Browsing by Subject "Micro-grid"
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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 Horizontal series fault comparison in AC(2011-12) Estes, Hunter Blake; Kwasinski, Alexis; Hebner, Robert E.; Grady, William M.This research focuses on empirical observations of horizontal series arc faults. These faults differ from ground faults, for series faults encompass the electromagnetic transient effects of arc formations in series with sustained current flow when there is a break in the circuit. This may happen intentionally (as in a breaker) or unintentionally (as in a loose, damaged, or severed cable). This paper studies some of those transient effects during arc ignition, propagation, and cessation. Emphasis is on dc systems, for series faults present some of the more challenging safety concerns relating to widespread dc micro-grid acceptance and proliferation. However, arc behavior is also compared to that of ac systems under “quasi-equivalent”, passive circuit parameters. Variables of study primarily include arc voltage, current, and their relationship to electrode spacing under dynamic conditions. Results indicate that interruptions in dc current, while appearing more chaotic from a localized standpoint, do not produce the fast-acting transients associated with ac disturbances. Additionally, if dc arcs propagate over a slowly increasing distance of separation, they can be modeled as quasi-static in nature. An equation model is developed and curve-fitting parameters match well with historically tabulated constants.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 Topology development and analysis for multiple input DC/DC converter(2011-05) Choung, Seung Hoon; Kwasinski, Alexis; Grady, William M.; Araposththis, Aristotle; Driga, Mircea D.; Son, You S.Nowadays, the number of applications which need more than one power source is increasing. Distributed generating systems or micro-grid systems normally use more than one power source or more than one kind of energy source. Also, to increase the utilization of renewable energy sources, diversified energy source combination is recommended. For example, a wind-photovoltaic generating system, a combination of a wind generator and photovoltaic array, can give a greater degree of freedom when choosing the install location. The combination of more power sources and diversified power sources makes it possible to obtain higher availability in a power system. A parallel connection of converters has been used to integrate more than one energy source in a power system. However, a multiple-input converter (MIC) can generally have the following advantages compare to a combination of several individual converters; (1) cost reduction, (2) compactness, (3) more expandability and (4) greater manageability. First, this research suggests MIC topology comparison criteria that can be used as a decision guide for choosing a MIC topology depending on the application. Even though there are some MIC topology classification methods such as by the kind of combining methods, the classification methods are not enough to choose one particular topology. The comparison criteria presented in this dissertation are practical enough to decide which topology is suitable and should be chosen. Second, a new MI modified inverse Watkins-Johnson converter (MIMIWJC) without a coupled inductor is proposed. The circuit configuration of this converter and its operation principles are described, including the open-loop and closed-loop circuit. For control purposes, a small signal model of the proposed converter is developed using Middlebrook’s extra element theorem. In addition, two possible control methods are introduced in this dissertation. Finally, the theoretical analysis of the proposed converter is verified with simulations and experiments.