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abstract: This dissertation presents a new hybrid fault current limiter (FCL) topology that is primarily intended to protect single-phase power equipment. It can however be extended to protect three phase systems but would need three devices to protect each individual phase. In comparison against the existing fault current limiter technology, the salient fea-tures of the proposed topology are: a) provides variable impedance that provides a 50% reduction in prospective fault current; b) near instantaneous response time which is with-in the first half cycle (1-4 ms); c) the use of semiconductor switches as the commutating switch which produces reduced leakage current, reduced losses, improved reliability, and a faster switch time (ns-µs); d) zero losses in steady-state operation; e) use of a Neodym-ium (NdFeB) permanent magnet as the limiting impedance which reduces size, cost, weight, eliminates DC biasing and cooling costs; f) use of Pulse Width Modulation (PWM) to control the magnitude of the fault current to a user's desired level. g) experi-mental test system is developed and tested to prove the concepts of the proposed FCL. This dissertation presents the proposed topology and its working principle backed up with numerical verifications, simulation results, and hardware implementation results. Conclu-sions and future work are also presented. / Dissertation/Thesis / Ph.D. Electrical Engineering 2013

For effective operation of microgrid systems (MGSs), it is important to understand the major types of power grid failures and how to deal with them. Detecting the fault, locating it, and isolating the faulty line are important to avoid damaging components and interrupting the service for customers. This will also improve the reliability and protection level of the system during fault conditions. Among the most successful protection methods to limit fault currents in power systems is the fault current limiter (FCL). The FCL improves the reliability of the system, voltage stability, and the fault current reduction. However, limited researches consider its applications inMGSs. The location and impedance size of the FCL play a major role in limiting fault currents in the system. Several studies concluded that installing FCLs near all generators, transformers, or loads in the system enhanced the performance of the system during fault conditions. However, increasing the number of FCLs in the system leads to an increase in cost. This dissertation proposes several effective approaches to specify the optimal locations and impedance values of the required number of installed FCLs in a grid-connected MGS. These FCLs improve the reliability and the protection level of the system by limiting fault currents during fault conditions. The goal is to reduce the required number of installed FCLs in the system. These installed FCLs must be able to reduce fault currents under the interrupting ratings of circuit breakers in the system. This goal will lead to lower the cost of installed protection devices in the system. In order to achieve this goal, this dissertation presents a novel fault management approach, sensitivity analysis, and an optimization model to find the optimal solutions. The study of this dissertation is meant to be used during the planning stage of power distribution system design. The results of this dissertation prove the robustness of the proposed approaches. This enhances the system’s performance while minimizing the required number of installed FCLs. Their sizes limit fault currents within safe ranges. Thus, the FCL significantly improves the reliability and protection scheme of the grid-connected MGS.

Limitadores de corrente supercondutores resistivos (LCSR) são dispositivos com características elétricas próximas ao ideal quando ocorre a transição do estado supercondutor para o estado normal, limitando a corrente de falta através da inserção rápida de uma resistência na rede. A viabilidade técnica e econômica destes limitadores aumentou após o desenvolvimento dos supercondutores de alta temperatura crítica HTS. A primeira geração de fitas HTS (BSCCO) consiste em multifilamentos de material supercondutor embutidos em uma matriz de prata. A densidade de corrente crítica típica é de aproximadamente 140 A/mm2. Contudo, a resistência alcançada pelo LCSR quando atinge o estado normal não é suficiente para limitar a corrente de falta, sendo necessários longos comprimentos de fitas (~5 km) para que a corrente seja limitada. O recente desenvolvimento de fitas supercondutoras de YBCO com filmes finos texturizados e com substrato de alta resistividade superou o desempenho das fitas de BSCCO para esta aplicação. Com uma densidade de corrente crítica de 136 A/mm2, as fitas de YBCO são bem conhecidas pelo seu alto valor de índice n (aproximadamente 30), rápida resposta de transição após falta e alta resistência elétrica no estado normal. Neste trabalho foram analisados o comportamento elétrico e magnético das fitas de BSCCO e YBCO (em amostras curtas de 0,2 m) utilizando diferentes técnicas de caracterização das propriedades das fitas HTS. A partir destes resultados, medidas com pulsos de corrente DC e AC com intensidades de 3 a 7 vezes a corrente crítica do sistema (I=720 A a 1700 A) com duração de 1 a 5 ciclos da rede em 60 Hz (16 a 80 ms) foram realizados de forma a obter as características do tempo de recuperação em um elemento contendo 4 fitas em paralelo (0,4 m). Os resultados permitiram o projeto, construção e medidas em corrente AC de um protótipo LCSR (Ipico = 2 kA) contendo 16 elementos sob condições normais de operação da rede (220 V - 60 Hz). / Resistive Superconducting fault current limiter (SFCL) are devices with electrical behavior near the ideal when it changes its state from the superconducting to the normal state, limiting the fault current by the insertion of a fast transition resistance in the grid. The technical and economical feasibility of these limiters arose after the development of the high critical temperature superconductors HTS. First generation HTS tapes (BSCCO) consist of multifilamentary composite tapes embedded in a silver matrix. The typical critical current density is approximately 14 kA/cm2. However, the resistance reached by the SFCL when normal state occurs is not high enough to limit the fault current, making necessary long lengths of tapes (~5 km) for limiting purposes. The recently development of coated conductors composites with high resistivity metal substrate have succeeded the BSCCO tapes which are based on YBCO textured film. Upon carrying a critical current density of 13.6 kA/cm2, YBCO tapes are well known for their high n-index value (approximately 30), fast transition response after fault, and high electrical resistance in the normal state. In this work, we will present the electrical and magnetic performance of BSCCO and YBCO tapes (short samples of 0.2 m) using several techniques to characterize the properties of the HTS tapes. It was also performed analysis with DC and AC currents peaks with strength of 3 to 7 times the critical current (I=720 A to 1700 A) lasting 1 to 5 cycles (16 to 80 ms) in order to verify the recovery characteristics in an element with 4 tapes in a parallel connection. And with those characteristics we designed and constructed and measured in AC current a SFCL prototype (Ipeak = 2 kA) consisting in 16 elements to use in the grid (220 V - 60 Hz) under operational conditions.

Limitadores de corrente supercondutores resistivos (LCSR) são dispositivos com características elétricas próximas ao ideal quando ocorre a transição do estado supercondutor para o estado normal, limitando a corrente de falta através da inserção rápida de uma resistência na rede. A viabilidade técnica e econômica destes limitadores aumentou após o desenvolvimento dos supercondutores de alta temperatura crítica HTS. A primeira geração de fitas HTS (BSCCO) consiste em multifilamentos de material supercondutor embutidos em uma matriz de prata. A densidade de corrente crítica típica é de aproximadamente 140 A/mm2. Contudo, a resistência alcançada pelo LCSR quando atinge o estado normal não é suficiente para limitar a corrente de falta, sendo necessários longos comprimentos de fitas (~5 km) para que a corrente seja limitada. O recente desenvolvimento de fitas supercondutoras de YBCO com filmes finos texturizados e com substrato de alta resistividade superou o desempenho das fitas de BSCCO para esta aplicação. Com uma densidade de corrente crítica de 136 A/mm2, as fitas de YBCO são bem conhecidas pelo seu alto valor de índice n (aproximadamente 30), rápida resposta de transição após falta e alta resistência elétrica no estado normal. Neste trabalho foram analisados o comportamento elétrico e magnético das fitas de BSCCO e YBCO (em amostras curtas de 0,2 m) utilizando diferentes técnicas de caracterização das propriedades das fitas HTS. A partir destes resultados, medidas com pulsos de corrente DC e AC com intensidades de 3 a 7 vezes a corrente crítica do sistema (I=720 A a 1700 A) com duração de 1 a 5 ciclos da rede em 60 Hz (16 a 80 ms) foram realizados de forma a obter as características do tempo de recuperação em um elemento contendo 4 fitas em paralelo (0,4 m). Os resultados permitiram o projeto, construção e medidas em corrente AC de um protótipo LCSR (Ipico = 2 kA) contendo 16 elementos sob condições normais de operação da rede (220 V - 60 Hz). / Resistive Superconducting fault current limiter (SFCL) are devices with electrical behavior near the ideal when it changes its state from the superconducting to the normal state, limiting the fault current by the insertion of a fast transition resistance in the grid. The technical and economical feasibility of these limiters arose after the development of the high critical temperature superconductors HTS. First generation HTS tapes (BSCCO) consist of multifilamentary composite tapes embedded in a silver matrix. The typical critical current density is approximately 14 kA/cm2. However, the resistance reached by the SFCL when normal state occurs is not high enough to limit the fault current, making necessary long lengths of tapes (~5 km) for limiting purposes. The recently development of coated conductors composites with high resistivity metal substrate have succeeded the BSCCO tapes which are based on YBCO textured film. Upon carrying a critical current density of 13.6 kA/cm2, YBCO tapes are well known for their high n-index value (approximately 30), fast transition response after fault, and high electrical resistance in the normal state. In this work, we will present the electrical and magnetic performance of BSCCO and YBCO tapes (short samples of 0.2 m) using several techniques to characterize the properties of the HTS tapes. It was also performed analysis with DC and AC currents peaks with strength of 3 to 7 times the critical current (I=720 A to 1700 A) lasting 1 to 5 cycles (16 to 80 ms) in order to verify the recovery characteristics in an element with 4 tapes in a parallel connection. And with those characteristics we designed and constructed and measured in AC current a SFCL prototype (Ipeak = 2 kA) consisting in 16 elements to use in the grid (220 V - 60 Hz) under operational conditions.

[pt] O primeiro material supercondutor de alta temperatura foi descoberto em 1986. Desde então foram sintetizados mais de 150 novos supercondutores (cupratos, bismutatos, boretos e fulerenos) com temperatura crítica superior à temperatura de 23,3 K, a temperatura crítica mais alta entre os materiais supercondutores de baixa temperatura. Muitos destes novos materiais supercondutores têm provocado interesse acadêmico, sendo utilizados, por exemplo, para a compreensão do fenômeno de supercondutividade a alta temperatura. No entanto, há famílias supercondutoras que possuem propriedades atraentes para o desenvolvimento de novas tecnologias voltadas particularmente para sistemas elétricos de potência, medicina e transporte (veículo Maglev). Dois supercondutores da família BSCCO, Bi2Sr2CaCu2O8+x (Bi-2212) e Bi2Sr2Ca2Cu3O10+x (Bi-2223), estão entre os mais pesquisados para aplicação em escala industrial no setor elétrico. Uma projeção do mercado de dispositivos para o setor elétrico, à base de supercondutores de alta temperatura, aponta que este mercado deve movimentar em torno de 47 bilhões de dólares em 2020, tendo como destaques, com quase 90% deste mercado, limitadores de corrente de curtocircuito, transformadores e SMES (Sistema Magnético Supercondutor de Armazenagem de Energia). A presente tese se propôs a contribuir para o desenvolvimento de uma nova tecnologia de produção de limitadores de corrente de curto-circuito (LCC) à base da fase supercondutora Bi-2212. Esta tecnologia baseia-se na produção de formas maciças (blocos) da fase Bi-2212, para a confecção de LCC, através da fusão parcial e solidificação peritéticas desta fase. Este método difere substancialmente do método de fusão completa, usualmente empregado para a obtenção destes blocos. Para o desenvolvimento e o aperfeiçoamento desta nova tecnologia foi necessário estudar a fusão e a solidificação peritéticas da fase Bi- 2212. Com este intuito, para acompanhar esses dois processos foram utilizadas técnicas de observação in situ, assim como análise de amostras temperadas. As propriedades supercondutoras foram estudadas por meio das caracterizações eletromagnéticas. Com base nestes estudos foi desenvolvido um ciclo térmico que proporciona uma densidade de corrente crítica da fase Bi-2212 na forma maciça superior a 1000A/cm2, em corrente direta, a 77K e sem campo magnético externo. Este valor confirma a viabilidade do método para a produção de material para LCC. / [en] High temperature superconductivity was discovered in 1986. Since then more than 150 new superconductors (cuprates, bismuthates, borides and fullerite) with critical temperature higher then 23,3K (the highest critical temperature for low temperature superconductors) have been synthesized. Many of them rise interest from the scientific viewpoint and are suitable for investigations focusing the phenomena of high temperature superconductivity. However, some superconducting families display properties that are attractive for applications in electrical power systems, medicine and transport. Two superconductors from the BSCCO family, Bi2Sr2CaCu2O8+x (Bi-2212) e Bi2Sr2Ca2Cu3O10+x (Bi-2223), are among the most studied for application in electrical power systems at industrial scale. One prognostic for the market of superconducting electrical devices points out that this market will represent more than 47 billion dollars in the year 2020. More than 90% of this market will be dominated by devices such as: fault current limiters (FCL), transformers and superconducting magnetic energy storage systems (SMES). The present thesis aims to contribute for the development of a new technology for production of fault current limiters, based on the Bi-2212 phase, the partial melt method. This method involves peritectic fusion and solidification of the Bi-2212 phase and is substantially different from the complete melt processing usually used for production of Bi- 2212 blocks. The peritectic fusion and solidification of the Bi-2212 phase were investigated by quenching and in situ techniques. Superconducting properties were also studied by electromagnetic measurements. Based on these studies, a thermal cycle was established which results in a critical current density in bulk form, of the Bi-2212 phase, superior to 1000A/cm2, in direct current, at 77K and zero field. This value confirms the potential of the method to produce material for FCL.

In today’s technological world, electrical energy is one of the most important forms of energy and is needed directly or indirectly in almost every field. Increase in the demand and consumption of electrical energy leads to increase in the system fault levels. It is not possible to change the rating of the equipment and devices in the system or circuits to accommodate the increasing fault currents. The devices in electronic and electrical circuits are sensitive to disturbance and any disturbance or fault may damage the device permanently so that it must be replaced. The cost of equipment like circuit breakers and transformers in power grids is very expensive. Moreover, replacing damaged equipment is a time and labour consuming process, which also affects the reliability of power systems. It is not possible to completely eliminate the faults but it is possible to limit the current during fault in order to save the equipment and devices in the circuits or systems. One solution to this problem is to use a current limiting device in the system. There are many different types of approaches used for limiting fault currents Two different approaches to limit fault currents have been discussed by the author. One is Passive Magnetic Current Limiter (MCL) and another is High Temperature Superconductor Fault Current Limiter (HTSFCL). Both are passive devices and they do not need any sensor or external sources to perform their current limiting action. The first device consists of two ferrite cores and a permanent magnet which is sandwiched between the two saturated cores and it is called Magnetic Current Limiter. Experimental results with the MCL in circuit are discussed. Both field and thermal models of the MCL have been simulated using finite element software, FEMLAB. The demonstration of the High Temperature Superconductor Fault Current Limiter (HTSFCL) in power systems has been explained. The MATLAB simulation of the HTSFCL has been done and the results with and without the fault are shown. Power System Analysis Toolbox (PSAT) software has been used to locate the optimum or the best location of HTSFCL in a nine bus system. It has been shown that it is possible to find a solution that limits the fault current in power systems. Depending on the size of the system, either the MCL or the HTSFCL can be implemented. The location of the HTSFCL is to be carefully selected to achieve optimum results.

Ce mémoire traite de la limitation de courant à partir de supraconducteurs HTc. L'utilisation de la transition naturelle d'un état supraconducteur vers un état dissipatif est envisagée pour limiter les courants de défaut sur les réseaux de distribution électrique. Cette application de la supraconductivité est très prometteuse puisque aucune solution conventionnelle ne réalise cette fonctionnalité pour la haute tension. Le gain réalisé par l'installaton de limiteurs de courant serait une densification des réseaux électriques et une amélioration de la qualité de l'énergie distribuée. Cette étude se scinde en deux points. Le premier concerne l'étude expérimentale du comportement de différents matériaux supraconducteurs HTc pour la limitation de courant. Cette caractérisation a été menée en régime nominal alternatif (mesures des pertes) et en régime de défaut. Dans le cadre de cette étude, le matériau YBCO mono domaine a fait l'objet d'une analyse approfondie à haute température. Un travail de modélisation numérique visant à prédire les conséquences d'une transition inhomogène et à estimer les pertes en régime assigné a également été mené. Afin de valider la faisabilité d'un appareillage haute tension, un démonstrateur 1 kV / 100 A constitué de 43 méandres d'YBCO texturé a été testé à une température de 90,5 K. Cette étude s'est également focalisée sur la dynamique de la transition des matériaux supraconducteurs. Nos expériences semblent montrer que la transition est plus homogène au voisinage de Tc. Cette observation est principalement justifiée par la faible valeur de Jc au voisinage de Tc, par des considérations thermiques (meilleure propagation du front de transition) et magnétiques (ancrage magnétique et mécanisme avalancheux de la transition différents près de Tc). Des résultats expérimentaux mettent également le doigt sur l'importance des échanges thermiques latéraux sur la transition des supraconducteurs. L'analyse menée montre que l'énergie dissipée au sein du matériau étudié semble l'être de façon localisée.

With the rapid increase in electrical loads, a considerable percentage of global power systems have been stretched to their designed capacity, leading to a sharp rise in the fault current. High fault current levels have severe impact on power systems. They can increase the likelihood of overcurrent damage, which may destroy electrical facilities or even cause system blackout. The resistive-type superconducting fault current limiter (RSFCL) is a device that can help to mitigate the increasing level of fault currents. It can also contribute to the performance, stability and efficiency of electricity grids. In order to promote the RSFCL more effectively, it is essential to study the device itself and conduct simulations regarding the performance and applicability from the system point of view. Chapter 1 and Chapter 2 of this thesis introduced fundamentals of superconductivity and RSFCLs, respectively. In Chapter 3, a power system model was built, and the transient analysis of short-circuit currents was given. Then, the operating principle of RSFCL was explained, and a step-resistance RSFCL model was introduced. To validate the current limiting performance of the SFCL module, wind farm protection schemes were studied under various fault scenarios. After thorough analysis it was concluded that the optimal allocating strategy of SFCLs was the installation of one SFCL at the integrating point of the system model. Chapter 4 presented a comprehensive study on the performance and optimal allocation strategy of RSFCLs. The two power system models used in this chapter were built based on the UK network standard. To assess the impact of incorporating SC material properties on the performance of SFCLs, three different models were compared throughout the study. Although computing time can be reduced when step-resistance and exponential equation models were used, such simplifications led to strong overestimations of the SFCL performance and resulted in wrong conclusion of optimal installing strategies. For both power system models, the simultaneous use of three SFCLs was the best protection strategy in terms of the performance, economic efficiency and reliability of the overall grids. To draw this conclusion, all the potential combinations of two, three, four, and five SFCLs were studied under a wide number of fault scenarios and measuring strategies. In Chapter 5, a series of experiments were performed to study the magnetic field-angular dependence of the critical current of different commercial YBCO samples. We selected ten 2G-HTS tapes with broad differences in width, fabrication process, and laminar structure. The obtained I_c (B,θ) characteristics of HTS samples were applied in the simulation of RSFCLs, showing unneglectable effect on the first peak limiting performance during faults. This study is helpful to the design and simulation of RSFCLs and other HTS applications which require superconducting wires operating inside magnetic fields. In Chapter 6, we presented a High Frequency (HF) AC-assisted quench study of the YBCO conductor. The differences in quench processes triggered by HF AC field were recorded and studied. We found that AC signals of 10 kHz can trigger quench of the HTS tape. Also, the device proved to be effective at guaranteeing uniform quench of the sample. After then, we placed the experimental device in a magnet, to study whether the quench process can be accelerated by external DC magnetic field. It was found that the DC field can reduce the quench time, but its performance was dependent on the amplitude of transport current and the characteristics of HF AC signals. In addition, the angle between the DC magnetic field and the tape surface showed a huge impact on the quench time. Finally, a comprehensive field-angular dependence study about the quench time of the YBCO sample was conducted, revealing the same pattern as J_c (θ,B) dependence of the tape, but with a greater anisotropy. To summarize, this technique showed outstanding performance regarding quench acceleration and tape protection, and therefore has huge potential to be applied in RSFCLs.

Die vorliegende Arbeit beschäftigt sich nach der Analyse und Optimierung der Stromzuführungen auf Flüssigstickstoff-Temperaturniveau, hauptsächlich mit der Untersuchung einer Gemisch-Stickstoff-Kaskade als Kälteversorgungssystem eines resistiven HTSL-Kurzschlussstrombegrenzers. Unter einem Kurzschlussstrombegrenzer versteht man einen veränderlichen elektrischen Widerstand, welcher, gegebenenfalls in Serie mit konventionellen, mechanischen Stromunterbrechern, direkt in den zu schützenden Stromkreis eingebaut ist. Als veränderlicher elektrischer Widerstand kann z.B. ein supraleitendes Element verwendet werden. Im normalen Betriebsfall setzt dieser dem fließenden Strom praktisch keinen Widerstand entgegen. Die Dimensionierung wird so gewählt, dass im Falle eines Überstroms ein rascher Übergang in den normalleitenden Zustand erfolgt. Durch den nun vorhandenen Widerstand wird der Stromfluss sehr effektiv begrenzt, bis beispielsweise nachgeschaltete mechanische Schutzeinrichtungen ansprechen. Sobald dies erfolgt ist, kann der Supraleiter regenerieren und in den Ausgangszustand zurückkehren. Die Verwendung von HTSL-Material als Supraleiter erscheint hier höchst vorteilhaft, da damit ein Arbeiten auf Flüssigstickstoff-Temperaturniveau erlaubt wird. Ein entscheidender Punkt für die erfolgreiche Einführung der HTSL-FCL ist die Bereitstellung einer geeigneten Kälteversorgung. Dies kann entweder durch regelmäßiges Nachfühlen mit flüssigem Stickstoff oder durch den Einsatz einer Kältemaschine zur Stickstoffrekondensation realisiert werden. Beim Einsatz einer Kältemaschine hat man einen von der Stickstoffnachlieferung unabhängigen Betrieb mit geschlossenem Kühlsystem. Die Energiedissipation in dem HTSL-Element selbst ist unter Normalbedingungen vernachlässigbar klein. Für die Auslegung des Kühlsystems ist von Bedeutung, dass der größte Teil der Wärmelast durch die metallischen Stromzuführungen verursacht wird. Die Auslegung des Kühlsystems muss sich daher an der thermodynamischen Analyse der Stromzuführungen orientieren. Das Ziel dieser Arbeit bestand in der Analyse von Kühlmethoden für solche Stromzuführungen hinsichtlich ihrer Effektivität und Wirtschaftlichkeit. Ein neues Kühlsystem auf der Basis einer Gemisch-Stickstoff-Kaskade wird vorgeschlagen als Alternative zu den derzeit in Frage kommenden Kühltechniken. Es wurde folgende Vorgehensweise gewählt: 1. Verschiedene Kühlmethoden zur Kühlung von SZF werden aufgeführt und thermodynamisch bewertet. 2. Kühlsysteme, basierend auf einer Gemisch-Stickstoff-Kaskade, werden vorgeschlagen und deren Charakteristika mittels numerischer Simulation bestimmt. 3. Ein auf der Basis einer Gemisch-Kältemaschine gebauter Stickstoffverflüssiger wird in Betrieb genommen und getestet. 4. Die Funktion und die Effektivität der Gemisch-Stickstoff-Kaskade zur Kühlung der Stromzuführungen werden bewertet. 5. Zugehörige Kühlsysteme auf der Basis verschiedener Kühltechniken werden analysiert 6. In einer Wirtschaftlichkeitsbetrachtung werden die Kühlsysteme einander gegenübergestellt Insgesamt kann als Ergebnis festgehalten werden, dass der Einsatz der vorgestellten Gemisch-Stickstoff-Kaskadenanlagen zur Kälteversorgung im vorgestellten Anwendungsfall eine durchaus wettbewerbsfähige, voraussichtlich sogar überlegene Alternative zu den sonst verfügbaren Methoden darstellt.