Voltage sags are short duration of voltage reduction caused by system faults, overloads and starting of large motors. Voltage sags are the main causes of trips of various sensitive equipment. In order to understand the voltage-tolerance performance of some process control equipment, this thesis presents test results of some sensitive equipment (such as computers, AC contactors, high intensity discharge lamps and programmable logic controller) and provides their voltage tolerance curves. A number of magnitudes and durations recommended by IEC 61000- 4- 11 are used to perform the tests. With the performance information in hands, power quality requirements of different types of equipment and customer, and area of vulnerability for sensitive loads could be estimated.

A wide range of ecological communities ranging from polar terrestrial to tropical marine environments are affectedby global climate change. Over the last century, atmospheric temperature has increased by an average of 0. 60 C andis expected to rise by 1.1- 6.40C over the next 100 years. This rising temperature has increased the intensity andfrequency of weather extremes due to which a large number of species are facing risk of extinction. Studies haveshown that species existing on lower latitude are more sensitive to temperature variability compared to speciesexisting on higher latitude but temperature is increasing rapidly in higher latitude compare to lower latitude. Thisuneven distribution of temperature sensitive species and warming rate has highlighted the need for combined studiesof temperature variability and sensitiveness of species to predict how the ecosystems will respond to increasinglyvariable climate. Using a generalized Rosenzweig-MacArthur model, I explored how temperature variability andsensitivity of species will affect the extinction risks of species and how the connectance and species-richness ofecological communities will govern this response. This study showed that the risk of extinction of species mostlydepends on their sensitivity to temperature deviation from the optimum value and level of temperature variability.Among these two, sensitivity of species to temperature deviation was most prominent factor affecting extinction risk.In this study, connectance did not show any effect on mean extinction risk and time taken by a certain proportion ofspecies to reach pre-defined extinction thresholds. But, species-richness showed some effect on mean extinction riskof species. It was found that risk of extinction of species in species-rich communities was higher compared tospecies-poor communities. Species-rich communities also took shorter time before they lost 1/6 of the species. Thepresent study also suggests a possible tipping point due to increasing temperature variability in near future. In furtherstudies, different sensitivity of species at different trophic levels and the possible evolution of sensitivity of speciesshould also be consider while predicting how ecological communities will respond to changing climate in the longrun.

Since large scale unscheduled tripping of wind power generation could lead to power system stability problem. Thus network interconnection regulations become more rigid when the wind power penetration reaches a non-neglible portion of the total power generation. This thesis presents a comparison of five different low voltage ride through (LVRT) capability enhancement technologies, i.e., additional rotor resistance, DC bus chopper, crowbar on rotor, the combination of above schemes, and grid voltage support by controlling grid side converter. System simulations are performed under Digsilent environment with model and control blocks provided by the package. Additional models are developed to implement the LVRT enhancement schemes studied. A Doubly-Fed Induction Generator (DFIG) with pitch control is used to simulate different system fault scenarios with different voltage sag magnitude and duration time. Simulation results indicate that different enhancement schemes provide various levels in relieving DC bus overvoltage, rotor winding overcurrent, and overspeed problems, and the method combines all tested schemes seems to provide the best result.

Large off-shore wind farms raise the concern of widespread tripping of off-shore wind generator in the presence of system faults and corresponding voltage dips that could potentially cause system wide blackout. In this thesis an offshore wind farm and three different types of power transmission are modeled and studied using simulation software. Off-shore wind farm composed of fixed speed induction generators and HVAC interconnection, HVAC interconnection plus STATCOM and HVDC interconnections are studied. Onshore grid faults are simulated for each interconnection. Voltage tolerance curves are established to assess fault ride through capability of each interconnection and compared with different grid transmission ride through capacity required by grid operator.

When more induction generator based wind farms are integrated into the power system, the system voltage dips and stability problems may arise due to the draw of reactive power by induction generators. The power system short-circuit event induced wind turbine trips could result in power imbalance and lead to power system instability. This thesis studies the influence of two compensation techniques on the wind turbine low voltage ride-through (LVRT) capability. One of which is based on a parallel compensation by a static synchronous compensator (STATCOM), and the other one is a series compensation by a dynamic voltage restorer (DVR). In this study, Matlab tools and models are used to simulate an active-stall controlled fixed-speed induction generator connected to a power system. Two system configurations are used to simulate three phase faults and compare the improvement of wind turbine LVRT capability due to the two studied compensation techniques. Simulation results indicate that wind turbine compensated by DVR would have better LVRT performance than that by STATCOM in dealing with the low voltage situations due to system faults.

Voltage sag (dip), a sudden reduction of the voltage magnitude within a short duration in power system, is one of major concerns of power quality problems. The main reason of the increased concerns for voltage sag problems is that the losses caused by voltage sag events are high and not negligible. Reliability indices have been used for many years to quantify the effect of sustained interruptions on the electric power system. Power quality indices reflecting the severity of various power quality problems, such as flicker, harmonics, voltage swell and sag conditions, power factor, losses, electromagnetic interference, and other phenomena, are still under development. The representation and classification of voltage sags have been studied recently by standard-setting organizations. In order to find compatibility between service quality and the equipment adopted and a least cost solution for possible power quality problems, the concept of system disturbance level and equipment immunity level was proposed in IEC 61000-3-7 but without clear definitions. A novel voltage sag index based on fuzzy logic technique to quantify system disturbance and equipment immunity levels is proposed in this dissertation. This approach takes network vulnerability, equipment sensitivity and uncertainties in measuring voltage sags into account, thereby, providing meaningful information for both the utility and customers. Using the proposed method, the probabilistic distribution of system disturbances can be obtained from the single event indices of all events recorded and the probabilistic distribution of equipment sag immunity capability can be evaluated based on the device voltage sag tolerance curve. This dissertation also presents a novel framework for predicting the number of equipment disruptions due to voltage sags in a unit of time by using the disturbance and immunity levels concepts. In the proposed approach, the number of disruptions is computed by using the unreliability concept. The area of overlapping between the distributions of site disturbance and equipment immunity levels, which indicates the number of possible disruptions, is calculated based on interference theory and reliability computations. The presented methodology can be used as a planning tool to quantify the system disturbances and equipment sensitivity. It can also be used to perform cost analysis of the compatibility of equipment with an electric power system. To minimize the costs due to voltage sags, it is always a good strategy to maintain a minimum overlap between the equipment immunity level and site disturbance level to have satisfactory operation of the equipment. The tool achieved in this work can be used to perform such analyses.

Le concept de niche a été défini par Hutchinson comme un espace multidimensionnel de variables environnementales où l'espèce survie. Au cours de ce travail, je me suis intéressée à différentes limites conceptuelles et opérationnelles du concept de niche. En m'appuyant sur le cas de l'adaptation à la salinité chez le genre Artemia – branchiopode extrêmophile – nous nous sommes intéressés : (i) aux mécanismes à l'origine du maintien du polymorphisme génétique d'une large population clonale, qui m'a permis de m'interroger sur l'échelle utile à l'application du concept de niche ; (ii) à l'impact de la flore bactérienne sur l'adaptation de l'artémie aux faibles salinités, qui m'a permis d'évaluer plus globalement l'impact que les interactions biotiques peuvent avoir dans le contexte multidimensionnel de la niche ; (iii) aux effets de la plasticité et de la qualité d'habitat sur l'adaptation de l'artémie aux fortes salinités, qui pose des questions opérationnelles sur l'évaluation de l'influence de ces facteurs sur la niche. Le travail détaillé dans ce manuscrit s'appuie sur des méthodologies variées et a apporté des éléments de réponses aux problématiques posées. Tout d'abord, nous avons pu montrer que la diversité génétique d'une large population clonale était structurée par des déterminants environnementaux tels que la salinité ou la température. Ce travail a aussi montré que dans le cas d'une population asexuée, le concept de niche pouvait s'appliquer à un groupe d'individu génétiquement proche et pouvant (selon le mode de reproduction) appartenir à une lignée commune. Ensuite, nous avons montré que la niche de la flore intestinale de l'artémie facilite la digestion des algues, mais contraint leur tolérance aux faibles salinités. Il faut alors envisager que les interactions biotiques peuvent avoir différents effets (parfois même contradictoires) sur les différents axes de la niche de l'espèce focale. Enfin, nous avons apporté des solutions méthodologiques pour évaluer séparément l'impact de la plasticité et de la qualité d'habitat sur l'adaptation des artémies aux fortes salinités. Au final, nous avons apporté des solutions conceptuelles et/ou opérationnelles permettant de solidifier le concept de niche qui est une notion clé en écologie évolutive. / Hutchinson defined the niche concept as the multidimensional space of environmental variables where the specie survives. During this work, I focused on several conceptual and operational limits of this concept. Basing our work on the adaptation to salinity of the genus Artemia – an extremophile branchipod – we studied: (i) mechanisms involved in the polymorphism maintenance in a large clonal population, which asked the question of the scaling in the use of the niche concept; (ii) impact of the gut microbiota on the adaptation to low salinities, which asked the question on the impact of biotic interactions on the niche; (iii) the effects of habitat quality and phenotypic plasticity on the tolerance to high salinities, which asked operational questions on the evaluation of theses factors and there impact on the niche. The work detailed in this manuscript is based on a large variety of methodologies and helped providing elements of answers to solve the problematic. First, we showed that the important diversity found in the large clonal population was structured by environmental variables such as salinity and temperature. Therefore, in the case of a large asexual population, the niche concept can apply to a group of genetically close individual that might share (depending on the reproduction mode) common ancestry. Then, we showed that the niche of the gut microbiota, associated with Artemia for algae digestion, constrained their host tolerance to low salinities. Thus, biotic interactions may have different effects (even conflicting sometimes) on the different axes of their host niche. Finally, we provided some methodological solutions to evaluate separately the impact of plasticity and habitat quality on the adaptation of Artemia to high salinities. In the end, we provided conceptual and/or operational solutions that strengthen the evolutionary ecology key concept of the niche.