Stability-constrained design for low voltage DC microgrids

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018.Cataloged from PDF version of thesis.Includes bibliographical references (pages 91-93).Microgrids are a promising solution to reducing the energy access gap. However, microgrids are inherently fragile systems as they are not globally stable. This thesis considers two voltage instabilities that can arise in DC microgrids as a result of tightly controlled loads in the presence of inductive delays. First, we examine the instability that arises when a constant power load is added to or removed from a network where the topology is unknown. While removing the dependence of the stability certificate on the interconnection structure creates a major technical challenge, it is beneficial as it allows for ease in network modification as the needs of a community vary. We thus develop conditions on individual power sources and loads such that a network comprised of many arbitrarily-connected units will be stable. We use Brayton-Moser potential theory to develop design constraints on individual microgrid components that certify transient stability-guaranteeing that the system will return to a suitable equilibrium after a change to the overall network loading. We find that stability can be guaranteed by installing a parallel capacitor at each constant power load, and we derive an expression for the required capacitance. Second, we analyze the small-signal instabilities in microgrids containing arbitrary, rather than constant power, loads. This network representation allows for a more accurate representation of DC loads controlled by power converters which have a destabilizing negative incremental impedance over a finite frequency band in contrast to constant power loads which have a negative incremental impedance over all frequencies. While there are many established methods for the small-signal stability certification of DC networks, these methods do not explicitly account for the influence of network. In contrast, we develop a method for stability assessment of arbitrary DC grids by introducing the Augmented Power Dissipation and showing that it's positive definiteness is a sufficient condition for stability. We present an explicit expression for this quantity through load and network impedances and show how it could be directly used for stability certification of complex networks.by Kathleen Alison Cavanagh.S.M

Network Topology Invariant Stability Certificates for DC Microgrids with Arbitrary Load Dynamics

DC microgrids are prone to small-signal instabilities due to the presence of tightly regulated loads. This paper develops a decentralized stability certificate which is capable of certifying the small-signal stability of an islanded DC network containing such loads. Utilizing a novel homotopy approach, the proposed standards ensure that no system eigenmodes are able to cross into the unstable right half plane for a continuous range of controller gain levels. The resulting "standards" can be applied to variety of grid components which meet the specified, but non-unique, criteria. These standards thus take a step towards offering plug-and-play operability of DC microgrids. The proposed theorems are explicitly illustrated and numerically validated on multiple DC microgrid test-cases containing both buck and boost converter dynamics

Chemotaxis of Cell Populations through Confined Spaces at Single-Cell Resolution

Cell migration is crucial for both physiological and pathological processes. Current in vitro cell motility assays suffer from various drawbacks, including insufficient temporal and/or optical resolution, or the failure to include a controlled chemotactic stimulus. Here, we address these limitations with a migration chamber that utilizes a self-sustaining chemotactic gradient to induce locomotion through confined environments that emulate physiological settings. Dynamic real-time analysis of both population-scale and single-cell movement are achieved at high resolution. Interior surfaces can be functionalized through adsorption of extracellular matrix components, and pharmacological agents can be administered to cells directly, or indirectly through the chemotactic reservoir. Direct comparison of multiple cell types can be achieved in a single enclosed system to compare inherent migratory potentials. Our novel microfluidic design is therefore a powerful tool for the study of cellular chemotaxis, and is suitable for a wide range of biological and biomedical applications

Reperfusion therapy for ST elevation acute myocardial infarction 2010/2011: current status in 37 ESC countries

Aims Primary percutaneous coronary intervention (PPCI) is the preferred reperfusion therapy in ST-elevation myocardial infarction (STEMI). We conducted this study to evaluate the contemporary status on the use and type of reperfusion therapy in patients admitted with STEMI in the European Society of Cardiology (ESC) member countries. Methods and results A cross-sectional descriptive study based on aggregated country-level data on the use of reperfusion therapy in patients admitted with STEMI during 2010 or 2011. Thirty-seven ESC countries were able to provide data from existing national or regional registries. In countries where no such registries exist, data were based on best expert estimates. Data were collected on the use of STEMI reperfusion treatment and mortality, the numbers of cardiologists, and the availability of PPCI facilities in each country. Our survey provides a brief data summary of the degree of variation in reperfusion therapy across Europe. The number of PPCI procedures varied between countries, ranging from 23 to 884 per million inhabitants. Primary percutaneous coronary intervention and thrombolysis were the dominant reperfusion strategy in 33 and 4 countries, respectively. The mean population served by a single PPCI centre with a 24-h service 7 days a week ranged from 31 300 inhabitants per centre to 6 533 000 inhabitants per centre. Twenty-seven of the total 37 countries participated in a former survey from 2007, and major increases in PPCI utilization were observed in 13 of these countries. Conclusion Large variations in reperfusion treatment are still present across Europe. Countries in Eastern and Southern Europe reported that a substantial number of STEMI patients are not receiving any reperfusion therapy. Implementation of the best reperfusion therapy as recommended in the guidelines should be encourage

The Biodiversity of the Mediterranean Sea: Estimates, Patterns, and Threats

The Mediterranean Sea is a marine biodiversity hot spot. Here we combined an extensive literature analysis with expert opinions to update publicly available estimates of major taxa in this marine ecosystem and to revise and update several species lists. We also assessed overall spatial and temporal patterns of species diversity and identified major changes and threats. Our results listed approximately 17,000 marine species occurring in the Mediterranean Sea. However, our estimates of marine diversity are still incomplete as yet—undescribed species will be added in the future. Diversity for microbes is substantially underestimated, and the deep-sea areas and portions of the southern and eastern region are still poorly known. In addition, the invasion of alien species is a crucial factor that will continue to change the biodiversity of the Mediterranean, mainly in its eastern basin that can spread rapidly northwards and westwards due to the warming of the Mediterranean Sea. Spatial patterns showed a general decrease in biodiversity from northwestern to southeastern regions following a gradient of production, with some exceptions and caution due to gaps in our knowledge of the biota along the southern and eastern rims. Biodiversity was also generally higher in coastal areas and continental shelves, and decreases with depth. Temporal trends indicated that overexploitation and habitat loss have been the main human drivers of historical changes in biodiversity. At present, habitat loss and degradation, followed by fishing impacts, pollution, climate change, eutrophication, and the establishment of alien species are the most important threats and affect the greatest number of taxonomic groups. All these impacts are expected to grow in importance in the future, especially climate change and habitat degradation. The spatial identification of hot spots highlighted the ecological importance of most of the western Mediterranean shelves (and in particular, the Strait of Gibraltar and the adjacent Alboran Sea), western African coast, the Adriatic, and the Aegean Sea, which show high concentrations of endangered, threatened, or vulnerable species. The Levantine Basin, severely impacted by the invasion of species, is endangered as well

Effects of eight neuropsychiatric copy number variants on human brain structure

Many copy number variants (CNVs) confer risk for the same range of neurodevelopmental symptoms and psychiatric conditions including autism and schizophrenia. Yet, to date neuroimaging studies have typically been carried out one mutation at a time, showing that CNVs have large effects on brain anatomy. Here, we aimed to characterize and quantify the distinct brain morphometry effects and latent dimensions across 8 neuropsychiatric CNVs. We analyzed T1-weighted MRI data from clinically and non-clinically ascertained CNV carriers (deletion/duplication) at the 1q21.1 (n = 39/28), 16p11.2 (n = 87/78), 22q11.2 (n = 75/30), and 15q11.2 (n = 72/76) loci as well as 1296 non-carriers (controls). Case-control contrasts of all examined genomic loci demonstrated effects on brain anatomy, with deletions and duplications showing mirror effects at the global and regional levels. Although CNVs mainly showed distinct brain patterns, principal component analysis (PCA) loaded subsets of CNVs on two latent brain dimensions, which explained 32 and 29% of the variance of the 8 Cohen’s d maps. The cingulate gyrus, insula, supplementary motor cortex, and cerebellum were identified by PCA and multi-view pattern learning as top regions contributing to latent dimension shared across subsets of CNVs. The large proportion of distinct CNV effects on brain morphology may explain the small neuroimaging effect sizes reported in polygenic psychiatric conditions. Nevertheless, latent gene brain morphology dimensions will help subgroup the rapidly expanding landscape of neuropsychiatric variants and dissect the heterogeneity of idiopathic conditions

Analysis and Development of a Low-Order Wavefront Sensor for Exoplanet Detection Applications

In light of the recent high-profile planet-finding missions such as Kepler, the hunt for exoplanets has become an area of increased interest and research. In order to locate an Earth-mass planet in the habitable zone of a Sun-like star, 1010 contrast must be achieved. Various means exist for achieving this contrast; however, this assumes that the light collected by the telescope is perfect. In practice, the wavefronts collected tend not to be perfect but slightly aberrated. These aberrations decrease the achievable contrast of the imaging system. Therefore, in addition to striving to find ways to increase the contrast of images, it is necessary to actively investigate manners in which the wavefront aberrations can be sensed and corrected. This thesis explores two methods of low-order wavefront sensing (LOWFS). The first method was proposed by Olivier Guyon in 2009 and consists of reimaging the core of the focal plane electric field to a defocused sensor. This thesis demonstrates that a defocused sensor plane is required such that the even-order aberrations can be sensed, only odd aberrations can be sensed using a focused sensor. A computational analysis showed that the focused sensor was highly effective at predicting odd aberrations in an ideal environment and in the presence of noise. The defocused sensor can only predict a maximum of 8 of the 12 aberrations considered. In the presence of noise, the efficacy of the Guyon¿s defocused sensor deteriorated even more showing that it is not an effective method for sensing low-order aberrations. The second LOWFS method explored was a novel technique which utilized interferometry to extract the aberrations. The sensing equations were developed and a one-dimensional simulation performed. This simulation was able to retrieve all aberrations in the wavefront with relative accuracy, including two which were very similar to each other. This method is still in its development; however, it shows great promise for the effective estimation of low-order aberrations