Browsing by Subject "Power electronics"
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Item Analysis, modeling, and control of highly-efficient hybrid dc-dc conversion systems(2012-12) Zhao, Ruichen; Kwasinski, Alexis; Aristotle, Arapostathis; Grady, William; Akella, Maruthi; Driga, MirceaThis dissertation studies hybrid dc-dc power conversion systems based on multiple-input converters (MICs), or more generally, multiport converters. MICs allow for the integration of multiple distributed generation sources and loads. Thanks to the modular design, an MIC yields a scalable system with independent control in all sources. Additional characteristics of MICs include the improved reliability and reduced cost. This dissertation mainly studies three issues of MICs: efficiency improvement, modeling, and control. First, this work develops a cost-effective design of a highly-efficient non-isolated MIC without additional components. Time-multiplexing (TM) MICs, which are driven by a time-multiplexing switching control scheme, contain forward-conducting-bidirectional-blocking (FCBB) switches. TM-MICs are considered to be subject to low efficiency because of high power loss introduced by FCBB switches. In order to reduce the power loss in FCBB switches, this work adopts a modified realization of the FCBB switch and proposes a novel switching control strategy. The design and experimental verifications are motivated through a multiple-input (MI) SEPIC converter. Through the design modifications, the switching transients are improved (comparing to the switching transients in a conventional MI-SEPIC) and the power loss is significantly reduced. Moreover, this design maintains a low parts-count because of the absence of additional components. Experimental results show that for output power ranging from 1 W to 220 W, the modified MIC presents high efficiency (96 % optimally). The design can be readily extended to a general n-input SEPIC. The same modifications can be applied to an MI-Ćuk converter. Second, this dissertation examines the modeling of TM-MICs. In the dynamic equations of a TM-MIC, a state variable from one input leg is possible to be affected by state variables and switching functions associated with other input legs. In this way, inputs are coupled both topologically and in terms of control actions through switching functions. Coupling among the state variable and the time-multiplexing switching functions complicate TM-MICs’ behavior. Consequently, substantial modeling errors may occur when a classical averaging approach is used to model an MIC even with moderately high switching frequencies or small ripples. The errors may increase with incremental number of input legs. In addition to demonstrating the special features on MIC modeling, this dissertation uses the generalized averaging approach to generate a more accurate model, which is also used to derive a small-signal model. The proposed model is an important tool that yields better results when analyzing power budgeting, performing large-signal simulations, and designing controllers for TM-MICs via a more precise representation than classical averaging methods. Analyses are supported by simulations and experimental results. Third, this dissertation studies application of a decentralized controller on an MI-SEPIC. For an MIC, a multiple-input-multiple-output (MIMO) state-space representation can be derived by an averaging method. Based on the averaged MIMO model, an MIMO small-signal model can be generated. Both conventional method and modern multivariable frequency analysis are applied to the small-signal model of an MI-SEPIC to evaluate open-loop and closed-loop characteristics. In addition to verifying the nominal stability and nominal performance, this work evaluates robust stability and robust performance with the structured singular value. The robust performance test shows that a compromised performance may be expected under the decentralized control. Simulations and experimental results verify the theoretical analysis on stability and demonstrate that the decentralized PI controller could be effective to regulate the output of an MIC under uncertainties. Finally, this work studies the control of the MIMO dc-dc converter serving as an active distribution node in an intelligent dc distribution grid. The unified model of a MIMO converter is derived, enabling a systematical analysis and control design that allows this converter to control power flow in all its ports and to act as a power buffer that compensates for mismatches between power generation and consumption. Based on the derived high-order multivariable model, a robust controller is designed with disturbance-attenuation and pole-placement constraints via the linear matrix inequality (LMI) synthesis. The closed-loop robust stability and robust performance are tested through the structured singular value synthesis. Again, the desirable stability and performance are verified by simulations and experimental results.Item Digital current mode control for multiple input converters(2012-08) Ding, Guanyu, 1987-; Kwasinski, AlexisIn this thesis, the possibility of applying digital current mode control on multiple-input (MI) converters is studied. As for MI topologies having a central energy transfer inductor, the predictive constant on-time current-mode control can greatly reduce both the design and digital realization efforts needed. By doing digital constant on-time current-mode control, the control of MI buck and MI buck-boost converters can be simplified into an equivalent-single-input converter control problem. The small signal models of digital constant on-time controlled single-input (SI), MI buck and SI, MI buck-boost converters in both CCM and DCM are proposed. Simulations and experiments were built to verify the proposed models.Item Isolated multiple-input single ended primary inductor converter (SEPIC) and applications(2010-05) Yu, Sheng Yang; Kwasinski, Alexis; Grady, William M.This document explores the isolated multiple-input single ended primary inductor converter (IMISEPIC) and discusses its application. This thesis proposes the following control methods such as current feed-forward control, voltage feedback control and maximum power point control to analyze the IMISEPIC. Zero-ripple technique is also applied to IMISEPIC in order to increase the converter’s life-time. Design strategy and concerns about the IMISEPIC are also presented, and simulations and circuit experiments are conducted to verify the analysis. Finally, the discussion about control limitation is used for future design consideration.Item A multiple-input single ended primary inductor converter for modular micro-grids with hybrid low-power sources(2010-05) Zhao, Ruichen; Kwasinski, Alexis; Grady, MackThis thesis studies a multiple-input single ended primary inductor converter (MI SEPIC) topology. The configuration allows the integration of different low-power distributed generation sources, such as individual photovoltaic modules, fuel cells, and small residential wind generators, into a common dc main bus. The current source interface allows the integration of all types of sources without the addition of filters; sources that require a nearly constant input current, such as fuel cells. In addition to discussing the circuit’s main models and operation, the thesis evaluates the stability under a decentralized PI control scheme through small signal analysis. The analysis is verified with simulations and experiments with prototypes. A derived circuit topology, the isolated MI SEPIC, is also explored here. In addition, a nonlinear control scheme, Lyapunov-based control, is implemented to stabilize an MI SEPIC.Item Power Electronic Topologies with High Density Power Conversion and Galvanic Isolation for Utility Interface(2015-01-26) Krishnamoorthy, Harish SarmaThe past decade has seen a significant increase in the number of applications where power electronic converters play a major role. Renewable energy systems such as wind turbines, solar photovoltaics, etc. employ power converters to interface with the utility grid. More and more power converters are being used in transportation sector such as in electric vehicles, locomotives, aircrafts, ships and submarines. Advancements in power converter topologies and devices have constantly pushed the limits and standards applicable in different markets towards better efficiency, lower cost and higher power density. Especially for large power systems such as wind turbine generators, adjustable speed drives, locomotives, etc., achieving smaller footprint at low cost and high efficiency has become a major challenge. These factors generate the major impetus towards the research undertaken in this dissertation. In applications that require integration with the utility grid, the bulkiest components are usually the transformers, inductors and DC electrolytic capacitors. Instead of using a line frequency transformer to interface any power electronic system with the utility grid directly, it is possible to use a power converter to transform the line frequency AC into a higher frequency AC that can be fed to a medium or high frequency transformer. These transformers are much smaller and lighter compared to line frequency transformers. This dissertation elucidates these concepts in detail in the first section as well as at the beginning of each subsequent section, along with a summary of such techniques already proposed in the literature. The sections in this dissertation propose and discuss several architectures (approaches) adhering to the earlier stated concepts that enable higher power density energy conversion for applications such as wind turbines, adjustable speed drives, data centers, energy storage systems, etc. Detailed operational analysis, design example, control strategy, simulation results and experimental results are shown for each concept or topology. The advantages and drawbacks are also discussed. Finally in this dissertation, the medium or high frequency transformers that can be used in the proposed approaches are analyzed in detail using ANSYS Maxwell software in terms of material, saturation, loss and size. Further, these numbers are used to estimate the relative size advantage and efficiency that can be achieved using higher frequency transformer compared to a line frequency transformer for utility interface applications. It will be shown that for many high power applications, medium frequency transformer based circuit designs can be more efficient and simpler alternatives for high frequency transformer based approaches. The specific contributions along with future research opportunities of the proposed concepts are summarized at the end.Item Realization, comparison, and topology investigation of multiple-input converters for distributed generation applications(2012-12) Yu, Sheng-Yang; Kwasinski, AlexisThis dissertation systematically explores multiple-input converters (MICs) configuration and topologies, and then proposes improvements on certain beneficial MICs—time-sharing MICs and soft-switching MICs for distributed generation (DG) applications with high voltage transfer ratio. Compared with other MIC families which are derived from same input and output cells, time-sharing MICs have the fewest circuit components. However, time-sharing MICs lack for bi-directional power flow capability due to their special input switches requirement. In addition, their hard-switching characteristic leads to a low efficiency problem when isolation is necessary. The dissertation investigates into time-sharing MIC input switch selection, which leads to a new driving strategy and new input switch combinations. With the new input switch combinations, bi-directional and high efficiency time-sharing MICs are made possible. Besides isolated time-sharing MIC, Soft-switching MICs might also be a common choice for high voltage transfer ratio DG applications. However, the enormous amount of circuit components makes the soft-switching MICs become less attractive. An input cell reduction method is introduced in this dissertation to greatly reduce the component count of isolated MICs, including soft-switching MICs. In addition to the improvement on existing MIC families, a new push-pull connected MIC family is proposed in this dissertation as another choice of high voltage transfer ratio DG applications. Moreover, a comparison among MIC families is made to provide a sense of topologies selection in certain applications. Prototypes of time-sharing dual-input (DI) SEPICs, a push-pull connected DI-Boost converter, and a DI full-bridge (FB) converter are built to verify aspects discussed in this dissertation. Bi-directional power flow capability of time-sharing MIC is confirmed with a time-sharing DI-SEPIC and a soft-switching time-sharing MIC is realized by an isolated time-sharing DI-SEPIC with an active clamping leg. Maximum power point tracking control feasibility in these converters is evaluated with real photovoltaic modules that are connected to the push-pull connected DI-Boost converter that uses a perturb-and-observe method. Finally, an efficiency comparison is made between time-sharing MIC and push-pull connected MIC.Item Single-phase nonlinear power electronic loads: modeling and impact on power system transient response and stability(2008-05) Rylander, Matthew Robert, 1981-; Grady, W. M.This dissertation examines single-phase nonlinear power electronic loads. The transient response of power electronic loads is unlike traditional linear loads. Therefore, a composite power electronic transient load model is developed. The load model dynamics are validated with actual utility voltage sag response data, laboratory controlled load response testing, and power electronic load dynamic simulations. The power electronic load model is applied in the University of Texas at Austin power system. The system transient response is unique and considerably different from what it would be with traditional linear loads. The power electronic load can be friendly or unfriendly to the system depending on the fault and system configuration.