82 research outputs found
Voltage Stabilization in Microgrids via Quadratic Droop Control
We consider the problem of voltage stability and reactive power balancing in
islanded small-scale electrical networks outfitted with DC/AC inverters
("microgrids"). A droop-like voltage feedback controller is proposed which is
quadratic in the local voltage magnitude, allowing for the application of
circuit-theoretic analysis techniques to the closed-loop system. The operating
points of the closed-loop microgrid are in exact correspondence with the
solutions of a reduced power flow equation, and we provide explicit solutions
and small-signal stability analyses under several static and dynamic load
models. Controller optimality is characterized as follows: we show a one-to-one
correspondence between the high-voltage equilibrium of the microgrid under
quadratic droop control, and the solution of an optimization problem which
minimizes a trade-off between reactive power dissipation and voltage
deviations. Power sharing performance of the controller is characterized as a
function of the controller gains, network topology, and parameters. Perhaps
surprisingly, proportional sharing of the total load between inverters is
achieved in the low-gain limit, independent of the circuit topology or
reactances. All results hold for arbitrary grid topologies, with arbitrary
numbers of inverters and loads. Numerical results confirm the robustness of the
controller to unmodeled dynamics.Comment: 14 pages, 8 figure
Secondary Frequency and Voltage Control of Islanded Microgrids via Distributed Averaging
In this work we present new distributed controllers for secondary frequency
and voltage control in islanded microgrids. Inspired by techniques from
cooperative control, the proposed controllers use localized information and
nearest-neighbor communication to collectively perform secondary control
actions. The frequency controller rapidly regulates the microgrid frequency to
its nominal value while maintaining active power sharing among the distributed
generators. Tuning of the voltage controller provides a simple and intuitive
trade-off between the conflicting goals of voltage regulation and reactive
power sharing. Our designs require no knowledge of the microgrid topology,
impedances or loads. The distributed architecture allows for flexibility and
redundancy, and eliminates the need for a central microgrid controller. We
provide a voltage stability analysis and present extensive experimental results
validating our designs, verifying robust performance under communication
failure and during plug-and-play operation.Comment: Accepted for publication in IEEE Transactions on Industrial
Electronic
Input-Output Performance of Linear-Quadratic Saddle-Point Algorithms With Application to Distributed Resource Allocation Problems
Saddle-point or primal-dual methods have recently attracted renewed interest as a systematic technique to design distributed algorithms, which solve convex optimization problems. When implemented online for streaming data or as dynamic feedback controllers, these algorithms become subject to disturbances and noise; convergence rates provide incomplete performance information, and quantifying input-output performance becomes more important. We analyze the input-output performance of the continuous-time saddle-point method applied to linearly constrained quadratic programs, providing explicit expressions for the saddle-point norm under a relevant input-output configuration. We then proceed to derive analogous results for regularized and augmented versions of the saddle-point algorithm. We observe some rather peculiar effects-a modest amount of regularization significantly improves the transient performance, while augmentation does not necessarily offer improvement. We then propose a distributed dual version of the algorithm, which overcomes some of the performance limitations imposed by augmentation. Finally, we apply our results to a resource allocation problem to compare the input-output performance of various centralized and distributed saddle-point implementations and show that distributed algorithms may perform as well as their centralized counterparts
Synchronization of coupled limit cycles
A unified approach for analyzing synchronization in coupled systems of
autonomous differential equations is presented in this work. Through a careful
analysis of the variational equation of the coupled system we establish a
sufficient condition for synchronization in terms of the geometric properties
of the local limit cycles and the coupling operator. This result applies to a
large class of differential equation models in physics and biology. The
stability analysis is complemented with a discussion of numerical simulations
of a compartmental model of a neuron.Comment: Journal of Nonlinear Science, accepte
Subject specific demands of teaching: Implications for out-of-field teachers
This chapter provides a framework for thinking about the subject-specific nature of teaching in terms of the
knowledge, modes of inquiry and discursive practices that delineate one subject from another in the
traditional school curriculum. The chapter will explore how these disciplinary traits are translated into
teaching as curriculum, knowledge and pedagogy, and how this subject-specificity of teaching is
juxtaposed against the more generic aspects of teaching. The chapter explores the idea that if a teacher’s
expertise can be situated within a field, then they can also be positioned out-of-field. Implications for
teaching out-of-field are discussed in terms of the subject-specific knowledge, processes and skills, and the
difficulties associated with teacher practice. English and Australian illustrations of teacher practices from
in-field and out-of-field situations are provided, in particular highlighting the demands of moving across
subject boundaries. Cross-fertilisation is especially evident when subjects are integrated, therefore, the
issues associated with integrated curriculum are discussed where the traditional subject boundaries are
being challenged as schools are reorganised to integrate subjects through, for example, STEM teaching, or
holistic curriculum designs
Eyg and Ey Pax proteins act by distinct transcriptional mechanisms in Drosophila development
Discrete gradient flows for general curvature energies
We consider the numerical approximation of theL2–gradient flow of general curvatureenergies∫G(|~κ|) for a curve inRd,d≥2. Here the curve can be either closed, or it can be open andclamped at the end points. These general curvature energies, and the considered boundary conditions,appear in the modelling of the power loss within an optical fibre. We present two alternative finiteelement approximations, both of which admit a discrete gradient flow structure. Apart from beingstable, in addition, one of the methods satisfies an equidistribution property. Numerical resultsdemonstrate the robustness and the accuracy of the proposedmethods
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