Browsing by Author "Lestas, Ioannis"

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    Decentralized stability conditions for DC microgrids: Beyond passivity approaches
    (Elsevier, 2022-12-22) Laib, Khaled; Watson, Jeremy; Ojo, Yemi; Lestas, Ioannis
    We consider the problem of ensuring stability in a DC microgrid by means of decentralized conditions. Such conditions are derived which are formulated as input–output properties of locally defined subsystems. These follow from various decompositions of the microgrid and corresponding properties of the resulting representations. It is shown that these stability conditions can be combined together by means of appropriate homotopy arguments, thus reducing the conservatism relative to more conventional decentralized approaches that often rely on a passivation of the bus dynamics. Examples are presented to demonstrate the efficiency and the applicability of the results derived.
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    A Distributed Scheme for Voltage and Frequency Control and Power Sharing in Inverter-Based Microgrids
    (IEEE, 2023-01-30) Ojo, Yemi; Watson, Jeremy; Laib, Khaled; Lestas, Ioannis
    Grid-forming inverter-based autonomous microgrids present new operational challenges as the stabilizing rotational inertia of synchronous machines is absent. The design of efficient control policies for grid-forming inverters is, however, a nontrivial problem where multiple performance objectives need to be satisfied, including voltage/frequency regulation, current limiting capabilities, and active power sharing and a scalable operation. We propose, in this article, a novel control architecture for frequency and voltage control, which allows current limitation via an inner loop, active power sharing via a distributed secondary control policy, and scalability by satisfying a passivity property. In particular, the frequency controller employs the inverter output current and angle to provide an angle droop-like policy, which improves its stability properties. This also allows us to incorporate a secondary control policy for which we provide an analytical stability result, which takes line conductances into account (in contrast to the lossless line assumptions in the literature). The distinctive feature of the voltage control scheme is that it has a double-loop structure that uses the dc voltage in the feedback control policy to implement a power-balancing strategy to improve performance. The performance of the control policy is illustrated via simulations with detailed nonlinear models in a realistic setting.
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    On the Synchronization of the Kuramoto-Type Model of Oscillators With Lossy Couplings
    (IEEE, 2023-01-02) Ojo, Yemi; Laib, Khaled; Lestas, Ioannis
    We consider the problem of synchronization of coupled oscillators in an advanced Kuramoto model that includes coupling conductances, and is characterized by nonhomogeneous coupling weights and noncomplete coupling graphs. Lyapunov analysis once such coupling conductances and aforementioned properties are included becomes nontrivial and more conventional energy-like Lyapunov functions are not applicable or are conservative. Small-signal analysis has been performed for such models, but due to the fact that we have convergence to a manifold, stability analysis via a linearization is on its own inconclusive for the nonlinear model. In this letter, we provide a formal derivation using centre manifold theory that if a particular condition on the equilibrium point associated with the coupling conductances and susceptances holds, then the synchronization manifold for the nonlinear system considered is asymptotically stable. Our analysis is demonstrated with simulations.
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    A Scalable Control Design for Grid-Forming Inverters in Microgrids
    (IEEE, 2021-08-18) Watson, Jeremy; Ojo, Yemi; Laib, Khaled; Lestas, Ioannis
    Microgrids are increasingly recognized as a key technology for the integration of distributed energy resources into the power network, allowing local clusters of load and distributed energy resources to operate autonomously. However, microgrid operation brings new challenges, especially in islanded operation as frequency and voltage control are no longer provided by large rotating machines. Instead, the power converters in the microgrid must coordinate to regulate the frequency and voltage and ensure stability. We consider the problem of designing controllers to achieve these objectives. Using passivity theory to derive decentralized stability conditions for the microgrid, we propose a control design method for grid-forming inverters. For the analysis we use higher-order models for the inverters and also advanced dynamic models for the lines with an arbitrarily large number of states. By satisfying the decentralized condition formulated, plug-and-play operation can be achieved with guaranteed stability, and performance can also be improved by incorporating this condition as a constraint in corresponding optimization problems formulated. In addition, our control design can improve the power sharing properties of the microgrid compared to previous non-droop approaches. Finally, realistic simulations confirm that the controller design improves the stability and performance of the power network.