This paper presents a new droop-based control strategy for hybrid microgrids (HMG) with improved power sharing. When ac microgrids (AC-MG) and dc microgrids (DC-MG) are present in a distribution grid, there is an opportunity to interconnect them via an interlinking converter (IC) and form a HMG. Power sharing and adequate voltage/frequency control are the main functions of AC-MG and DC-MG control systems in standalone mode. When it comes to a HMG, active power sharing throughout the whole system must also be properly achieved by controlling the IC active power throughput. Furthermore, the possibility of participation of IC in AC-MG reactive power adds some complexity to a HMG control system. In this paper, a new decentralized control strategy is presented for a HMG which relies on regulating the voltage magnitude of a common bus in each microgrid. In this regard, new droop characteristics for sources across both microgrids as well as IC are proposed. The proposed droop characteristics result in better active/reactive power sharing across both microgrids and at the same time results in better voltage regulation. The derivation of new droop characteristics is thoroughly discussed in this paper. Simulation results are used to show the improvement achieved.

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The increasing penetration of dc distributed energy resources (DERs) and dc loads has motivated the utilization of dc microgrids (MGs). In this paper, a new two-level control scheme is proposed for accurate power sharing and appropriate voltage regulation in dc MGs during islanded operation mode. In the primary control level, a P-V ̇ droop method is proposed to eliminate the dependency of power sharing among DERs on line resistances. Since the P-V ̇ droop deviates the voltage derivative to a nonzero value, a voltage derivative restoration mechanism is adopted in the secondary control level to provide voltage stability. The secondary control level also compensates the voltage deviations by cascading an outer voltage control loop with the inner voltage derivative restoration loop. The secondary control signal is broadcasted to each DER via a unidirectional lowbandwidth communication link. Small signal stability of the proposed scheme is analyzed by studying the locus of the system eigenvalues. To verify the efficacy of the proposed method and compare it with the conventional scheme, real-time simulations in OPAL-RT lab setup are provided. keywords-Distributed energy resources, DC microgrids, power management, voltage regulation.

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This paper is concerned with the analysis of an instability phenomenon, known as limit cycle, in hybrid microgrids (HMGs). The rapid growth of distributed energy resources and their integration into existing distribution networks has opened a new era for electricity generation and distribution markets. The appearance of microgrids, both in the form of ac and dc, is the result of this integration. When both ac and dc microgrids (AC-MGs and DC-MGs) are in the vicinity of each other, they can be interconnected via ac-dc converters, also known as interlinking converters (ICs). Such a structure allows more efficient use of all resources in the system by enabling energy exchange between DC-and AC-MGs. In this paper, it is shown how the reactive power compensation method in IC leads to unstable operation of the HMG. Detailed analysis of this instability reveals that it is caused by a phenomenon known as limit cycle. By knowing the roots of instability, it is possible to eliminate it. Then we studied how the limit cycle that occurred is avoided by nonlinear control techniques support. The analytical studies are backed up by the simulation of a sample HMG. Résumé-Cet article porte sur l'analyse d'un phénomène d'instabilité, connu sous le nom de cycle limite dans les micro-réseaux hybrides (MRHs). La croissance rapide des ressources énergétiques distribuées et de leur intégration dans les réseaux de distribution existants a ouvert une nouvelle ère aux marchés de production et de distribution d'électricité. L'apparition de micro-réseaux, à la fois sous forme de courant alternatif et continu, est le résultat de cette intégration. Lorsque les deux micro-réseaux CA et CC (CA-MRs et CC-MRs) se trouvent à proximité l'un de l'autre, ils peuvent être reliés entre eux par des convertisseurs CA-CC, aussi connu comme convertisseurs interconnectés (CI). Une telle structure permet une utilisation plus efficace des ressources dans le système en permettant l'échange d'énergie entre CC-et CA-MRs. Dans cet article, il a été montré comment la méthode de compensation de puissance réactive dans un CI conduit à un fonctionnement instable du MRHs. Une analyse détaillée de cette instabilité révèle qu'elle est causée par un phénomène connu sous le nom de cycle limite. En connaissant les racines de l'instabilité, il est possible de l'éliminer. Ensuite, nous avons étudié la façon dont le cycle limite produit est évité par le support des techniques de contrôle non linéaire. Les études analytiques sont soutenues par la simulation d'un échantillon de MRH.