Thesis (Ph.D.)--University of Wollongong, 2005. / Typescript. Includes bibliographical references: leaf 188-200.

Superconducting Magnetic Energy Storage (SMES) offers many advantages over conventional forms of energy storage. The higher unit costs of SMES make it economically feasible only for large-scale applications (5000 MWh or more). Early studies showed that low aspect ratio solenoids have the lowest overall costs and most of the subsequent research and conceptual design was centered around low to moderate aspect ratio solenoidal designs. Toroids, poloids and force-reduced magnets are some alternate magnet structures that can be used for SMES but have not received much attention. These structures have some advantages over solenoidal designs such as lower peak fields with greater energy storage capability (poloid), lower support structure requirement (force-reduced magnet) and zero external field (toroid). For some applications of SMES such as pulsed power for fusion reactors or particle accelerators, where the large external field of the solenoid may be unacceptable but the fast response and high efficiency of SMES are required, alternate magnet structures or geometries must be used even if the costs are somewhat higher. Therefore it is useful to study the relative costs of each magnet structure in order to choose a suitable magnet structure for a given application. Four magnet structures viz. the solenoid, toroid, poloid and a force-reduced magnet are evaluated for their energy storage capability, support structure requirements and stray field characteristics. The variation of these parameters with the geometry of the magnet as well as the size of the SMES system are also studied. The objective of this study is to provide a quantitative comparison of important magnet parameters as well as to develop a simple procedure for the preliminary magnetic design of SMES magnets of any size, based on the detailed analysis of a reference design. Due to the high costs of SMES particularly for smaller sizes, it is important to optimize magnet design as well as to look for new magnet configurations to make SMES more cost effective. Superconductor and support structure material are major components of the overall cost of SMES magnets. Various methods of optimizing these parameters are explored. Force-reduced magnets have attracted some controversy in SMES research due to various claims made for and against them. The virial theorem and its implications for force-reduced magnets are analyzed with reference to a specific force-reduced configuration and it is shown that the claims that force-reduced magnets do not offer any savings in structure are unjustified. Methods for further reducing the structure requirements in force-reduced magnets and toroidal magnets are discussed. Due to the unique and highly desirable characteristic of zero stray field of toroidal magnets, it is important to seek ways of increasing its energy storage capability. A variational problem is formulated to determine the optimal cross-sectional shape which maximizes the stored energy with a given quantity of superconductor. The optimal shape stores 16 p.c. more energy than the circular cross section toroid and is shown to be identical to the structurally superior constant tension D shape. The desired characteristics of an ideal SMES magnet are shown to be a uniform magnetic field within a closed magnet configuration. A twisted toroidal magnet combining the features of an ideal solenoid and the general toroidal configuration is studied as a candidate for the ideal magnet. Geometric arguments are used to prove that no such closed surface can be found in three dimensions with the minimum smoothness conditions required from physical considerations. / Ph. D.

Electromagnetic transient and dynamic modeling and simulation studies are presented for a StatCom-SMES compensator in power systems. The transient study aims to better understand the transient process and interaction between a high power/high voltage SMES coil and its power electronics interface, dc-dc chopper. The chopper is used to attach the SMES coil to a StatCom. Following the transient study, the integration of a StatCom with SMES was explored to demonstrate the effectiveness of the combined compensator in damping power oscillations. The transient simulation package PSCAD/EMTDC has been used to perform the integrated modeling and simulation studies. A state of the art review of SMES technology was conducted. Its applications in power systems were discussed chronologically. The cost effective and feasible applications of this technology were identified. Incorporation of a SMES coil into an existing StatCom controller is one of the feasible applications, which can provide improved StatCom operation, and therefore much more flexible and controllable power system operation. The SMES coil with the following unique design characteristics of 50MW (96 MW peak), 100 MJ, 24 kV interface has been used in this study. As a consequence of the high power/ high voltage interface, special care needs to be taken with overvoltages that can stress the insulation of the coil. This requires an investigation of transient overvoltages through a detailed modeling of SMES and its power electronics interface. The electrical model for the SMES coil was developed based on geometrical dimensions of the coil. The interaction between the SMES coil and its power electronics interface (dc-dc chopper for the integration to StatCom) was modeled and simulated to identify transient overvoltages. Transient suppression schemes were developed to reduce these overvoltages. Among these are MOV implementation, surge capacitors, different configurations of the dc-dc chopper. The integration of the SMES coil to a StatCom controller was developed, and its dynamic behavior in damping oscillations following a three-phase fault was investigated through a number of simulation case studies. The results showed that the addition of energy storage to a StatCom controller can improve the StatCom-alone operation and can possibly reduce the MVA rating requirement for the StatCom operating alone. The effective location selection of a StatCom-SMES controller in a generic power system is also discussed. / Ph. D.

This dissertation is the result of research and development of a power conditioning system for Superconductive Magnetic Energy Storage System. The dominant challenge of this research was to develop the power conditioning system that can match slowly varying dc voltage and dc current on the super conductive magnet side with the ac voltages and ac currents on the utility side. At the same time the power conditioning system was required to provide a bi-directional power flow to the superconductive magnet. The focus of this dissertation is a three-level diode clamped dc-ac converter which is a principle part of the power conditioning system. Accordingly, this dissertation deals with the space vector modulation of three-level converters and introduces a computationally very efficient three-level space vector modulation algorithm that is experimentally verified. Furthermore, the proposed space vector modulation algorithm is successfully generalized to allow equally efficient, real time implementation of space vector modulation to dc-ac converters with virtually any number of levels. The most important advantage of the proposed concept is in the fact that the number of instructions required to implement the algorithm is almost independent from the number of levels in a multilevel converter. More on the side of the control of multilevel converters, the particular attention in this dissertation is paid to the problem of charge balance in the split dc-link capacitors of three-level neutral-point-clamped converters. It is a known fact that although the charge balance in the neutral point can be maintained on a line cycle level, a significant third harmonic current flows into the neutral point for certain loading conditions, causing the neutral point voltage ripple. The logical consequence of that ripple is the deteriorated quality of the output voltage waveforms as well as the increased voltage stress on the switching devices. This was the motivation to more carefully explore the loading conditions that cause the unbalance, as well as to study the fundamental limitations of dc-link capacitor charge balancing algorithms. As a part of that work, a new model of the neutral point current in the rotating coordinate frame is developed as a tool in investigation of theoretical limitations and in providing some intuitive insight into the problem. Additionally, the low frequency ripple is quantified and guidelines are offered that can help size the dc-link capacitors. Because the study of the neutral point balance identified the loading conditions, that under some possible system constraints, cause an unavoidable neutral point voltage ripple, a feed forward type of control method is developed next. The proposed feed forward algorithm can effectively prevent the neutral point voltage ripple from creating distortions in the converter output voltage under all loading conditions and without causing additional disturbance in the neutral point voltage. The feed forward method is developed for a sine triangle as well as for the space vector type PWM algorithm. The simulation results that include the full dynamic model of the converter and load system validate the feed forward approach and prove that the feed forward algorithm can effectively compensate the effect of the neutral point voltage ripple. The simulation results are than experimentally verified. / Ph. D.

Superconducting Magnetic Energy Storage (SMES) has branched out from its application origins of load leveling, in the early 1970s, to include power quality for utility, industrial, commercial and military applications. It has also shown promise as a power supply for pulsed loads such as electric guns and electromagnetic aircraft launchers (EMAL) as well as for vital loads when power distribution systems are temporarily down. These new applications demand more efficient and compact high performance power electronics. A 250 kW Power Conditioning System (PCS), consisting of a voltage source converter (VSC) and bi-directional two-quadrant DC/DC converter (chopper), was developed at the Center for Power Electronics Systems (CPES) under an ONR funded program. The project was to develop advanced power electronic techniques for SMES Naval applications. This thesis focuses on system analysis and development of a demonstration test plan to illustrate the SMES systems' ability to be multitasked for implementation on naval ships. The demonstration focuses on three applications; power quality, pulsed power and vital loads. An integrated system controller, based on an Altera programmable logic device, was developed to coordinate charge/discharge transitions. The system controller integrated the chopper and VSC controller, configured applicable loads, and dictated sequencing of events during mode transitions. Initial tests with a SMES coil resulted in problems during mode transitions. These problems caused uncontrollable transients and caused protection to trigger and processors to shut down. Accurate models of both the Chopper and VSC were developed and an analysis of these mode transition transients was conducted. Solutions were proposed, simulated and implemented in hardware. Successful operation of the system was achieved and verified with both a low temperature superconductor here at CPES and a high temperature superconductor at The Naval Research Lab. / Master of Science