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This study presents a control method for DC–DC switching power converters to avoid the instability caused by the interaction between a tightly controlled switching converter and an input low-pass filter. The proposed method is based on the Lyapunov stability criterion. A non-linearised model is established, and the instantaneous quantities of the state variables are considered. The switching command is formulated by calculating the stability of the system. The inductor current and voltage across the filter capacitor, along with the state variables in the switching converter, follow the corresponding references. A boost converter is considered to demonstrate the associated effectiveness. The results of several simulations and experiments pertaining to the start-up, tracking, and regulation stages validate the proposed control approach. Moreover, the robustness of the approach against variations in the parameters of the controlled system is examined experimentally. To demonstrate the superiority of the proposed method, the method is compared with the proportional–integral, fuzzy, and flatness-based controllers.

International audience; Most of researchers have already studied and discussed the power sharing and synchronization of several generation systems connected to a unique point of common coupling (PCC) to which the loads are also connected. A high penetration rate of distributed generation systems (DGs) based on renewable energies has for logic consequence the development and setting up of networked multi-PCC microgrids. In this paper an improved droop control method for synchronization as well as active and reactive power sharing of different DGs in multiple PCC islanded microgrids is proposed while the real characteristics of the line feeders are taken into account. The simulation results confirm the feasibility and effectiveness of the proposed strategies for synchronization and interconnection of different microgrid DGs, while insuring accurate sharing of the DGs active and reactive powers. 

International audience; During the lifespan of a Polymer Electrolyte Membrane Fuel Cell (PEMFC) system, some heterogeneities between the cells constituting the stack can appear. The voltage of one particular cell in a stack may decrease because of specific aging or local malfunctioning such as drying. As a result, more heat is generated in this cell leading to an increase in its temperature and thus an additional voltage loss. This snowball effect can result in the failure of the cell. Therefore, the lifetime of a PEMFC stack can be increased by applying energy management to its cells. Notable that the output voltage of a cell is lower than a stack. Hence, a high conversion ratio converter is necessary to implement such energy management. An efficient way to increase the output voltage is to connect the output capacitors of the converters such as the boosts in series. Ensuring the converters' controllability is a key point to implement energy management. In this paper, an equalizer system is proposed to ensure the controllability of the boost converters. The balancing speed and the low number of switches are the main advantages of this system. The validity of the proposed system is verified through simulation and experiments.

International audience; This paper proposes a control method for a switching power converter. This converter is connected to a dc-link of a dc microgrid system via an LC input filter. This control method is based on a Lyapunov stability analysis; all state variables are acquired for evaluating stability analysis, including the output current for calculating the load resistance. To reduce a number of sensor counts, the input current is estimated. Moreover, loss occurrence in the converter is observed to reduce static errors of the controlled state variables, especially the output voltage. The losses are modelled as a series input voltage source observed from the dc-dc converter and a parallel current source. The latter enables us to eliminate the output current sensor. Using information from the estimated current source, a load resistance can be calculated. A Lyapunov function candidate is chosen to ensure that the system, including the observer, operates in a stable manner. Based on evaluating the time-derivative of the chosen Lyapunov function, we obtain the switching command that makes the system operate stably in each sampling cycle. In this paper, the proposed technique is applied on a boost converter. The simulation and as well experimental results validate the proposed control.

Microgrids are designed to operate at a fundamental frequency (50 or 60 Hz). However, certain types of loads generate voltage and/or current harmonics that increase power system heat losses. These harmonic losses reduce system efficiency, cause apparatus overheating and increase power costs. Besides that, the Distributed Generation (DG) is expected to play an important role in future power systems, since it is able to reduce losses, improve power quality to end users. In this paper we propose new harmonic droop controller to compensate the harmonic currents produced by each DG to feed the nonlinear loads. Furthermore, the proposed controller is able to damp the total harmonic distortion of the voltage at the common connection point (PCC). Modeling and simulations incorporate to define the best control parameters for a proper operation. Experimental results are given to show the efficiency of the proposed algorithm.

International audience; It is well known that the interaction between cascaded individually designed power conversion systems can cause instability. To overcome this issue, a Hamiltonian energy control scheme is proposed, which is based on passivity control theory and port-controlled Hamiltonian framework. A complementary PI adjustment term is also included in the control algorithm to eliminate the steady-state output voltage error caused by the parameter uncertainty. The proposed control approach is applied to three different cascade structures. First, the cascade structure between dc/dc converters is considered, and the detailed controller design is given. Second, the cascade connection of a single converter and its LC filter is studied. By placing the LC filter into the Hamiltonian model of the controlled converter system, the dynamic and potential instability caused by the filter can be adjusted. Finally, the cascade structure between subsystems including filters and converters, which are common in microgrids, is studied. By using the Hamiltonian function (storage function) as the Lyapunov function candidate, the large-signal stability of each controlled converter system is proved. When the cascade structure contains multiple controlled converter systems, the stability of the entire cascaded system is guaranteed by the superposition of multiple Lyapunov functions. A 3.5 kW 220-270-350 V test bench is built in the laboratory to demonstrate the application of the proposed control approach to these three cascade structures.

The lifetime and cost are two bottlenecks in the widespread use of fuel cells. Increasing the lifetime of the fuel cell can also counteract the cost justification bottleneck. However, this depends on the method of extending the life of the fuel cell. The durability of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) can be improved using a management system. This management system must control the operating condition of cells or cell groups in such a way that the electrochemical and fluidic instabilities can be avoided to improve the lifetime of a stack. Developing this Fuel Cell Management System (FCMS) is challenging due to the coupling between cells inside a stack. In this paper, an FCMS is proposed. It can detect the instability of cell groups and change their reference power based on the detected instabilities. Since a mathematical model has a high ability to describe the phenomena occurring in a fuel cell, this paper uses a model to describe its operating conditions. The reference power of the cell groups can be changed using a developed power electronics structure. The last piece of the FCMS puzzle is to develop a management strategy. This strategy is developed in this paper. Eventually, the proposed system is evaluated experimentally. The experimental results validate the effectiveness of the proposed system.

International audience; This paper describes a method to control a dc-dc switching power converter with an LC input filter working under continuous conduction mode. It is known that the interaction between the low-pass filter and the tightly controlled switching converter occurs, and it might lead to an instability operation. The idea of this method relies on the Lyapunov stability criterion. The switching command of this control is defined by the manner of pre-evaluating the stability of the controlled system. All state variables are controlled in the same time. Therefore, the responses of this controlled system are fast while guaranteeing a stable operation. The proposed control is applied to a boost converter to demonstrate the implementation and its performance. The simulation and experimental results validate the proposed control approach.

Si la naissance de la bande dessinée s’accompagne d’une théorisation à travers les travaux d’un Töpffer cherchant à circonscrire l'ensemble des possibles qui s'offrent à lui, force est de constater que la veine va bien vite se tarir. Rapidement exclue du champ critique, la bande dessinée semble ne bénéficier d'aucune analyse sérieuse qui soit de nature à lui forger des repères. L'Histoire montre que c'est de manière empirique qu'elle a travaillé sa substance, parfois avec génie (McCay, Hergé…) souvent sans âme, à l’exception de quelques tentatives que le présent article entend présenter.If the birth of comic strips is accompanied by a theorical effort through the work of a Töpffer seeking to circumscribe all possible available to them, it is clear that the vein will soon dry up. Quickly removed from the critical field, the comic seems to benefit from any serious analysis that could help authors and readers in their practises. Through History, Comics has worked empirically it its substance, sometimes with genius (McCay, Hergé ...) often soulless, with the exception of some attempts that this article intends to present.

In this paper, we investigated the power sharing issues in mesh islanded microgrids that contain several distributed generators (DGs) and loads connected to different points of common coupling (PCC). Firstly, an improved decentralized droop control algorithm is proposed to achieve the active and reactive power sharing of different DGs in reconfigurable mesh islanded microgrids. Accurate power sharing was obtained even though line parameters or the mesh microgrid configuration were unknown. Secondly a state-space model of the whole mesh microgrid was developed, considering several generators with their decentralized controllers, line feeders, and dynamic loads. This model was used to design parameters of droop controllers, to study the asymptotic stability and the robustness properties of the system. All strategies and analyses were validated by simulation based on the generic microgrid detailed in the standard IEEE 9bus test feeder.

International audience; This paper presents the stability analysis of dc distributed power systems with multiple converter-controlled loads. The load converters are tightly controlled, behaving as constant power loads with low-damped LC filters. The dynamic behavior of the system in high frequency range is often not studied with the classical tools based on conventional averaging techniques. However, dc power systems with the reduced size filter, and consequently, the high resonant frequency, are widely used in transportation applications. In this paper, the stability analysis of the system is established based on a discrete-time model of the system, taking into account the switching frequency and intrinsic nonlinearities of the system model. The impacts of the filter parameters and interactions among the constant power loads are investigated with the proposed discrete-time method. Moreover, an active stabilizer is developed and included in the dynamic model of the system, in order to extend the stability margin. The theoretical observations are then validated experimentally on a laboratory hardware prototype.

Mid-altitude, mid-latitude mountains are complex environments owing to their Pleistocene glacial heritage, the importance of geomorphic processes on the steep slopes, and the climatic conditions that are often close to periglacial. These factors, along with the fragmentation of the alpine habitats, enhance the topographic and floristic diversity of these environments in northeastern North America. Through case studies, this synthesis underlines the interactions between the geosphere (glacial, paraglacial, and periglacial processes), the atmosphere (climatic fluctuations), and the biosphere (vegetation establishment and evolution to the present day) that explain the low elevation of the northeastern North American alpine environment and that testify to its complexity. Vegetation established earlier in the southern ranges, following the same general trend as the Laurentian Ice Sheet recession. However, local factors such as ice retreat, response to global-scale climate changes, and paraglacial processes acted in synergy to increase the resilience and to influence the occurrence of alpine landscapes. The establishment of the latter environment can therefore be considered to be azonal. Finally, our findings highlight the lack of a conceptual framework, systemic studies, and multi-proxy reconstructions of alpine environments located at the limit of bioclimatic zones controlled by the equilibrium between biostatic and rhexistatic regimes.

Voltage imbalance poses a challenge to photovoltaic systems. A modular structure based on a three-level boost converter is proposed to address this problem. The three-level boost converter offers advantages, such as a low current ripple and voltage stress, over a classic boost converter. These advantages offset the use of additional elements in the proposed converter circuit. Two capacitors are used to enable the innovative connection between multiple sources and the three-level boost converter. The second capacitor of the first module is shared with the first capacitor of the second module. This structure is used in conjunction with a controller to balance the voltages in the system. The operating modes of the two-module system in a nominal case are introduced. The controller is based on an indirect sliding model, wherein the input current of each energy source and the output voltage balance are considered. The performance of the current and voltage controllers are studied in two scenarios. The first case involves increasing the reference current and the presence of four sliding surfaces related to the current control and voltage balance, whereas the second case involves the presence and absence of two sliding surfaces related to the voltage balance. The dynamic response of this controller is also compared with the Proportional Integral(PI) controller. Large-signal modeling of the two-module system and the accuracy of this model in two cases of radiation change and panel temperature change is investigated. The robustness of the system is investigated using this large-signal model in two cases that involve changing the inductors and capacitors of the system. A topology consisting of two conventional boost converters is chosen to compare energy stored and efficiency with the proposed topology. The capacitance of the system is calculatedfor two topologies. The energy stored in the two systems is compared. The two-module system is simulated using Simulink MATLAB software. The simulation and experimental results validate the proposed system.

Online knowledge of dc–dc converters behavior is always of great interest. Based on a reliable model of the converters, some great improvement can be achieved. First, the control of the converters can be designed more precisely, especially while thinking in nonlinear theories model-based controls, and it can help to improve the energy management and the efficiency. Also, the knowledge of the converters gives some really useful indications about their state of health and, then, represents a good diagnosis tool and fault detection possibility. This paper proposes a modeling of the converters and a new state observer dedicated to an online estimation of the model parameters. The proposed average models include parameter modeling the losses and their estimation. They are validated on two different converters: the classical dc–dc boost converter and the current-fed dual-bridge dc–dc converter (CFDB—also called isolated boost). It is shown that the model of this last converter is strongly nonlinear, which impacts on the estimation. Simulations and experimental validation are given both on the boost and the isolated boost, and comparisons with the Luenberger state observer and extended Kalman filter are given to underline the interest of the proposed parameter estimation in terms of convergence for nonlinear systems and convergence rapidity.

International audience; In this paper, a practical online open-switch fault detection method is presented. The proposed open-switch fault detection method is used to detect the open-circuited fault switch in the parallel inverters system. This method can relieve the severe effects of the over-current stress, to avoid the secondary fault to the electrical components in the parallel inverters system. This method is based on the detection of the resulting zero-sequence current, under the fault conditions. The proposed algorithm is activated by detecting the zero-sequence current between the parallel units, under the open-switch fault conditions. The sign of the resulting zero-sequence current is related to the upper or the lower open-switch fault case. The faulty phase identification relies on the analysis and the investigation of only the current measurements information of the faulty inverter. This method can get rid of the effects of the load, to obtain a reliable identification. The effectiveness of the proposed open-switch fault detection algorithm, is theoretically analyzed and validated by the simulations and the experiments.

A robust tool compensating stability problems occurring in interconnected voltage-source rectifier loads with input passive LCL filter is developed. An LCL filter can reduce the harmonics introduced by PWM switching frequency and guarantee low currents THD using small component values comparing to the well-known L filter solution. Nevertheless, the additional poles introduced by the LC part induce resonance in the system and may lead to instability. To deal with this issue, a large-signal stabilizing supervisor based on the direct Lyapunov theorem is proposed in this contribution. The use of Lyapunov theory allows constructing feedback stabilizing signals which keep both the damping performance and robustness of the proposed stabilizer. The stability phenomenon is highlighted and the effectiveness of the proposed solution is validated by experimental results.

This paper develops a robust tool for handling ac side stability issues occurring in LCL-filter-based grid-connected converters.LCL filters give advantages in cost and dynamic as smaller inductors employed compared with L filters for the same required damping of the switching harmonics. Nevertheless, the additional poles introduced by the LC part induce resonance in the system leading to stability problems. To deal with this stability issue, a new damping method based on the generalized circle criterion is proposed in this paper. The large signal nature of the applied theory allows obtaining high damping efficiency and robustness under large variations of the grid impedance. A theoretical analysis and the experimental results are presented to show the effectiveness of the proposed compensating method.

In this paper, a new current sharing technique on a general case of N paralleled DC-DC boost converters is presented. The proposed optimization is based on the knowledge of individual boost parameters. Every loss through the structure are modeled by equivalent resistors. Using an accurate online estimation of those resistors, the losses through each individual converter can be determined. Then, a new current sharing scheme is defined aiming to maximize the global efficiency of the overall structure. To verify the proposed method, simulations and experiments have been realized on a three-parallel boost converters structure.

The Spanner project reports that one can build practical large-scale systems that combine strong semantics with geo-distribution. In this review manuscript, we provide insight on how Spanner's concurrency control provides both read-only transactions which avoid locking data, and strong consistency.

DC distribution power systems are vulnerable to instability because of the destabilizing effect of converter-controlled constant power loads (CPLs) and input filters. Standard stability analysis tools based on averaging linearization techniques can be used only when the switching frequency of the converter is significantly higher than the cutoff frequency of the filter. However, dc distribution systems with a reduced size filter, and consequently a high cutoff frequency, are common in transportation applications. Conventional methods fail to detect instabilities in the system because they do not take into account the switching effect. To overcome this drawback, this paper proposes a discrete-time method to analyze the stability of dc distribution systems. This model is applied here to a dc power system with a CPL. The switching effects and the nonlinearities of the system model are taken into account with a simple discretization approach. The proposed method is able to predict the dynamic properties of the system, such as slow-scale and fast-scale instabilities. An active stabilizer is also included in the system model in order to extend the stability margin of the system. Finally, these observations are validated experimentally on a laboratory test bench. © 20xx IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.

International audience; In this paper, a global study in terms of control architecture is applied to parallel voltage-source inverters. Parallelism of inverters represents very interesting advantages for the industrial applications to meet the high-power requirements, and it is possible to apply maintenance during the operation without interruptible operation. The main objective of this paper is to guarantee a reliable operation of the parallel inverters system during healthy and faulty conditions due to full disconnection of any inverter. Special precautions are recommended to avoid the negative effects of the circulating currents, which are caused essentially by the asynchronous pulse width modulation or existence of dispersion of the system component characteristics. To deal with this crucial problem, one-loop flatness-based control is proposed to the control of N parallel inverters and allows obtaining low total harmonic distortion. The proposed control uses the advantages of the flatness to ensure high power quality at the point of common coupling and equal currents distribution between the parallel inverters and minimize the impact of a full disconnection of any inverter on the performance. The proposed algorithm is theoretically analyzed and validated experimentally.

International audience; The usual method of estimating the onset of bifurcation and studying non-linear phenomena in power electronic devices consists of defining the discrete-time model. This model provides more precise results than an averaged model. However, the discrete-time model cannot be developed easily for some particular systems. In this study, a continuous-time averaging model is developed for a boost converter in the continuous-conduction mode (CCM). This proposed model is used to obtain the eigenvalues of the system to predict the stable operation of the period-1 orbit. It is observed that the loss of the asymptotic stability of the proposed model corresponds to the onset of period-doubling bifurcation in the discrete-time model. The generalisation of this method is investigated by applying the proposed method to a DC active filter converter connected in parallel with a CCM boost converter. Using this model, it is possible to obtain the eigenvalues of the system to investigate robustness properties with regards to system parameter variations. To confirm the validity of the proposed method, simulation and experimental results are presented.

International audience; Small LC filters connected to tightly controlled power loads may lead to instabilities. With small LC filters comes a high cutoff frequency, and the conventional stability analysis based on an average model may not be precise enough. Other tools based on another model are required to study this type of system. In this paper, a new discrete-time model is proposed to investigate the behavior of the studied system and to take into account the switching effect. Then, the stability analysis of the limit cycle is done using Floquet multipliers. This system is composed of a dc-dc boost converter, an LC filter, and a controlled power load. Simulation and experimental results are given to validate the proposed model and analysis.

Recently, hybrid electrical power sources composed of storage elements and renewable energy sources are known to have made great development. These energy sources are connected to a dc bus and need a dc-to-ac converter to transfer the produced energy to the grid. Three-leg voltage source inverters equipped with an output LC filter are often used. The main objective of this stage is to generate a three-phase sinusoidal voltage with defined amplitude and to ensure the smallest harmonic distortion rate of the output voltage for any load conditions. To satisfy the defined objectives, we present in this paper a new control method based on differential flatness control technique. The main interest of this control method is the possibility to define the behavior of the state variable system in the steady state as well as in transients. The use of only one control loop allows obtaining high dynamic properties of the system which ensure small harmonic distortion rate of the output voltage. Experimental results under balanced, unbalanced, and nonlinear load conditions are presented and validate the effectiveness of the proposed control methods.

This paper presents an energy management method in an electrical hybrid power source (EHPS) for electric vehicular applications. The method is based on the flatness control technique (FCT) and fuzzy logic control (FLC). This EHPS is composed of a fuel cell system as the main source and two energy storage sources (ESSs)-a bank of supercapacitors (SCs) and a bank of batteries (BATs)-as the auxiliary source. With this hybridization, the volume and mass of the EHPS can be reduced, because the high energy density of BAT and high power density of SC are utilized. In the proposed novel control strategy, the FCT is used to manage the energy between the main and the auxiliary sources, and the FLC is employed to share the power flow in the ESS between the SC and the BAT. The power sharing depends on the load power and the state of charge of the SC and the BAT. EHPS is controlled by the regulation of the stored electrostatic energy in the dc buses. The main property of this strategy is that the energy management in the power source is carried out with a single general control algorithm in different operating modes, consequently avoiding any algorithm commutation. An EHPS test bench has been assembled and equipped with a real-time system controller based on a dSPACE. The experimental results validate the efficiency of the proposed control strategy.

Employing fuel cell (FC) as main source requires increasing and regulating its output voltage. In this paper, nonisolated dc-dc converter with high voltage ratio is proposed to interface between the FC and high-voltage dc bus. To take into account the low-voltage-high-density characteristics of power sources, a cascaded structure composed of two subconverters in cascade has been chosen and allows obtaining high voltage ratio. The choice of each subconverter is based on source requirements and its performances. Consequently, in this paper, a converter consisting of two-interleaved boost converter is chosen as first subconverter and a three-level boost converter is chosen as second subconverter. Control of the whole system is realized by energy trajectory planning based on flatness properties of the system. The design of trajectories is explained and allows respecting the fuel-cell constraints as main power source. To ensure correct design of the energy trajectories, a noninteger power-law function is used to model the static characteristic of the FC. This law allows investigating the effect of humidity and temperature on the dynamics of the proposed system. The control of both current and voltage balance across the output serial capacitors of the three-level boost converter is ensured by nonlinear controllers based on a new nonlinear model.

Fuel-cell vehicles (FCVs) with energy storage (ES) device(s) could result in improved lifetime, performance, fuel economy, and reduced cost. This paper presents the utilization of an ES device consisting of a supercapacitor bank for future electric vehicles with a hydrogen fuel cell (FC) as the main power source. The study mainly focuses on the innovative control law based on the flatness properties for a FC/supercapacitor hybrid power source. Utilizing the flatness principle, we propose simple solutions to the hybrid energy-management and stabilization problems. A supercapacitor module, as a high dynamic and high-power density device, functions to supply energy to regulate the dc-bus energy. The FC, as a slower dynamic source in this system, functions by supplying energy to keep the supercapacitor module charged. To ensure energy-efficient operation of the FC stack, the output current ripple of the FC stack is minimized by parallel boost converters with an interleaving switching technique for a high-frequency ripple by the supercapacitor for a low-frequency ripple. To authenticate the proposed control laws, a test bench is realized in the laboratory. The control algorithm (energy and current control loops) is digitally implemented by dSPACE controller DS1103. Experimental results with small-scale devices (a proton exchange membrane FC (PEMFC) of 500 W, 50 A, and 10 V and a supercapacitor bank of 250 F, 32 V, and 500 A) substantiate the excellent performance during load cycles.

International audience; Cet article présente une nouvelle techniqued'équilibrage pour les tensions d'entrée d'un onduleur multiniveauxde type "Neutral Point Clamped (NPC)". Cette techniquepeut être utilisée dans des applications liées aux énergies renouvelables.Pour ces types d'applications, un onduleur classique2-niveaux est général utilisé, cependant lorsqu'il y a deuxsources, il est plus avantageux d'utiliser un onduleur NPC. Lesystème considéré dans ce travail est donc constitué de deuxsources photovoltaïques, deux convertisseurs DC/DC boost etun onduleur NPC connecté à un bus AC. Pour la définition dupoint de fonctionnement de chaque source, une stratégie de typeDistributed-MPPT(DMPPT) offre une amélioration du rendementénergétique annuel entre 6.9%-11.1% par rapport au Centralised-MPPT(CMPPT). En utilisant la technique d'équilibrage proposéeassociée à une configuration de type DMPPT, un MPPT indépendantpour les deux sources peut être obtenue pour une plus grandeplage de fonctionnement et ainsi augmenter l'efficacité énergétiqueannuelle globale de l'installation. Des résultats expérimentauxobtenus sur un banc de test du laboratoire permettent de validerles résultats de simulation et le modèle présentés dans cet article.

In this paper, the modeling and study of a new hybrid current controller is presented. It ensures high dynamic response with a fixed-frequency operation mode, a zero static error, and high robustness properties in regard to system parameters variations. To model the proposed nonlinear current controller, different tools are developed. In a first step, a high-frequency average model is proposed. It allows studying the average dynamic properties (bandwidth, time response, and overflow). To investigate the behavior of the current ripple due to the switching effect, a second model, based on the construction of a 3D bifurcation diagram and the definition of a form function, is established. This model allows studying the nature of the cycle described by the state trajectory and proving that the system operates with a fixed switching frequency. Design rules of the control parameters of this controller are explained and its robustness properties are tested by numerical simulations and validated by experimental tests.

It is known that power electronic circuits such as DC–DC converters are rich in non-linear phenomena. It is often necessary to determine when the system becomes unstable. The authors investigate a continuous-time averaging model for a modulated-hysteresis current controller. This model is applied to a boost converter working in continuous conduction mode to estimate the bifurcation point and to study the stability of the controlled system. This controller ensures a fixed-switching frequency, a zero static error and high robustness properties of the system parameter variations. Simulation and experimental results are presented to validate the proposed approach.

This paper presents the behaviour of a proton exchange membrane fuel cell (PEMFC) connected to a static dc–dc converter. Two different models of the PEM fuel cell are obtained from static measurements and impedance spectroscopy. The paper points out the necessity to use different models according to the type of study performed on the system. The comparison of models at high semiconductors switching frequency (25 kHz) is illustrated. Various experimental results obtained on a 500 W PEMFC test bench are compared with simulation ones to illustrate the accuracy of the proposed models.

Wide area network testbeds allow researchers and engineers to test out new equipment, protocols and services in real-life situations, without jeopardizing the stability and reliability of production networks. The Data TransAtlantic Grid (DataTAG) testbed, deployed in 2002 between CERN, Geneva, Switzerland and StarLight, Chicago, IL, USA, is probably the largest testbed built to date. Jointly managed by CERN and Caltech, it is funded by the European Commission, the U.S. Department of Energy and the U.S. National Science Foundation. The main objectives of this testbed are to improve the Grid community's understanding of the networking issues posed by data-intensive Grid applications over transoceanic gigabit networks, design and develop new Grid middleware services, and improve the interoperability of European and U.S. Grid applications in High-Energy and Nuclear Physics. In this paper, we give an overview of this testbed, describe its various topologies over time, and summarize the main lessons learned after two years of operation.

Photovoltaic (PV) systems are one of the main actors in distributed power generation. In particular, in urban contexts, the PV generators can be subjected to mismatching phenomena due to the different orientation of the modules with respect to the sun rays or due to shadowing. In these cases, the maximum power point tracking (MPPT) function must be designed carefully. In this paper, architecture, including one dc/dc converter for each PV generator, is considered. The converters' output terminals are series connected to a high-voltage dc bus, where also a bidirectional dc/dc converter managing the power from/to a storage device is plugged. The functional constraints deriving from the dc/dc converters' connection, the mismatching phenomena, the MPPT capabilities of the inverter, connected with its input terminals at the dc bus, are taken into account in order to determine the best operating point of the system as a whole. The real-time constrained optimization problem is solved by using the particle swarm optimization method, which needs the knowledge of the actual current versus voltage curve of each PV generator. The practical impact of this need is also discussed in the paper. The feasibility and the performances of the proposed approach are experimentally validated by using a laboratory prototype.

Since the mid 1990s, network and systems management has steadily evolved from centralized paradigms, where the management application runs on a single management station, to distributed paradigms, where it is distributed over many nodes. In this survey, our goal is to classify all these paradigms, especially the new ones, in order to help network and systems administrators design a management application, and choose between mobile code, distributed objects, intelligent agents, etc. Step by step, we build an enhanced taxonomy based on four criteria: the delegation granularity, the semantic richness of the information model, the degree of specification of a task, and the degree of automation of management.

Winegrowers' syndicates in Languedoc were a long time indépendant of national agricultural syndicalism. They demanded that the State organize the market to maintain the price of wine and defend the income of winegrowers'. They used radical actions and somtimes violence. The Montredon shooting (March 1976) forced them to change. Since 1976, wingrowers'syndicalism has changed its discourse and its practices. It accept now the market, want to improve the quality of the wines and want to create groups of producers, only ways to improve winegrowers'income., En Languedoc, les syndicats de viticulteurs ont longtemps fait le choix de l'autonomie vis-à- vis du syndicalisme agricole national. Leur principale revendication portait sur l'intervention de l'État sur le marché et la défense du prix du vin afin de protéger le revenu des producteurs. Pour les imposer, ils ont utilisé l'action directe parfois violente, la fusillade de Montredon (1976) les oblige à changer. Depuis cette date, le syndicalisme viticole a modifié son discours et ses pratiques. Il accepte les lois du marché, défend la nécessité de l'action économique et la reconversion qualitative, seuls moyens susceptibles d'améliorer le revenu des vignerons., Martin Jean Philippe. Viticulture du Languedoc : une tradition syndicale en mouvement. In: Pôle Sud, n°9, 1998. La "grande transformation" du Midi Rouge, sous la direction de William Genieys . pp. 71-87.