 dimers. In the presence of Pd–Au alloys, the NO–H2 interaction comprises a simple NO dissociation causing the formation of surface NO2 species. On Pd–Au tip samples, atom probe tomography proves the occurrence of significant surface enrichment of palladium atoms in the presence of NO gas, but it is not sufficient to drift the behavior of the surface to that of pure palladium. Accordingly, external control parameters could be changed to tune the surface composition of Pd–Au catalysts and thus their activity and/or selectivity.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Morphological changes and surface intermediates during C2H2 exposure on nickel nanocrystals: surface carbide as an early stage of carbon nanotubes nucleation

We present an imaging study of the reshaping of Ni 3D single crystals when exposed to C2H2 at 873K. This has been achieved using Field Emission/Ion Microscopy techniques which provide high resolution micrographs of the apex of sharp metallic tips. The latter realistically model a single catalytic grain and reactive gases can be introduced in the analysis chamber, providing direct in situ imaging of surface reactions. When adding C2H2(g) to a H2(g) atmosphere, at 873K, the tip morphology switches from a strongly facetted form to a new equilibrium shape where step edges-containing crystal planes, like {113}, extend. We propose, in agreement with other authors’ hypothesis, that carbon preferentially adsorbs on step edges and therefore induce the extension of related crystal planes. These regions could then act as preferential sites for graphene sheets nucleation. We have then used an atom-probe apparatus, which consists in a FIM coupled with a ToF mass spectrometer, to analyze the adsorbed species and the composition of the (sub)surface during the ongoing experiment. We have associated this reshaping phenomenon with the formation of a surface carbide as NiC and NiC2 compounds were detected. We propose that the formation of this surface carbide, along with the induced morphological changes, is a prerequisite for graphene - and thus CNTs - nucleation on nickel catalysts.info:eu-repo/semantics/nonPublishe

Field Emission study of the carbon nanotubes growth mechanism

The outstanding properties of carbon nanotubes (CNTs) provide high potenti- al for numerous applications. However, these properties are tightly related to the intimate structure of the CNTs. Accurate control of this structure is mandatory but not yet achieved in the different synthesis methods. This lack of control most probably arises from an insufficient knowledge of the nucleation and growth me- chanism at the atomic level. The goal of this study is to image and chemically probe model catalysts at the nanoscale in CNTs CVD conditions, aiming at unra- veling the initial stages of the still unknown growth mechanism.info:eu-repo/semantics/nonPublishe

A new training simulator for improved voltage control of the Hydro-Québec system

This paper reports on the development of a training simulator focusing on voltage control and stability. It has been derived from a quasi steady-state simulation software already used and validated in planning and operational planning studies. The computational engine has been provided with a user interface built on the client-server architecture, allowing the simulator to run on a network of PCs in a very flexible way. New displays can be set up easily and quickly. This tool is presently used by Hydro-Québec to improve the operator’s ability to control transmission voltages. Other features, uses and benefits are reported

Complex oscillation patterns during the catalytic hydrogenation of NO2 over platinum nanosized crystals

We studied the catalytic reduction of NO2 with hydrogen on platinum model catalysts by means of field emission techniques. Atomic resolution field ion microscopy (FIM) was used to characterize Pt samples conditioned as tips. The catalytic reaction was investigated with video frequency resolution using field electron emission microscopy (FEM). Kinetic instabilities eventually merging into self-sustained periodic oscillations were observed. Data analyses revealed monomodal and bimodal oscillations as well as the coexistence of both mono-and bimodal regimes. Fourier transforms and interpeak intervals were used to characterize these different modes. We propose a mechanism that qualitatively explains the emergence of oscillations and the appearance of bimodal oscillations. © 2014 American Chemical Society.SCOPUS: ar.jinfo:eu-repo/semantics/publishe