Thesis (Ph. D.)--University of Wisconsin--Madison, 1983. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 432-463).

This thesis first outlines the testing undertaken on a partial core superconducting transformer under open circuit, short circuit, full load and endurance test conditions. During the endurance test, a failure occurred after 1 minute and 35 seconds. During the failure, voltage dipping and rapid liquid nitrogen boil off was observed. This prompted a failure investigation which concluded that the lack of cooling in the windings was the most probable cause to the failure. Full core transformer and superconductor theories are then introduced. A copper winding transformer model, based on a Steinmetz equivalent circuit and a reverse design method, is described. A superconductor loss model which outlines the different types of losses experienced under AC conditions is used to determine the resistance of the windings in the Steinmetz equivalent circuit. This resistance changes with the magnitude of current and the strength of the magnetic field that is present in the gaps between each layer of the windings. An alternative leakage flux model is then presented, where the flux is modelled based on the combination of the reluctance of the core and the air surrounding the windings. Based on these theories, an iterative algorithm to calculate the resistance of the superconductor is developed. A new design of a 15kVA single phase full core superconducting transformer, operating in liquid nitrogen, is presented. The issues with building the superconducting transformer are outlined. First, a copper mockup of the superconducting transformer was designed where the mockup would have the same tape and winding dimensions as the superconducting transformer, which means the same core can be used for two different sets of windings. This led to designing a core that could be easily taken apart as well as reassembled. Construction of the core, the copper windings and the superconductor windings ensued. The process of cutting the core laminations, insulating the copper and superconductor tapes, and making the steel fasteners and terminations are described. The copper mockup and superconducting transformers was then tested under open circuit, short circuit, different load and endurance conditions at both liquid nitrogen and room temperatures. These test results were then compared with the those from two models. The comparison showed a significant inaccuracy in the reactances in the models. This introduced a correction factor into the superconductor model which ii made it more accurate. However, further work is required to explain and quantify the correction factors for the copper transformer model under different load conditions.

The main bulk of electric power systems use a three¿phase configuration with an alternating current flow. Pursuing the optimal performance of these networks and avoiding possible technical problems, it is preferred balanced operating conditions, i.e., the currents of all phases have the same magnitude and form a direct sequence with a phase of 120º. However, if several single¿phase loads (such as high speed traction systems) are present in these systems, the operating condition turns unbalanced, provoking an asymmetric voltage supply. This undesirable working condition can be avoided by the use of two reactances connected with the single-phase load using a triangle configuration, in such a way that the total current consumption turns out to be balanced. This approach is commonly known as Steinmetz circuit or symmetrizing circuit. Due to the reactances of the symmetrizing circuit, the connection of the circuit in an electric network changes the system frequency response, appearing new resonances of several types and presenting impedance values too small or too large. Moreover, the quantity of nonlinear loads (i.e., loads with consume nonsinusoidal currents, such as arc furnaces, power electronics devices like high-speed rail systems...) is increasing nowadays. The harmonic current injection by these charges may interact with the resonances caused by the Steinmetz circuit, resulting in a large harmonic distortion in voltage. The aim of this doctoral thesis is to analyze the presence of several types of resonances in order to avoid this problem. It is used a simplified scheme of a network in which the single¿phase load and the symmetrized circuit are connected. This system also encloses nonlinear loads that generate harmonic currents. This set is analyzed from the viewpoint of nonlinear loads and of the network for parallel and series resonance frequencies location, respectively. By obtaining these resonant frequencies and knowing the harmonic injection of nonlinear loads, the trouble of voltage supply distortion can be anticipated and avoided.

Ce travail est le résultat d'une collaboration entre le laboratoire LAPLACE, la "Seconda Università degli Studi di Napoli" (SUN) et la Société National des Chemins de fer Français SNCF. Le sujet de recherche concerne l'utilisation de dispositifs électroniques de puissance dans les sous stations ferroviaires 25kV/50Hz afin d’améliorer la qualité de l'énergie électrique. Dans le transport ferroviaire, le système d'électrification monophasé 25kV/50Hz est largement diffusé en particulier pour les lignes ferroviaires à grande vitesse. Bien qu'aujourd'hui les systèmes d’alimentation en courant continu soient encore largement utilisés, l'adoption du courant alternatif monophasé offre des avantages économiques pour les infrastructures d'environ 30% en termes d'investissement, d'exploitation et d'entretien. Initialement, compte tenu de la simplicité du circuit, il n'y avait aucune nécessité d'intégrer de l'électronique de puissance dans les sous stations. Toutefois, au cours de la décennie passée, l'intérêt pour ces équipements est apparu car ils peuvent apporter une solution d'optimisation du réseau lorsque le trafic augmente ou lorsqu’une nouvelle sous station est envisagée. Deux principaux types de dispositifs sont installés aujourd'hui sur le réseau ferré français : les compensateurs de puissance réactive et les compensateurs de déséquilibre de tension. Cette thèse présente de nouvelles topologies de compensateurs basées sur le concept d’impédances contrôlées par gradateur MLI. Comparées aux solutions existantes, ces topologies ont des caractéristiques particulièrement intéressantes en termes de pertes dans les semi-conducteurs et de volume des composants réactifs. Le manuscrit contient trois parties principales: La première partie présente le principe de l’électrification en 25kV/50Hz et souligne l’intérêt d’installer des moyens de compensation statique dans les sous stations. Après une description des solutions actuellement utilisées, le concept d’impédance contrôlée par gradateur MLI (CCI : Chopper Controlled Impedance ) est ensuite présenté. La deuxième partie du travail concerne l'utilisation du concept de CCI pour la compensation de puissance réactive. La sous-station SNCF de Revest est considérée comme cas d’étude. Celle-ci est équipée d'un transformateur monophasé de 60MVA dont le primaire est connecté à une ligne de transport 225kV. Deux topologies de compensateur de puissance réactive, basées sur des montages abaisseur ou élévateur de tension sont présentées. Le dimensionnement des gradateurs est effectué sur la base d'une campagne de mesures réalisée à la sous station. Des simulations numériques utilisant des formes d’ondes réelles de courant et de tension sont présentées. Des résultats expérimentaux effectués à la plateforme de test de la SNCF sur un prototype de 1,2MVAR permettent de valider le concept de CCI. La dernière partie du travail concerne le problème du déséquilibre de tension en amont de la sous station. Un circuit de Steinmetz « actif », toujours basée sur des gradateurs MLI, est présenté et étudié. La sous station SNCF d'Evron est alors considérée comme cas étude. Celle-ci comporte un transformateur de 32MVA et est connectée à une ligne de transmission 90kV. Les mesures effectuées sur le site permettent le dimensionnement du compensateur ainsi que l’utilisation des formes d'onde réelles de courant et de tension dans les simulations numériques. Une comparaison avec des solutions classiques basées sur des onduleurs 2 niveaux et 3 niveaux souligne les avantages de la solution proposée. Ainsi, les résultats des calculs et des simulations montrent que l'énergie stockée dans les éléments réactifs est réduite d’un facteur six et que les pertes dans les semi-conducteurs sont réduites de 40%. Des résultats expérimentaux obtenus sur une maquette de 1.5 kVA permettent de valider le principe du circuit de Steinmetz actif. / This work is the result of collaboration between the LAPLACE laboratory, the “Seconda Università degli Studi di Napoli” (SUN) and the French national railways operator SNCF. The research topic treated herein concerns the use of power electronic devices in 25kV/50Hz railways substations to achieve power quality improvements. In railway transportation, single-phase 25kV-50Hz electrification system is widely diffused especially for high-speed railway applications. Although electrified DC systems are still widely applied, the adoption of AC single-phase system offers economical advantages for the infrastructures of about 30% in terms of investment, exploitation and maintenance. In early ages, due to its very simple diagram, there was no necessity to integrate power electronics in substations. However, for the last decade, the interest in power electronic equipments raised since they can provide the solution for network optimization when traffic increases or when a difficulty is foreseen for a substation implementation. Two types of devices are implemented today on the French Railway Network: Reactive Power compensators and Voltage Unbalance compensators. This thesis presents an investigation into new topologies based on the concept of “Chopper Controlled Impedances”(CCI). Compared to existing solutions, the new topologies show interesting features in terms of semi-conductor losses reduction and volume of reactive components. The manuscript is developed through three main parts: Firstly, the French railways system is introduced and the interest in installing power electronic compensators in substations is highlighted. After a brief description of currently used solutions, the CCI concept is presented: the use of Pulse Width Modulated AC Choppers allows achieving structures which behave as variable impedances. In the second part, the use of CCI structures in reactive power compensation is investigated. The SNCF substation of Revest is under study. It is equipped by a 60MVA single phase transformer with the primary side connected to a 225kV transmission line. Based on the step-down or step-up functioning mode of CCIs, two topologies of reactive power compensator are presented. The converter design is developed on the base of a measurement campaign carried out at the substation. Numerical simulations using real current and voltage waveforms are presented. Finally, experimental results carried out at the SNCF test platform on a 1.2MVAR prototype are shown. In the last part, the problem of voltage unbalance is treated. Using the concept of CCI, the feasibility of an active Steinmetz circuit based on AC choppers is explored. As a case study, the substation of Evron is considered. It is a 32MVA substation connected to a 90kV transmission line. Measurements carried out on the substation site allow the compensator design and the possibility to consider real waveforms for current and voltage in numerical simulations. A comparison with classical solution based on two levels VSI and three levels NPC-VSI highlights the advantages of the proposed solution. Calculation and simulation results show that the stored energy in reactive elements is reduced by a factor six whereas the semiconductor losses are 40% lower. Experimental results obtained on a scaled demonstrator (1.5 kVA) validate the principle of the active Steinmetz circuit.

The UK Government has set an ambitious target to achieve 15% of final energy consumption from renewable sources by 2020. High Voltage Direct Current (HVDC) technology is an attractive solution for integrating offshore wind power farms farther from the coast. In the near future, more windfarms are likely to be connected to the UK grid using HVDC links. With the onset of this fairly new technology, new challenges are inevitable. This research is undertaken to help assist with these challenges by looking at possibilities of problems with respect to faster AC/DC interaction modes, especially, on the impact of inrush currents which occur during fault-recovery transients. In addition to that, possible mitigation strategies are also investigated. Initially, the relative merits of different transformer models are analysed with respect to inrush current transient studies. The most appropriate transformer model is selected and further validated using field measurement data. A detailed electro-magnetic-transient (EMT) model of a grid-connected MMC-HVDC system is prepared in PSCAD/EMTDC to capture the key dynamics of fault-recovery transformer inrush currents. It is shown that the transformer in an MMC system can evoke inrush currents during fault recovery, and cause transient interactions with the converter and the rest of the system, which should not be neglected. It is shown for the first time through a detailed dynamic analysis that if the current sensors of the inner-current control loops are placed at the converter-side of the transformer instead of the grid-side, the inrush currents will mainly flow from the grid and decay faster. This is suggested as a basic remedial action to protect the converter from inrush currents. Afterwards, analytical calculations of peak flux-linkage magnitude in each phase, following a voltage-sag recovery transient, are derived and verified. The effects of zero-sequence currents and fault resistance on the peak flux linkage magnitude are systematically explained. A zero-sequence-current suppression controller is also proposed. A detailed study is carried out to assess the key factors that affect the maximum peak flux-linkage and magnetisation-current magnitudes, especially with regard to fault specific factors such as fault inception angle, duration and fault-current attenuation. Subsequently, the relative merits of a prior-art inrush current mitigation strategy and its implementation challenges in a grid-connected MMC converter are analysed. It is shown that the feedforward based auxiliary flux-offset compensation scheme, as incorporated in the particular strategy, need to be modified with a feedback control technique, to alleviate the major drawbacks identified. Following that, eight different feedback based control schemes are devised, and a detailed dynamic and transient analysis is carried out to find the best control scheme. The relative merits of the identified control scheme and its implementation challenges in a MMC converter are also analysed. Finally, a detailed EMT model of an islanded MMC-HVDC system is implemented in PSCAD/EMTDC and the impacts of fault-recovery inrush currents are analysed. For that, initially, a MMC control scheme is devised in the synchronous reference frame and its controllers are systematically tuned. To obtain an improved performance, an equivalent control scheme is derived in the stationary reference frame with Proportional-Resonant controllers, and incorporated in the EMT model. Following that, two novel inrush current mitigation strategies are proposed, with the support of analytical equations, and verified.