957 research outputs found
On the Robust Control and Optimization Strategies for Islanded Inverter-Based Microgrids
In recent years, the concept of Microgrids (MGs) has become more popular due to a significant integration of renewable energy sources (RESs) into electric power systems. Microgrids are small-scale power grids consisting of localized grouping of heterogeneous Distributed Generators (DGs), storage systems, and loads. MGs may operate either in autonomous islanded mode or connected to the main power system. Despite the significant benefits of increasing RESs, many new challenges raise
in controlling MGs. Hence, a three layered hierarchical architecture consisting of
three control loops closed on the DGs dynamics has been introduced for MGs. The
inner loop is called Primary Control (PC), and it provides the references for the DG’s
DC-AC power converters. In general, the PC is implemented in a decentralized way
with the aim to establish, by means of a droop control term, the desired sharing of
power among DGs while preserving the MG stability. Then, because of inverterbased DGs have no inertia, a Secondary Control (SC) layer is needed to compensate
the frequency and voltage deviations introduced by the PC’s droop control terms.
Finally, an operation control is designed to optimize the operation of the MGs by
providing power setpoints to the lower control layers.
This thesis is mainly devoted to the design of robust distributed secondary frequency and voltage restoration control strategies for AC MGs to avoid central controllers and complexity of communication networks. Different distributed strategies
are proposed in this work: (i) Robust Adaptive Distributed SC with Communication delays (ii) Robust Optimal Distributed Voltage SC with Communication Delays and (iii) Distributed Finite-Time SC by Coupled Sliding-Mode Technique. In all
three proposed approaches, the problem is addressed in a multi-agent fashion where
the generator plays the role of cooperative agents communicating over a network
and physically coupled through the power system. The first approach provides an
exponentially converging voltage and frequency restoration rate in the presence of
both, model uncertainties, and multiple time-varying delays in the DGs’s communications. This approach consist of two terms: 1) a decentralized Integral Sliding
Mode Control (ISMC) aimed to enforce each agent (DG) to behaves as reference
unperturbed dynamic; 2) an ad-hoc designed distributed protocol aimed to globally, exponentially, achieves the frequency and voltage restoration while fulfilling
the power-sharing constraints in spite of the communication delays. The second
approach extends the first one by including an optimization algorithm to find the
optimal control gains and estimate the corresponding maximum delay tolerated by
the controlled system. In the third approach, the problem of voltage and frequency
restoration as well as active power sharing are solved in finite-time by exploiting
delay-free communications among DGs and considering model uncertainties. In this approach, for DGs with no direct access to their reference values, a finite-time
distributed sliding mode estimator is implemented for both secondary frequency
and voltage schemes. The estimator determines local estimates of the global reference values of the voltage and frequency for DGs in a finite time and provides this
information for the distributed SC schemes.
This dissertation also proposes a novel certainty Model Predictive Control (MPC)
approach for the operation of islanded MG with very high share of renewable energy sources. To this aim, the conversion losses of storage units are formulated by
quadratic functions to reduce the error in storage units state of charge prediction
Best effort measurement based congestion control
Abstract available: p.
Linear time-delay systems: the complete type functionals approach
[EN] Recent results on Lyapunov-Krasovskii functionals of complete type for linear time-delay systems are presented. The main concepts and results are introduced for the single delay system case, and necessary and sufficient stability conditions expressed in terms of the Lyapunov delay matrix are explained. The use of complete type functionals in analysis and controller design is discussed. The contribution focuses mainly at results of researchers in Mexico.[ES] Se introducen resultados recientes del enfoque de funcionales de Lyapunov-Krasovski de tipo completo para sistemas lineales con retardos. Se explican brevemente los principales conceptos y resultados para el caso de sistemas con un retardo asà como las condiciones necesarias y suficientes de estabilidad expresadas en terminos del análogo de la matriz de Lyapunov. Las extensiones de este tipo de condiciones de estabilidad a otras clases de sistemas con retardos son expuestas brevemente. Tambien se presentan aplicaciones existentes del efoque de funcionales de tipo completo a problemas de analisis y de diseño de controladores. El trabajo se enfoca a contribuciones de investigadores de Mexico a este tema de estudio.Este trabajo ha sido realizado parcialmente gracias al apoyo del Conacyt, México, Proyecto A1-S-24796.Mondié, S.; Gomez, M. (2022). Contribuciones al estudio de sistemas lineales con retardos: el enfoque de funcionales de tipo completo. Revista Iberoamericana de Automática e Informática industrial. 19(4):381-393. https://doi.org/10.4995/riai.2022.16828OJS38139319
The Virtual Bus: A Network Architecture Designed to Support Modular-Redundant Distributed Periodic Real-Time Control Systems
The Virtual Bus network architecture uses physical layer switching and a combination of space- and time-division multiplexing to link segments of a partial mesh network together on schedule to temporarily form contention-free multi-hop, multi-drop simplex signalling paths, or 'virtual buses'. Network resources are scheduled and routed by a dynamic distributed resource allocation mechanism with self-forming and self-healing characteristics. Multiple virtual buses can coexist simultaneously in a single network, as the resources allocated to each bus are orthogonal in either space or time. The Virtual Bus architecture achieves deterministic delivery times for time-sensitive traffic over multi-hop partial mesh networks by employing true line-speed switching; delays of around 15ns at each switching point are demonstrated experimentally, and further reductions in switching delays are shown to be achievable. Virtual buses are inherently multicast, with delivery skew across multiple destinations proportional to the difference in equivalent physical length to each destination. The Virtual Bus architecture is not a purely theoretical concept; a small research platform has been constructed for development, testing and demonstration purposes
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Performance Evaluation of Classical and Quantum Communication Systems
The Transmission Control Protocol (TCP) is a robust and reliable method used to transport data across a network. Many variants of TCP exist, e.g., Scalable TCP, CUBIC, and H-TCP. While some of them have been studied from empirical and theoretical perspectives, others have been less amenable to a thorough mathematical analysis. Moreover, some of the more popular variants had not been analyzed in the context of the high-speed environments for which they were designed. To address this issue, we develop a generalized modeling technique for TCP congestion control under the assumption of high bandwidth-delay product. In a separate contribution, we develop a versatile fluid model for congestion-window-based and rate-based congestion controllers that can be used to analyze a protocol’s stability. We apply this model to CUBIC – the default implementation of TCP in Linux systems – and discover that under a certain loss probability model, CUBIC is locally asymptotically stable. The contribution of this work is twofold: (i) the first formal stability analysis of CUBIC, and (ii) the fluid model can be easily adapted to other protocols whose window or rate functions are difficult to model. We demonstrate another application of this model by analyzing the stability of H-TCP, another popular variant used in data science networks.
On a different front, a wide range of quantum distributed applications, which either promise to improve on existing classical applications or offer functionality that is entirely unobtainable via classical means, are helping to fuel rapid technological advances in the area of quantum communication. In view of this, it is prudent to model and analyze quantum networks, whose applications range from quantum cryptography to quantum sensing. Several types of quantum distributed applications, such as the E91 protocol for quantum key distribution, make use of entanglement to meet their objectives. Thus, being able to distribute entanglement efficiently is one of the most important and fundamental tasks that must be performed in a quantum network – without this functionality, many quantum distributed applications would be rendered infeasible. Modeling such systems is vital in order to better conceptualize their operation, and more importantly, to discover and address the challenges involved in actualizing them. To this end, we explore the limits of star-topology entanglement switching networks and introduce methods to model the process of entanglement generation, a set of switching policies, memory constraints, link heterogeneity, and quantum state decoherence for a switch that can serve bipartite (and in a specific case, tripartite) entangled states. In one part of this work, we compare two modeling techniques: discrete time Markov chains (DTMCs) and continuous-time Markov chains (CTMCs). We find that while DTMCs are a more accurate way to model the operation of an entanglement distribution switch, they quickly become intractable when one introduces link heterogeneity or state decoherence into the model. In terms of accuracy, we show that not much is lost for the case of homogeneous links, infinite buffer and no decoherence when CTMCs are employed. We then use CTMCs to model more complex systems. In another part of this work, we analyze a switch that can store one or two qubits per link and can serve both bipartite and tripartite entangled states. Through analysis, we discover that randomized policies allow the switch to achieve a better capacity than time-division multiplexing between bipartite and tripartite entangling measurements, but the advantage decreases as the number of links grows
The Role of Developmental Timing Regulators in Progenitor Proliferation and Cell Fate Specification During Mammalian Neurogenesis
Developmental timing is a key aspect of tissue and organ formation in which distinct cell types are generated through a series of steps from common progenitors. These progenitors undergo specific changes in gene expression that signifies both a distinct progenitor type and developmental time point that thereby specifies a particular cell fate at that stage of development. The nervous system is an important setting for understanding developmental timing because different cell types are produced in a certain order and the switch from stem cells to progenitors requires precise timing and regulation. Notable examples of such regulatory molecules include the RNA-binding protein LIN28, and its downstream target, miRNA let-7. Although LIN28 is known to regulate both cell fate and tissue growth, and at times to promote an undifferentiated state, thus far a unified understanding of LIN28’s biological role at the cellular level has not been attained. Here I address LIN28’s activity in mammalian postnatal neurogenesis. Constitutive expression of LIN28 in cells derived from the subventricular zone of the mouse caused several distinct effects: (1) the number of differentiated neurons was dramatically reduced while the relative abundance of two neuronal sub-types was significantly altered; (2) the population of proliferating neural progenitors in the SVZ was reduced while the proportion of neuroblasts was increased, (3) neuro-blast exit from the SVZ increased, and (4) the number of astrocytes was reduced while occasionally causing them to appear early. Thus, LIN28 acts at a post-stem cell/pre-differentiation step, and its continuous expression caused a precocious, not a reiterative phenotype, as is seen in other experimental systems. I made use of a circular RNA sponge that effectively inhibits let-7 activity to address the degree to which LIN28’s effects are due to its inhibition of let-7. Moreover, since LIN41 contributes to a subset of LIN28’s function in C. elegans, I explored whether LIN41 played a role in mammalian neurogenesis. I found that although LIN28 has a multifaceted role in the number and types of cells produced during postnatal neurogenesis, it appears that its action through let-7 accounts for only a fraction of these effects
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