No description available.

Thesis (M. S.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Meliopoulos, A. P. Sakis; Committee Member: Divan, Deepakraj M.; Committee Member: Harley, Ronald G.; Committee Member: Johnson, Ellis L.; Committee Member: Taylor, David G.

The objective of the proposed research is to quantify the limits of overstressed and aging circuit breakers in terms of probability of failure and to provide guidelines to determine network reconfigurations, generator commitment, and economic dispatch strategies that account for these limits. The proposed temporary power system operating strategies address circuit breaker adequacy issues and allow overstressed breakers to be operated longer and more reliably until they are replaced with adequate equipment. The expansion of electric networks with new power sources (nuclear plants, distributed generation) results in increased short-circuit or fault currents levels. As fault currents increase, they will eventually exceed circuit breaker ratings. Circuit breakers exposed to fault currents in excess of their ratings are said to be overstressed, underrated, or inadequate. Insufficient ratings expose overstressed breakers to increased failure probabilities. Extensive common-mode outages caused by circuit breaker failures reduce the reliability of power systems. To durably avoid outages and system unreliability, overstressed breakers must eventually be replaced. Large-scale replacements of overstressed breakers cannot be completed in a short time because of budgetary limits, capital improvement schedules, and manufacturer-imposed constraints. Meanwhile, to preserve the ability of old and overstressed breakers to safely interrupt faults, short-circuit currents must be kept within the limits imposed by the ratings and the age of these breakers by using the substation reconfiguration and generator commitment strategies described in this study. The immediate benefit of the above-mentioned operating strategies is a reduction of the failure probability of overstressed breakers obtained by avoiding the interruption of currents in excess of breaker ratings. Other benefits include (i) increased network reliability, (ii) restored operating margins with respect to existing equipment, and (iii) prioritized equipment upgrades that enhance the long-term planning of power systems.

No description available.

This thesis presents the development of a novel numerical model capable of evaluating suspended sediment dynamics under breaking waves, and is based in the open source Computational Fluid Dynamics software, OpenFOAM®. The hydrodynamics were determined by solving the incompressible, Reynolds-Averaged Navier-Stokes equations for a two-phase fluid using the Finite Volume method, along with a Volume of Fluid scheme that modelled the interface between the air and water phases. A new library of five turbulence models was developed to include weakly compressible effects through the introduction of density variations in the conservation equations. This library was thoroughly evaluated against existing physical data for surf zone dynamics. A skill score was applied, based on the MSE, to rank the models, with the nonlinear k−ε performing best overall, and the k−ω predicting turbulent kinetic energy most accurately. Furthermore, the numerical model was shown to predict the near-bed hydrodynamics well, through comparison with new in-house physical data obtained in the COAST laboratory. Suspended sediment concentrations were determined using an advection-diffusion methodology, with near-bed processes modelled using a flux based approach that balances entrainment and deposition. The model was validated against existing experimental data for steady state flow conditions, as well as for regular and breaking waves. The agreement was generally good, with the results indicating that the model is capable of capturing complicated processes such as sediment plumes under plunging breakers. The validated model was applied to investigate the properties of the sediment diffusivity, which is a vital parameter in suspended sediment dynamics. In physical experiments, sediment diffusivity is commonly estimated implicitly, based on the vertical concentration profile. In this work, this approach was applied to the numerical concentration predictions, and compared with the value directly determined within the model. The estimated value was generally acceptable providing that large horizontal concentration gradients were not present, and diffusion dominated flow advection. However, near the breaking point of waves, large errors were observed at mid-depth of the water column, which strongly correlates with a region of large flow advection relative to diffusion. Therefore, when using this estimation, caution is recommended since this approach can potentially lead to substantial discrepancies.

Compared to conventional AC-circuit breakers, a DC-breaker has to act fast and force the current down to zero. Many different DC-breaker topologies are available, and this thesis is focused on the hybrid DC-breaker comprising a mechanical switch and high power semiconductors. The main part of this thesis is focused on the current commutations in the hybrid DC-breaker. The two current commutations: from the mechanical switch to the semiconductor branch, and from the semiconductor to the metal oxide varistor, have completely different characteristics. When the mechanical switch opens, the metallic contacts separate and an electric arc is formed. As the voltage across the arc is higher than the voltage across the semiconductors, the current is pushed over to the semiconductor branch. The undesired stray inductance in the loop limits the current derivative and slows down the commutation. As the contacts keep separating, the arc voltage increases and eventually all current is conducted by the semiconductor and the arc ceases. For a hybrid DC-breaker, the worst case is a solid ground fault, as the fast rising current results in high current levels and makes the commutation from the mechanical switch to the semiconductor both difficult and slow. However, the fast rise of the current can be used to enhance the commutation by using coupled inductors in the two parallel branches. When the fault current rises in the semiconductor branch, the mutual coupling of the inductors causes the current in the mechanical switch to decrease and helps the commutation. The result is that the commutation time decreases with decreasing fault impedance, and makes the solid ground fault easier to handle. The commutation from the semiconductor to the metal oxide varistor is controlled by the turn-off of the semiconductor. When the semiconductor is turned off, it pulls the current down to zero with a rather constant current derivative regardless of the surrounding circuit and the system current is taken over by the metal oxide varistor. Hence, any inductance in the commutation loop will result in an over-voltage proportional to this inductance on top of the varistor voltage. By connecting a smaller metal oxide varistor, as a snubber, close to the semiconductor, the over-voltage can be controlled and the commutation from the snubber to the metal oxide varistor will be driven by the voltage difference between the two varistors. It is shown that for a 12 kV DC-system, a possible design of the mechanical switch in the hybrid DC-breaker comprises two contact gaps in series and opens with a velocity of 11 m/s. It has been experimentally verified that when starting the commutation at 4 kA, the commutation takes less than 700 us and is over before the switch has opened 1 mm. The thesis also contains proposed designs for an 80 kV DC-breaker that can be used as a modular solution for higher system voltages. For this higher voltage, the design will be a choice of the combination between the number of contact gaps in series and the opening velocity of the mechanical switch. / Till skillnad från konventionella brytare i växelspänningssystem behöver en brytare för likström (DC) reagera fortare och tvinga ner strömmen. Det finns många olika topologier för hur man kan designa en DC-brytare, men denna avhandling fokuserar på en hybridbrytare som består av en mekanisk kontakt och halvledarkomponenter. Huvuddelen av denna avhandling fokuserar på kommuteringen av strömmar mellan hybridbrytarens grenar. Brytförloppet består av två kommuteringar: från den mekaniska kontakten till halvledarna och från halvledarna till en metalloxidvaristor och dessa två kommuteringar har helt olika karakteristik. När den mekaniska kontakten öppnar bildas en ljusbåde. Eftersom ljusbågsspänningen är högre än spänningsfallet över halvledarna, flyttas strömmen över till halvledargrenen. Den oönskade induktansen som finns i kretsen kommer begränsa kommuteringen och förlänga tiden det tar att flytta strömmen från den mekaniska kontakten till halvledarna. Tack vare att den mekaniska kontakten öppnar med hög hastighet förlängs ljusbågen och ljusbågsspänningen fortsätter att öka tills all ström flyttats över till halvledarna och ljusbågen slocknar. För hybridbrytaren är ett solitt jordfel det värsta felfallet eftersom den snabbt ökande felströmmen leder till en svårare och mer utdragen kommutering till halvledarna. Den höga strömderivatan kan dock utnyttjas genom att installera två kopplade spolar i serie med den mekaniska kontakten och halvledarna. När strömmen ökar i halvledargrenen skapar den kopplade induktansen en motspänning som leder till en minskad ström genom den mekaniska kontakten och snabbar på kommuteringen. Resultatet är att kommuteringstiden blir kortare ju snabbare felströmmen växer. Kommuteringen från halvledarna till varistorn styrs av halvledarkomponenternas karakteristik. När halvledaren stänger av tvingas strömmen ner med en näst intill konstant derivata oberoende av komponenterna i kretsen och strömmen tas över av varistorn. Den oönskade induktansen i kretsen kommer då ge upphov till en överspänning proportionell till induktansen som ökar kraven på halvledaren. Genom att installera en liten varistor nära halvledaren kan överspänningen kontrolleras och kommuteringen kommer istället drivas av spänningsskillnaden mellan de två varistorerna. För ett 12 kV likströmssystem är en möjlig design av den mekaniska kontakten att ha två kontaktgap i serie och en öppningshastighet på 11 m/s. Experiment har verifierat att om kommuteringen startar vid 4 kA tar den midre än 700 us och är avslutad innan kontakten öppnats 1 mm. Avhandlingen innehåller även förslag på hur en 80 kV brytare kan designas för att användas som en modul i system med högre spänning. I det fallet är designen en avvägning mellan antalet kontakter i serie och öppningshastigheten på kontakten. / <p>QC 20170519</p>

Nesta dissertação, discorre-se sobre manobra sincronizada de disjuntores. Entre as atividades desenvolvidas para a sua elaboração estão a análise de transitórios causados por manobras em capacitores, reatores e linhas de transmissão, estudos de sobretensão e estudos para a determinação de instantes favoráveis às manobras, sob o ponto de vista de diminuição de sobretensões. Completa a dissertação a especificação de equipamento de sincronismo que, a partir de medições em tempo real de variáveis elétricas - que interferem nas sobretensões - e de variáveis climáticas - que interferem no tempo de acionamento dos contatos dos disjuntores - calculam o tempo de atraso necessário para o fechamento elétrico em instantes mais convenientes. / The scope of this dissertation is the study of synchronized switching of circuit breakers. The transients caused by capacitor, reactor and transmission line switching are analyzed, for overvoltage evaluation and to determined the favorable time in the wave that leads to smallest overvoltages. Finally the specification of an equipment is produced. The equipment acquire, in real time, the values of electrical variables (current and voltages) the ambient temperature and oil pressure in the circuit breaker operating mechanism, voltage in the auxiliary system and assign a delay in the breaker switching time in order to control overvoltages.

Nesta dissertação, discorre-se sobre manobra sincronizada de disjuntores. Entre as atividades desenvolvidas para a sua elaboração estão a análise de transitórios causados por manobras em capacitores, reatores e linhas de transmissão, estudos de sobretensão e estudos para a determinação de instantes favoráveis às manobras, sob o ponto de vista de diminuição de sobretensões. Completa a dissertação a especificação de equipamento de sincronismo que, a partir de medições em tempo real de variáveis elétricas - que interferem nas sobretensões - e de variáveis climáticas - que interferem no tempo de acionamento dos contatos dos disjuntores - calculam o tempo de atraso necessário para o fechamento elétrico em instantes mais convenientes. / The scope of this dissertation is the study of synchronized switching of circuit breakers. The transients caused by capacitor, reactor and transmission line switching are analyzed, for overvoltage evaluation and to determined the favorable time in the wave that leads to smallest overvoltages. Finally the specification of an equipment is produced. The equipment acquire, in real time, the values of electrical variables (current and voltages) the ambient temperature and oil pressure in the circuit breaker operating mechanism, voltage in the auxiliary system and assign a delay in the breaker switching time in order to control overvoltages.

This licentiate thesis investigates the decision-making process behind the regulation of winter shipping along the coast of the northern part of Sweden, the Norrland region, in the period 1940-1975. The licentiate thesis examines two aspects of this decision-making process. First, how the regulations in the field of winter shipping were designed in the period. Second, this work examines the underlying factors behind this regulatory outcome on the premise that the regulatory design in the field was the result of an interaction between the regulating actors in the government and their political and economic institutional context. As for the first issue, it is demonstrated that the period 1940-1975 was characterised by a regulatory ambition to expand winter shipping along the coast of Norrland. This meant that the government made substantial investments in ice-breakers during the period, which gradually expanded the shipping season until the target of year-round shipping even to the northernmost ports was established in the first part of the 1970s. Accordingly, those dues for ice-breaker services proposed by several committees that investigated the issue were never introduced. Instead, government-led ice-breaking has served to compensate Norrland as a peripheral region for its relatively high transport costs. Regarding the second issue, it is showed that the decision-making process was influenced by developments at different policy levels of the government hierarchy. In the period 1940-1964, when a public authority within the maritime sector emerged and was consolidated, developments at the maritime sector level affected the decision-making process to a large extent. In turn, the period after 1964 witnessed a change in government policy towards the Norrland region as a more interventionist regional and industrial policy than earlier was implemented. This meant that the decision-making process to a larger extent was influenced by factors originating from a macro policy level. During the decision-making process, actors at both the maritime sector level and the macro level emphasized the importance of government-regulated winter shipping for the regional industrialization of the Norrland region in terms that reflected the aims and interests of their policy levels. In this respect, actors in the maritime sector pointed to the role of winter shipping as a trade policy instrument while actors who represented the interests of regional development policy and industrial policy considered the expansion of winter shipping as crucial in achieving the general ambition to create a geographically egalitarian welfare state, characterised by high levels of growth and low unemployment.

<p>This licentiate thesis investigates the decision-making process behind the regulation of winter shipping along the coast of the northern part of Sweden, the Norrland region, in the period 1940-1975. The licentiate thesis examines two aspects of this decision-making process. First, how the regulations in the field of winter shipping were designed in the period. Second, this work examines the underlying factors behind this regulatory outcome on the premise that the regulatory design in the field was the result of an interaction between the regulating actors in the government and their political and economic institutional context.</p><p>As for the first issue, it is demonstrated that the period 1940-1975 was characterised by a regulatory ambition to expand winter shipping along the coast of Norrland. This meant that the government made substantial investments in ice-breakers during the period, which gradually expanded the shipping season until the target of year-round shipping even to the northernmost ports was established in the first part of the 1970s. Accordingly, those dues for ice-breaker services proposed by several committees that investigated the issue were never introduced. Instead, government-led ice-breaking has served to compensate Norrland as a peripheral region for its relatively high transport costs.</p><p>Regarding the second issue, it is showed that the decision-making process was influenced by developments at different policy levels of the government hierarchy. In the period 1940-1964, when a public authority within the maritime sector emerged and was consolidated, developments at the maritime sector level affected the decision-making process to a large extent. In turn, the period after 1964 witnessed a change in government policy towards the Norrland region as a more interventionist regional and industrial policy than earlier was implemented. This meant that the decision-making process to a larger extent was influenced by factors originating from a macro policy level.</p><p>During the decision-making process, actors at both the maritime sector level and the macro level emphasized the importance of government-regulated winter shipping for the regional industrialization of the Norrland region in terms that reflected the aims and interests of their policy levels. In this respect, actors in the maritime sector pointed to the role of winter shipping as a trade policy instrument while actors who represented the interests of regional development policy and industrial policy considered the expansion of winter shipping as crucial in achieving the general ambition to create a geographically egalitarian welfare state, characterised by high levels of growth and low unemployment.</p>