Thesis (Ph. D.)--State University of New York at Binghamton, Department of Computer Science, 2007. / Includes bibliographical references (leaves 197-211).

Γείωση ονομάζουμε την αγώγιμη σύνδεση των μεταλλικών εξαρτημάτων μιας εγκατάστασης ως προς τη γη, ώστε το δυναμικό αυτής να εξισωθεί με το δυναμικό της γης. Σκοπός ενός συστήματος γείωσης είναι να παρέχει ένα δρόμο χαμηλής αντίστασης σε ρεύματα από κεραυνούς ή σφαλμάτων προς τη γη και να μειώνει τις πιθανότητες ανάπτυξης επικίνδυνων βηματικών τάσεων ή τάσεων επαφής, καθορίζοντας ένα δυναμικό αναφοράς. Τα συστήματα γείωσης είναι πολύ σημαντικά και για αυτό το λόγο έχει μελετηθεί η συμπεριφορά τους από πολλούς ερευνητές και έχουν προταθεί διάφορα μοντέλα εξομοίωσης τους. Το σημαντικότερο κομμάτι από την μελέτη ενός συστήματος γείωσης είναι η μεταβατική κατάσταση, γιατί εκεί εμφανίζονται τα πιο επικίνδυνα φαινόμενα και οι τάσεις που εμφανίζονται παρουσιάζουν μεγαλύτερες τιμές καθώς και πιο απότομες κυματομορφές. Ένα φαινόμενο που εμφανίζεται όταν ένας κεραυνός πλήξει ένα πλέγμα γείωσης είναι ο ιονισμός του εδάφους. Το φαινόμενο αυτό παρουσιάζεται όταν το ηλεκτρικό πεδίο στις πλευρικές επιφάνειες των ηλεκτροδίων του πλέγματος ξεπεράσει το πεδίο ιονισμού του εδάφους. Αυτό δημιουργεί μη γραμμική συμπεριφορά στα συστήματα γείωσης, η οποία δεν είναι πλήρως κατανοητή ίσως εξαιτίας της εξαιρετικά σύνθετης διεργασίας, παρόλα αυτά μερικές προσεγγίσεις συνήθως υιοθετούνται όταν το φαινόμενο του ιονισμού του εδάφους συμπεριλαμβάνεται. Το φαινόμενο αυτό επηρεάζει την απόκριση των συστημάτων γείωσης. Έτσι, όταν υψηλά ρεύματα οδηγούνται στη γη, η προσομοίωση του ιονισμού του εδάφους πρέπει να είναι ένα θεμελιώδες τμήμα σε κάθε μοντέλο που έχει το χαρακτηρισμό του συστήματος γείωσης σε μεταβατικές συνθήκες. Σκοπός αυτής της εργασίας είναι η επιλογή του κατάλληλου μοντέλου για την εξομοίωση ενός πλέγματος γείωσης, συμπεριλαμβάνοντας το φαινόμενο του ιονισμού του εδάφους, και η μελέτη της μεταβατικής συμπεριφοράς του. Παρουσιάζονται τα αποτελέσματα τις εξομοίωσης ενός πλέγματος για είσοδο του ρεύματος κεραυνού σε διάφορους κόμβους του πλέγματος και συγκρίνονται μεταξύ τους. Στη συνέχεια γίνεται εξομοίωση δύο επιπλέον πλεγμάτων, μελετάται η συμπεριφορά τους για είσοδο του ρεύματος κεραυνού σε διάφορους κόμβους των πλεγμάτων και γίνεται σύγκριση μεταξύ τους. Η εργασία ολοκληρώθηκε με παρατηρήσεις επάνω στα αποτελέσματα των εξομοιώσεων και σύγκριση μεταξύ αυτών, σε ότι αφορά το σημείο εισόδου του ρεύματος κεραυνού καθώς και την επιλογή της μορφής του πλέγματος γείωσης. / Grounding, we call the conductive connection of metal elements of an installation as for the ground, in order that the potential of this is evened with the potential of the ground. Aim of a grounding system is to provide a street of low resistance in currents from lightning or faults to the ground and to decrease the probabilities of growth of dangerous incremental voltages or voltages of contact, determining a potential of report. The grounding systems are very important and for this reason have been studied their behavior from a lot of researchers and have been proposed various models of simulation. The more important piece than the study of a grounding system is the transient response, because there are presented the most dangerous phenomena and the voltages which appear present largest values as well as more abrupt curves. A phenomenon that is presented when a lightning affects a grounding grid is the soil ionization. This phenomenon could be happen when the electric field, in correspondence with the lateral surface of ground electrodes, overcomes the soil ionization gradient. This creates a no linear behavior in the grounding systems, which is not completely comprehensible perhaps because of the exceptionally complex process, nevertheless certain approaches usually is adopted when the phenomenon of ionization of ground is included. This phenomenon influences the impulse response of grounding systems. Thus, when high impulse currents are drained to earth, the simulation of soil breakdown must be a fundamental part of each model which has the characterization of grounding systems in transient conditions as the main goal. Aim of this work is the selection of a suitable model for the simulation of a grounding grid including the phenomenon of the soil ionization, and the study of its transient behavior. There are presented the results of the simulation of a grid for entry of current of lightning in various nodes of the grid and are compared from each other. Afterwards the simulation of two more grids is presented. We study their behavior for entry of current of lightning in various nodes of the grid and compare each other. The work was completed with observations on the results of the simulations and comparison between these, regarding the point of entry of current of lightning as well as the selection of the form of the grid.

Σκοπός αυτής της εργασίας είναι η εξομοίωση ενός πλέγματος γείωσης υποσταθμού μέσης τάσης και η μελέτη της μεταβατικής συμπεριφοράς του. / Grounding, is the connection of metal elements of an installation with the ground, in order that the potential of this, is evened with the potential of the ground. Aim of a grounding system is to provide a street of low resistance in currents from lightning or faults to the ground and to decrease the probabilities of growth of dangerous incremental voltages or voltages of contact, determining a potential of report. The grounding systems are very important and for this reason have been studied their behavior from a lot of researchers and have been proposed various models of simulation. The more important piece than the study of a grounding system is the transient response, because there are presented the most dangerous phenomena and the voltages which appear present largest values as well as more abrupt curves. Aim of this work is to study and simulate the earthing grid of a substation. Θere are presented the results of the simulation of an earthing grid in order for the thunder electricity to be inserted to the node where the “tower" of 150 kV is being earthed and in order for the shift circuit to be inserted to the node where the thansformer of the substation is being earthed.

Grid computing is a virtualized distributed computing environment aimed at enabling the sharing of geographically distributed resources. Grid resources have traditionally consisted of dedicated supercomputers, clusters, or storage units. With the present ubiquitous network connections and the growing computational and storage capabilities of modem everyday-use computers, more resources such as PCs, devices (e.g., PDAs and sensors), applications, and services are on grid networks. Grid is expected to evolve from a computing and data management facility to a pervasive, world-wide resource-sharing infrastructure. To fully utilize the wide range of grid resources, effective resource discovery mechanisms are required. However, resource discovery in a global-scale grid is challenging due to the considerable diversity, large number, dynamic behavior, and geographical distribution of the resources. The resource discovery technology required to achieve the ambitious global grid vision is still in its infancy, and existing applications have difficulties in achieving both rich searchability and good scalability. In this thesis, we investigate the resource discovery problem for open-networked global-scale grids. In particular, we propose a distributed semantics-based discovery framework. We show how this framework can be used to address the discovery problem in such grids and improve three aspects of performance: expressiveness, scalability, and efficiency. Expressiveness is the first characteristic that a grid resource-searching mechanism should have. Most existing search systems use simple keyword-based lookups, which limit the searchability of the system. Our framework improves search expressiveness from two directions: First, it uses a semantic metadata scheme to provide users with a rich and flexible representation mechanism, to enable effective descriptions of desired resource properties and query requirements. Second, we employ ontological domain knowledge to assist in the search process. The system is thus able to understand the semantics of query requests according to their meanings in a specific domain; this procedure helps the system to locate only semantically related results. The more expressive the resource description and query request, however, the more difficult it is to design a scalable and efficient search mechanism. We ensure scalability by reconfiguring the network with respect to shared ontologies. This reconfiguration partitions the large unorganized search space into multiple well-organized semantically related sub-spaces that we call semantic virtual organizations. Semantic virtual organizations help to discriminatively distribute resource information and queries to related nodes, thus reducing the search space and improving scalability. To further improve the efficiency of searching the virtual organizations, we propose two semantics-based resource-integrating and searching systems: GONID and OntoSum. These two systems address searching problems for applications based on different network topologies: structured and unstructured peer-to-peer overlay networks. Queries in the search systems are processed in a transparent way, so that users accessing the data can be insulated from the fact that the information is distributed across different sources and represented with different formats. In both systems, ontological knowledge is decomposed into different coarse-grained elements, and then these elements are indexed with different schemes to fit the requirements of different applications. Resource metadata reasoning, integrating, and searching are based on the index. A complex query can be evaluated by performing relational operations such as select, project, and join on combinations of the indexing elements. We evaluate the performance of our system with extensive simulation experiments, the results of which confirm the effectiveness of the design. In addition, we implement a prototype that incorporates our ontology-based virtual organization formation and semantics-based query mechanisms. Our deployment of the prototype verifies the system's feasibility and its applicability to real-world applications. / Science, Faculty of / Computer Science, Department of / Graduate

In this work, two topics of reservoir simulations are discussed. The first topic is the two-phase compositional flow simulation in hydrocarbon reservoir. The major obstacle that impedes the applicability of the simulation code is the long run time of the simulation procedure, and thus speeding up the simulation code is necessary. Two means are demonstrated to address the problem: parallelism in physical space and the application of sparse grids in parameter space. The parallel code can gain satisfactory scalability, and the sparse grids can remove the bottleneck of flash calculations. Instead of carrying out the flash calculation in each time step of the simulation, a sparse grid approximation of all possible results of the flash calculation is generated before the simulation. Then the constructed surrogate model is evaluated to approximate the flash calculation results during the simulation. The second topic is the wormhole propagation simulation in carbonate reservoir. In this work, different from the traditional simulation technique relying on the Darcy framework, we propose a new framework called Darcy-Brinkman-Forchheimer framework to simulate wormhole propagation. Furthermore, to process the large quantity of cells in the simulation grid and shorten the long simulation time of the traditional serial code, standard domain-based parallelism is employed, using the Hypre multigrid library. In addition to that, a new technique called “experimenting field approach” to set coefficients in the model equations is introduced. In the 2D dissolution experiments, different configurations of wormholes and a series of properties simulated by both frameworks are compared. We conclude that the numerical results of the DBF framework are more like wormholes and more stable than the Darcy framework, which is a demonstration of the advantages of the DBF framework. The scalability of the parallel code is also evaluated, and good scalability can be achieved. Finally, a mixed finite element scheme is proposed for the wormhole simulation.

No description available.

Several different time-integration algorithms for the Euler equations are investigated on two distributed-memory parallel computers using an explicit message-passing paradigm: these are classic Euler Explicit, four-stage Jameson-style Runge-Kutta, Block Jacobi, Block Gauss-Seidel, and Block Symmetric Gauss-Seidel. A finite-volume formulation is used for the spatial discretization of the physical domain. Both two- and three-dimensional test cases are evaluated against five reference solutions to demonstrate accuracy of the fundamental sequential algorithms. Different schemes for communicating or approximating data that are not available on the local compute node are discussed and it is shown that complete sharing of the evolving solution to the inner matrix problem at every iteration is faster than the other schemes considered. Speedup and efficiency issues pertaining to the various time-integration algorithms are then addressed for each system. Of the algorithms considered, Symmetric Block Gauss-Seidel has the overall best performance. It is also demonstrated that using parallel efficiency as the sole means of evaluating performance of an algorithm often leads to erroneous conclusions; the clock time needed to solve a problem is a much better indicator of algorithm performance. A general method for extending one-dimensional limiter formulations to the unstructured case is also discussed and applied to Van Albada’s limiter as well as Roe’s Superbee limiter. Solutions and convergence histories for a two-dimensional supersonic ramp problem using these limiters are presented along with computations using the limiters of Barth & Jesperson and Venkatakrishnan — the Van Albada limiter has performance similar to Venkatakrishnan’s. / Ph. D.

The integration of distributed generators (DGs) into the distribution grid has exacerbated voltage unbalance issues leading to greater risks of reducing equipment lifetime, equipment damages, and increased ohmic losses. Most approaches to regulating voltage in distribution systems only focus on voltage magnitude and neglect phasor discrepancies and do little to remedy voltage unbalance. To combat this, a novel Optimal Power Flow (OPF) is designed to help operate these resources in a manner that curtails voltage unbalance using the reactive power compensation capabilities of inverters. The OPF was run for a wide variety of loading conditions on a pair of systems using MATLAB and was shown to improve the voltage profile of the system in addition to minimizing losses in most cases. However, it is noted that the OPF loses exactness in highly stressed conditions and is unable to provide meaningful solutions / Master of Science / With the power grid getting greener and smarter by the day, a slew of new challenges arise to overcome. Distributed sources of energy like solar panels and batteries are being added to the grid right from the household level. While they are desirable for reducing our need for traditional sources of energy, the addition of these resources has been shown to cause issues in the quality of the power grid. This is particularly observed at the low-voltage domestic part of the grid where the resources cause issues with the voltage quality. The distribution grid is unbalanced by nature and adding these resources only amplifies this problem. To help mitigate voltage quality issues grid operators are starting to require voltage regulation capabilities from resources to be connected to the grid and a lot of work has been conducted to find the optimal strategies for operating these resources. However, existing paradigms for these sources only focus on fixing the voltage magnitude part of the power quality and neglect phasor relationships. This thesis aims to bridge this gap by developing a method to determine the optimal operation of these resources by using the voltage regulation capability to address both voltage magnitude and voltage unbalance issues in addition to optimal operation.

The goal of this thesis is to assess the impact of the smart grids implementation on PRE Distribuce, a third largest electricity distributor in the Czech Republic. The first part of the thesis presents an overview of the current energy value chain, including the division of tasks and responsibilities. Following this is the definition of the smart grid, its drivers, benefits and key barriers of its implementation. The analytical part provides a scenario analysis of the smart grid roll-out, where the first scenario assumed implementation of all technologies and components within five years and the second scenario assumes implementation in line with the depreciation of traditional components. Furthermore, the cash flow impact and NPV of the project under both scenarios is provided.

Esta dissertação tem por objetivo apresentar a proposta de uma política de escalonamento para grids computacionais. Essa política, intitulada Dynamic Max-Min2x, é orientada ao escalonamento de aplicações cujas tarefas não realizam comunicação entre si e visa a redução do tempo de resposta dessas aplicações através da utilização de atribuição dinâmica de tarefas e replicação das mesmas. Experimentos, feitos através de simulação, mostram que o tempo médio de resposta de aplicações utilizando-se a Dynamic Max-Min2x é inferior ao de outras políticas da literatura. Análises dos resultados desses experimentos apontam que esse tempo tende a ser mais atrativo principalmente quando as tarefas necessitam de muito processamento e quando há grande variação de carga no sistema, caracteristicas comuns em grids computacionais. Além disso, esta dissertação apresenta a implementação de um framework utilizando-se o Globus Toolkit, onde é possível a inserção de políticas de escalonamento para a submissão inteligente de tarefas em um grid computacional. / This Master thesis proposes a new grid scheduling policy called Dynamic Max-Min2x. This policy focuses on applications in which tasks do not communicate among themsenves and targets a response time reduction of these applications through the use of dynamic task distribution and replication techniques. Experiments, done using simulations, have shown that the response time related to Dynamic Max-Min2x is smaller than others policies found in literature. Analysis of the results have demonstrated that this time tends to become more attractive when tasks do not need much processing power and when there is a great load variation in the system, characteristics frequently found in grids. Furthermore, this thesis presents the implementation of a framework using Globus Toolkit, which makes possible the new scheduling policies insertion to provide an intelligent submission tasks in a computational grid system.