Coordinated Control of Distributed Energy Resources in Islanded Microgrids

As the penetration of the distributed energy resources (DERs) in the power grid increases,new challenges are revealed, including: stability issues, frequency fluctuations, voltage control, protection system coordination, etc. A systematic approach for dealing with those issues is to view the DERs and associated loads as a subsystem or a microgrid (MG). MGs can operate either in the grid connected or islanded modes. As opposed to the grid connected mode, the voltage and frequency regulation and load/generation balancing during islanded mode is solely dependent on the local generation units. Therefore, stable and reliable operation of islanded MGs requires a real time coordinated control scheme. Conventionally, such coordination is achieved by means of the active power-frequency and reactive powervoltage droop control schemes. The conventional droop method, which is based on P-f droop concept in power systems, lacks compatibility with the resistive nature of networks as well as the low inertia of electronically interfaced DER units in MGs. As a result, it features a slow dynamic response but also a low power quality due to frequency and voltage fluctuations. This PhD research proposes a novel droop concept based on the global positioning system (GPS) and voltage-current (V-I) droop characteristics for coordination of inverter-based DER units in islanded MGs. The concept of V-I droop control is introduced in Chapter 2. In this control approach, each DER is equipped with a GPS receiver, which produces a pulse at frequency of 1Hz (1PPS). Since all GPS receivers are locked to atomic clocks of the GPS satellites, the 1PPS signal can be utilized to synchronize the time reference of the DER units. Using the common time reference and fixing the frequency at the nominal value, all of the units can share a common synchronous rotating reference frame (SRRF). Furthermore, proportional load sharing is achieved by drooping the d and q axis components of the reference voltage with respect to the d and q axis components of current, respectively. The proposed scheme not only circumvents the issue of frequency fluctuations but also is in accordance with the fast dynamics of inverter-based DER units and resistive nature of the networks in islanded MGs. The V-I droop scheme, in its basic form, relies on availability of GPS signals at each of the DER units. With the intention of improving the MG robustness with respect to GPS signal failure, a new control strategy based on V-I droop concept is presented Chapter 3. In this method, an adaptive reactive power-frequency droop scheme is used as a backup for the V-I droop controller to ensure synchronization in case of a GPS signal failure. Droop control schemes in general, and the proposed V-I droop strategy in particular are characterized by non-ideal sharing of current among the DER units due to the variations of voltage along the MGs. In order to improve the sharing accuracy of the V-I droop scheme iv while regulating the average voltage at the nominal value, a new distributed secondary control method based on consensus protocol is proposed in Chapter 4. In this method, the daxis droop characteristics is altered so as to regulate the average microgrid voltage to the rated value but also guarantee proper sharing of active power among the DERs. Additionally, the q-axis component of voltage is adjusted to perform proper sharing of current. Generally, DERs might be supplied from different energy sources, including renewables and storage systems. The intermittency of renewable energy resources on one hand and the limited capacity of the energy storage systems on the other hand, necessitate modification of droop characteristics based on an energy management plan. In Chapter 5, a novel distributed secondary control strategy is introduced for power management of integrated photovoltaicbattery DER units in islanded MGs. The distributed secondary controllers are coordinated based on a leader-follower framework, where the leader restores the MG voltage to the rated value and the followers pursue energy management. Unbalanced and nonlinear loads, which are quite common in MGs, adversely affect the power quality and sharing accuracy. In order to mitigate those issues, two new solutions are proposed in this thesis. In the first approach (Chapter 6), a new supplementary droop control scheme is added to the V-I droop controller to reduce the voltage unbalance while preventing current and power overload under unbalanced loading conditions. In the second approach (Chapter 7), a hierarchical control scheme, consisting of primary (modified V-I droop) and distributed secondary control levels is introduced to mitigate harmonic distortions and prevent overcurrent stresses under nonlinear and unbalanced loading conditions. Finally, the conclusions and possible future work are addressed in Chapter 8

Pierre Bourdieu. Critique et réflexivité comme attitude analytique en sciences de gestion

L'objet de cet article est d'analyser l'apport potentiel des travaux de Pierre Bourdieu pour les sciences des organisations. Les principaux concepts de cette sociologie sont présentés, avant de mettre en évidence quelques pistes d'application possibles en sciences de gestion. La dimension résolument critique de l'œuvre, qui valorise la part de réflexivité dans le travail du chercheur pour renouveler les pratiques scientifiques, est enfin soulignée.sociologie critique, réflexivité, émancipation

A Review on New Coronavirus Mechanism of Action

The new coronavirus outbreak caused by the severe acute respiratory syndrome coronavirus2 (SARS-CoV2) has resulted in more than 126750 deaths worldwide so far and billions of expenses for the governments. SARS-CoV2, similar to SARS, was transmitted from bats and spread via human to human closed contacts. The virus uses its spike protein to bind to angiotensin-converting enzyme 2 receptors on target cells to replicate. Consequently, it spreads from infected cells to contaminate other cells. The body’s first response toward the virus is to activate the innate immune system, leading to the synthesis of inflammatory mediators. In the next step, the adaptive immune system appears where B lymphocytes produce antibodies specific for the virus, and CD8+ cells kill the infected cells directly. In this article, we try to explain the virus mechanism of action and immune response in detail. Although many questions remain unanswered, we expect this review could help in vaccine and treatment progression, Keywords: COVID-19; SARS-CoV2; Mechanism; Review

Mouvements sociaux, organisations et stratégies

En France, les « mouvements sociaux » renvoient souvent, dans le langage commun, à une réalité négative, synonyme de protestations sociales, qui seraient soit le produit principal du corporatisme de certains corps de métier (les « cheminots »), soit la voie que choisissent d’emprunter les salariés afin de réagir face à des situations désespérées comme les fermetures sauvages d’entreprises ou les délocalisations massives. [Premières lignes de l'article