All Years Seminars
[INMA] 2015-02-10 (14.00) : Plug-and-Play Operation of Microgrids
At Euler building (A002)
Speaker :
Dörfler, Florian
Abstract : Microgrids are low-voltage electrical networks composed of distributed generation, storage, load, and managed autonomously from the larger utility grid. Modeled after the hierarchical control architecture in bulk power systems, a layering of primary, secondary, and tertiary control has become the standard operation paradigm for microgrids. Despite this superficial similarity, the control objectives in microgrids across these three layers are varied and ambitious, and they must be achieved in real-time without time-scale separations, without centralized decision makers, and ideally in a model-free fashion. In this seminar, we explore different control strategies for these three layers and illuminate some possibly-unexpected connections and dependencies among them. We build upon decentralized primary droop control strategies and motivate the need for additional secondary regulation. We find that distributed averaging-based secondary control architectures using communication among the generation units offer the best combination of flexibility and performance. We further leverage these results in tertiary-level energy management tasks such as an economic generation dispatch. Surprisingly, we show that the minimizers of the economic dispatch optimization problem are in one-to-one correspondence with the set of steady-states reachable by droop control. This equivalence results in simple guidelines to select the droop coefficients, which include the well known criteria for power sharing. Finally, we stress the idealistic assumptions underlying any droop-based control architecture and propose a virtual oscillator control paradigm that contains droop control in the asymptotic limit. We illustrate the performance and robustness of all of our designs through hardware experiments.
[INMA] 2015-02-04 (16.00) : Local methods for nonlinear science and engineering
At Sud18 Auditorium
Speaker :
Sepulchre, Rodolphe
Abstract : Inaugural Lecture of the Chaire Franqui, see more information on
http://www.uclouvain.be/503702.html
[INMA] 2014-12-09 (14.00) : The importance of non-Markovianity in temporal networks
At Euler building (A002)
Speaker :
Antonios Garas (ETH Zurich)
Abstract : Typically, real complex systems have topologies that evolve with time. This poses limitations to the usual way of studying them through static, time-aggregated networks. In this talk I will discuss about a
recently introduced temporal-topological feature called betweenness preference, which describes the tendency of nodes to preferentially connect - in a temporal sense - particular pairs of neighbors. This preferential connectivity results to specific ordering of interactions in temporal networks, alters causality, makes the system non-Markovian, and affects the evolution of dynamical processes. Thus, I will describe a methodology to analytically predict causality-driven changes of diffusion speed in non-Markovian temporal networks. I will show that compared to the time-aggregated network, non-Markovian characteristics can lead to both a slow-down or speed-up of diffusion, which can even outweigh the decelerating effect of community structures in the static topology. Therefore, I will argue that non-Markovian properties of temporal networks constitute an important additional dimension of complexity in time-varying complex systems.
[INMA] 2014-12-02 (14.00) : Geometric methods for recommender systems
At Euler building (A002)
Speaker :
Absil, Pierre-Antoine
Abstract : The central topic of this talk is low-rank optimization, where the archetypal problem consists of minimizing a real-valued function defined on a set of matrices of fixed or bounded rank. The fact that the set of fixed-rank matrices admits Riemannian manifold structures endows the problem with a rich geometry. We will see how geometric concepts can be exploited to design efficient low-rank optimization methods. Upstream, this will lead us into the realm of Riemannian optimization. Downstream, we will glance through various applications of low-rank optimization, such as the maximal cut of a graph, sparse principal component analysis, and recommender systems, with an emphasis on the latter. In recommender systems, a collection of items are available to users. For example, as popularized by the Netflix prize, the items can be movies and the users can be customers. Each customer has rated some movies, and the task is to predict how much the customers would like the movies they did not rate, so as to make a personalized recommendation. One popular model posits that the matrix of ratings is approximately low-rank. This results in the mathematical problem of finding a matrix of low rank that is optimal, in the sense that it agrees as well as possible (according to some criterion) with measured entries. We will see that, while the best known methods for recommender systems take the form of meta-algorithms that aggregate results provided by various techniques, low-rank optimization for recommender systems has recently made important progress and can provide a useful basis for better predictors. This talk is based on joint work with Nicolas Boumal.
[INMA] 2014-11-25 (14.00) : A Discrete Geodesic Calculus for Shape Space Applications
At Euler building (A002)
Speaker :
Wirth, Benedikt
Abstract : The notion of shapes can be useful in imaging and computer vision applications, for instance in the context of prior information in form of shapes. Unfortunately, shapes are often difficult to deal with since the space of shapes usually has no linear structure. However, the space of shapes can frequently be equipped with a Riemannian metric, turning it into a Riemannian manifold, a structure that can be exploited computationally. We show how a corresponding set of computational tools on a (possibly infinite-dimensional)
Riemannian manifold, a discrete geodesic calculus, can be developed just based on a computationally cheap approximate distance measure. It starts from an energy for discrete paths on the manifold, which gives rise to a notion of discrete geodesics as well as a discrete logarithmic map, a variational definition of a discrete exponential map, and a discrete parallel transport. We investigate the fundamental properties of those discrete operations and their convergence against their continuous counterparts, and we present applications in a space of viscous fluid-like shapes. (Joint work with Martin Rumpf, University of Bonn)
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