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The main focus of this PhD thesis is fundamental investigations into control techniques of inverter-based microgrids. It aims to develop new and improved control techniques to enhance performance and reliability. It focuses on the modelling, stability analysis and control design of parallel inverters in a microgrid. In inverter-based microgrids, the paralleled inverters need to work in both grid-connected mode and stand-alone mode and should be able to transfer seamlessly between the two modes. In grid-connected mode, the inverters control the amount of power injected into the grid. In stand-alone mode, however, the inverters control the island voltage while the output power is dictated by the load. This can be achieved using droop control. Inverters can have different power set-points during grid-connected mode but in stand-alone mode they all need their power set-points to be adjusted according to their power ratings. However, during sudden unintentional islanding (due to loss of mains), transient power can flow from inverters with high power set-points to inverters with low power set-points, which can raise the DC link voltage of the inverters causing them to shut down. This thesis investigates the transient circulating power between paralleled inverters during unintentional islanding and proposes a controller to limit it. The controller monitors the DC link voltage and adjusts the power set-point in proportion to the rise in the voltage. A small signal model of an island microgrid has been developed and used to design the controller. The model and the controller design have been validated by simulation and practical experimentation. The results confirmed the performance of the proposed controller for limiting the DC link voltage and supporting a seamless mode transfer. The limitation of the droop controller, that is utilized to achieve load sharing between parallel-operated inverters in island mode, has also been addressed. Unequal output impedances among the distribution generation (DG) units lead to the droop control being inaccurate, particularly in terms of reactive power sharing. Many methods reported in the literature adopt low speed communications to achieve efficient sharing. However, the loss of this communication could lead to inaccuracy or even instability. An improved reactive power-sharing controller is proposed in this thesis. It uses the voltage at the point of common coupling (PCC) to estimate the inductance value of the output impedance including the impedance of the interconnecting power cables and to readjust the voltage droop controller gain accordingly. In an island microgrid consisting of parallel-connected inverters, the interaction between an inverter’s output impedance (dominated by the inverter’s filter and voltage controller) and the impedance of the distribution network (dominated by the other paralleled inverters’ output impedances and the interconnecting power cables) might lead to instability. This thesis studies this phenomenon using root locus analysis. A controller based on the second derivative of the output capacitor voltage is proposed to enhance the stability of the system. Matlab simulation results are presented to confirm the validity of the theoretical analysis and the robustness of the proposed controller. A laboratory-scale microgrid consisting of two inverters and local load has been built for the experimental phase of the research work. A controller for a voltage source inverter is designed and implemented. A dSPACE unit has been used to realize the controller and monitor the system in real time with the aid of a host computer. Experimental results of the two voltage source inverters outputs are presented.

Thesis (Ph. D.)--Ohio State University, 2007. / Full text release at OhioLINK's ETD Center delayed at author's request

Thesis (M.A. in National Security Affairs)--Naval Postgraduate School, March 2004. / Thesis advisor(s): Daniel Moran, James Russell. Includes bibliographical references (p. 41-45). Also available online.

Thesis--M. Phil., University of Hong Kong, 1978.

Thesis (Ph.D.) -- McMaster University, 1999. / Includes bibliographical references (leaves [188]-192). Also available via World Wide Web.

Thesis (Ph.D.)--University of Delaware, 2005. / Principal faculty advisor: Samuel L. Gaertner, Dept. of Psychology. Includes bibliographical references.

The aim of the study is to broaden the existing research that touches how power expresses itself and distributes in childrens relationship building. The purpose is to study childrens relationship building and how power and gender affects one another. The reason is to locate possible patterns that excel during an ongoing construction and development of a relationship. To understand what has been observed a sociological perspective has been used. The chosen method is a qualitative study. Observations of groups of children at two different preschools have been done. Semi-structured interviews with educators from the different preschools have also been conducted. The result of the study is that children constantly are trying to get control over their surroundings. In this process when children try to regain control, power may become a big part. Further results of the study is that the preschool and everyone who operates within it holds a major role when it comes to guiding the children and showing appropriate actions individually as well as together with other social human beings. Next the results of the study will be connected to previous research, the preschool’s curriculum and the Convention on the Rights of the Child will be presented. Lastly the conclusions of the study are put in relation to the professional role as a preschool-teacher which will be embraced in a few months.

Esta tese investiga os papéis políticos assumidos pela produção e o uso de estatísticas sobre crimes e criminosos na história recente das instituições que compõem o chamado sistema de justiça criminal brasileiro (polícias Civil e Militar, Ministério Público, Poder Judiciário e Estabelecimentos Carcerários), em São Paulo. Por meio da análise de documentos e formulários de coleta de dados, defende-se que, mesmo após a redemocratização e as pressões para a estruturação de procedimentos de controle público das agências estatais de pacificação social, o segredo permanece como modus-operandi desse sistema, no qual dados são produzidos, mas não há coordenação entre produtores e usuários; entre oferta e demanda da informação. Dessa forma, não havendo consenso sobre os papéis das estatísticas criminais disponíveis, há um movimento simultâneo de crescimento dos estoques de dados gerados como subproduto da adoção de modernas ferramentas de informática, de um lado, e, paradoxalmente, há o reforço da opacidade e da “experiência" institucional das práticas burocráticas no desenho e operação de políticas públicas de pacifi cação social, de outro. O resultado alcançado reforça a manutenção dos mecanismos de reprodução de verdades profissionais e institucionais nos modelos vigentes de segurança pública e justiça criminal, garantindo a permanência e a governabilidade em relação às pressões democráticas por mudanças nos padrões de policiamento e no acesso à justiça. / This thesis investigates the political roles taken by the production and use of crime and criminal offenders statistics on the recent history of the institutions that compose the Brasilian criminal justice system (the police, prosecution, judiciary and prisons) in São Paulo. Through the analysis of documents and data collecting forms, the thesis defends that, despite the redemocratization and the (political) pressure in order to organize the public control procedures of the governmental agencies of social pacifi cation, the secret remains the modus operandi of this system, in which data is produced but there is no coordination between producers and users or supply and demand of information. Along with the lack of consensus on the roles of the available criminal statistics, there is, on the one hand, an increment of data stores resulting from the constant use of technological tools. On the other hand, the opaqueness and institucional “experience" of bureaucratic practices are reinforced in planing and managing public policies of social pacifi cation. The consequence is the maintenance of mechanisms that reproduce institutional and professional truths in the current models of public law enforcement and criminal justice and that guarantee the permanence and governmentability in relation to democratic pressure for changes in the patterns of policing and access to justice.