Eleni Stai

Related publications (12)

Improved Network-Calculus Nodal Delay-Bounds in Time-Sensitive Networks

Jean-Yves Le Boudec, Ehsan Mohammadpour, Eleni Stai

In time-sensitive networks, bounds on worst-case delays are typically obtained by using network calculus and assuming that flows are constrained by bit-level arrival curves. However, in IEEE TSN or IETF DetNet, source flows are constrained on the number of packets rather than bits. A common approach to obtain a delay bound is to derive a bit-level arrival curve from a packet-level arrival curve. However, such a method is not tight: we show that better bounds can be obtained by directly exploiting the arrival curves expressed at the packet level. Our analysis method also obtains better bounds when flows are constrained with g-regulation, such as the recently proposed Length-Rate Quotient rule. It can also be used to generalize some recently proposed network-calculus delay-bounds for a service curve element with known transmission rate.

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2023

Admissibility of Uncertain Injections in Quadratic Algebraic Systems

Jean-Yves Le Boudec, Eleni Stai, Cong Wang

We study the admissibility problem in multivariate algebraic systems, such as ac electrical networks, where the power injection is quadratic in the state. The goal of such systems is to ensure that the state stays in some security set (e.g., magnitudes of nodal voltages and branch currents are within safety bounds). A common practice is to implicitly control the state by controlling the injection; a difficulty is that the number of states that correspond to a given injection can be zero or many. Further, the injection is subject to some uncertainty. The admissibility problem is whether it is possible to ensure that the state stays in the security set, given that the only available information is some uncertainty set that constrains the injection. We extend the recently proposed V-control theory, design a solution framework to test if a given uncertainty set is admissible, and develop a concrete method for ac electrical networks.

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2021

On the Solution of the Optimal Power Flow for Three-Phase Radial Distribution Networks With Energy Storage

Jean-Yves Le Boudec, Eleni Stai, Cong Wang

We study a multiperiod optimal power flow (OPF) problem in unbalanced three-phase radial distribution networks with batteries. To address this problem, we first take the resistance-line battery model that treats lossy batteries by adding resistive lines and virtual buses. Then, we derive a linearization for the power flow equations, called generalized three-phase simplified DistFlow, which accounts for shunt elements. Also, it is exact at a freely chosen point while having globally good performance. Using it, we extend the heuristic iterative OPF algorithm, CoDistFlow, to unbalanced three-phase networks. At convergence, three-phase CoDistFlow gives a solution that satisfies the exact nonlinear power-flow equations and the exact security constraints. To theoretically guarantee the convergence of three-phase CoDistFlow, we develop the forced convergent three-phase CoDistFlow. Moreover, we show how to adapt the three-phase CoDistFlow to solve a scenario-based OPF in the presence of uncertainties, whereas other existing OPF solution methods may not offer this possibility. Finally, we perform evaluations on the IEEE 123-bus test feeder and comparisons with Ipopt.

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2021

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We consider low run-time complexity power management in distribution grids with renewable energy sources (RESs) and batteries, where forecasts are unavailable. We propose iterative Lyapunov Real-time Control (iLypRC), a fast online algorithm, which does not need forecasts, for the real-time battery control. iLypRC is designed via Lyapunov optimization and applies either to follow a previously computed dispatch plan or to optimize the monetary cost, while it satisfies look-ahead state-of-energy constraints. iLypRC requires only bounds on the uncertain quantities for each real-time interval. iLypRC efficiently accounts for grid losses, battery efficiency, and grid constraints via iterative linearizations of the power flow equations. We compute a theoretical upper bound on the difference between the cost of iLypRC and the cost of an oracle. Finally, numerical evaluations (using real data) on a 4-bus distribution grid and a 34-bus real Swiss grid show that iLypRC achieves a cost very close to that of a model predictive control (MPC) with a good forecast, but iLypRC needs no forecast and has much lower run-time complexity. Also, when the MPC forecast is inaccurate, iLypRC outperforms MPC.

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2021

A Receding Horizon Control Approach For Re-Dispatching Stochastic Heterogeneous Resources Accounting for Grid and Battery Losses

Jean-Yves Le Boudec, Emil Namor, Mario Paolone, Fabrizio Sossan, Eleni Stai

In this paper, we propose a re-dispatch scheme for radial distribution grids hosting stochastic Distributed Energy Resources (DERs) and controllable batteries. At each re-dispatch round, the proposed scheme computes a new dispatch plan that modifies and extends the existing one. To do so, it uses the CoDistFlow algorithm and applies a receding horizon control principle, while accounting for hard time computation constraints that impact on the instantaneous update of a dispatch plan. CoDistFlow handles stochastic DERs and prosumers uncertainties via scenario-based optimization and the non-convexity of the AC Optimal Power Flow by iteratively solving suitably defined convex problems until convergence. We perform numerical evaluations based on real-data, obtained from a real Swiss grid. We show that, with our proposed re-dispatch scheme, the daily dispatch tracking error can decrease more than 80%, even for small battery capacities, and if re-dispatch is frequent enough, it can be eliminated. Finally, we show that re-dispatch should be performed as often as the market allows and the performance continues to improve.

2019

FCR: Fast and Consistent Controller-Replication in Software Defined Networking

Jean-Yves Le Boudec, Maaz Mashood Mohiuddin, Mia Primorac, Eleni Stai

We consider the problem of coordination among replicated SDN controllers, where the challenge is to ensure a consistent view of the network while reacting to network events in a prompt manner. Existing solutions are either consensus-based, which achieve consistency at the expense of high latency; or eventual-consistency-based, which have low latency at the expense of severe limitations on the types of applications and policies implementable by the controller. We propose the Fast and Consistent Controller-Replication (FCR) scheme. FCR is based on a deterministic agreement mechanism that performs agreement on the input of controllers, instead of agreement on the output as done in consensus mechanisms. We formally prove that FCR provides the same guarantees in terms of implementable applications and network policies, as any deterministic single-image controller. Through simulation and implementation, we show that these guarantees can be implemented with little latency overhead, compared to eventual-consistency approaches, and can be achieved significantly faster than consensus-based approaches.

2019

Improved Credit Bounds for the Credit-Based Shaper in Time-Sensitive Networking

Jean-Yves Le Boudec, Ehsan Mohammadpour, Eleni Stai

In Time-Sensitive Networking (TSN), it is important to formally prove per-flow latency and backlog bounds. To this end, recent works have applied network calculus and obtained latency bounds from service curves. The latency component of such service curves is directly derived from upper bounds on the values of the credit counters used by the Credit-Based Shaper (CBS), an essential building-block of TSN. In this paper, we derive and formally prove credit upper bounds for the CBS, which improve on existing bounds.

2019

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Network calculus is often used to prove delay bounds in deterministic networks, using arrival and service curves. We consider a FIFO system that offers a rate-latency service curve and where packet transmission occurs at line rate without pre-emption. The existing network calculus delay bounds take advantage of the service curve guarantee but not of the fact that transmission occurs at full line rate. In this letter, we provide a novel, improved delay bound which takes advantage of these two features. Contrary to existing bounds, ours is per-packet and depends on the packet length. We prove that it is tight.

2019

Dispatching Stochastic Heterogeneous Resources Accounting for Grid and Battery Losses

Jean-Yves Le Boudec, Mario Paolone, Lorenzo Enrique Reyes Chamorro, Fabrizio Sossan, Eleni Stai

We compute an optimal day-ahead dispatch plan for distribution networks with stochastic resources and batteries, while accounting for grid and battery losses. We formulate and solve a scenario-based AC Optimal Power Flow (OPF), which is by construction non-convex. We explain why the existing relaxation methods do not apply and we propose a novel iterative scheme, Corrected DistFlow (CoDistFlow), to solve the scenario-based AC OPF problem in radial networks. It uses a modified branch flow model for radial networks with angle relaxation that accounts for line shunt capacitances. At each step, it solves a convex problem based on a modified DistFlow OPF with correction terms for line losses and node voltages. Then, it updates the correction terms using the results of a full load flow. We prove that under a mild condition, a fixed point of CoDistFlow provides an exact solution to the full AC power flow equations. We propose treating battery losses similarly to grid losses by using a single-port electrical equivalent instead of battery efficiencies. We evaluate the performance of the proposed scheme in a simple and real electrical networks. We conclude that grid and battery losses affect the feasibility of the day-ahead dispatch plan and show how CoDistFlow can handle them correctly.

2018

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We compute bounds on end-to-end worst-case latency and on nodal backlog size for a per-class deterministic network that implements Credit Based Shaper (CBS) and Asynchronous Traffic Shaping (ATS), as proposed by the Time-Sensitive Networking (TSN) standardization group. ATS is an implementation of the Interleaved Regulator, which reshapes traffic in the network before admitting it into a CBS buffer, thus avoiding burstiness cascades. Due to the interleaved regulator, traffic is reshaped at every switch, which allows for the computation of explicit delay and backlog bounds. Furthermore, we obtain a novel, tight per-flow bound for the response time of CBS, when the input is regulated, which is smaller than existing network calculus bounds. We also compute a per-flow bound on the response time of the interleaved regulator. Based on all the above results, we compute bounds on the per-class backlogs. Then, we use the newly computed delay bounds along with recent results on interleaved regulators from literature to derive tight end-to-end latency bounds and show that these are less than the sums of per-switch delay bounds.

IEEE2018