A new formulation is required to provide a proper power flow analysis in islanded microgrids taking into consideration their special philosophy of operation. In this paper, a novel and generic three-phase power flow algorithm is formulated for islanded microgrids. The algorithm is novel since it adapts the real characteristics of the islanded microgrid operation; i.e., 1) some of the distributed generation (DG) units are controlled using the droop control methods and their generated active and reactive power are dependent on the power flow variables; 2) the steady-state system frequency is considered as one of the power flow variables. The proposed algorithm is generic, where the features of distribution systems, i.e., three-phase feeder models, unbalanced loads and load models have been taken in consideration. Further, all possible operation modes of DG units (droop, PV, or PQ) have been considered. The problem has been formulated as a set of nonlinear equations. A globally convergent Newton-trust region method has been proposed to solve this set of nonlinear equations. The proposed algorithm is a helpful tool to perform accurate steady state studies of the islanded microgrid. Different case studies have been carried out to test the effectiveness and the robustness of the proposed algorithm.

Recently, the concept of microgrids (clusters of distributed generation, energy storage units, and reactive power sources serving a cluster of distributed loads in grid-connected and isolated grid modes) has gained a lot of interest under the smart grid vision. However, there is a strong need to develop systematic procedure for optimal construction of microgrids. This paper presents systematic and optimized approaches for clustering of the distribution system into a set of virtual microgrids with optimized self-adequacy. The probabilistic characteristics of distributed generation (DG) units are also considered by defining two new probabilistic indices representing real and reactive power of the lines. Next, the advantages of installing both distributed energy storage resources (DESRs) and distributed reactive sources (DRSs) are investigated to improve the self-adequacy of the constructed micro-grids. The new strategy facilitates robust infrastructure for smart distribution systems operational control functions, such as self-healing, by using virtual microgrids as building blocks in future distribution systems. The problem formulation and solution algorithms are presented in this paper. The well-known PG&E 69-bus distribution system is selected as a test case and through several sensitivity studies, the effect of the total DESRs or DRSs capacities on the design and the robustness of the algorithm are investigated.

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The energy sector is moving into the era of distributed generation (DG) and microgrids (MGs). The stability and operation aspects of converter-dominated DG MGs, however, are faced by many challenges. Important among these, are: 1) the absence of physical inertia; 2) comparable size of power converters; 3) mutual interactions among generators; 4) islanding detection delays; and 5) large sudden disturbances associated with transition to islanded mode, grid restoration, and load power changes. To overcome these difficulties, this paper presents a new large-signal-based control topology for DG power converters that is suitable for both gridconnected and islanding modes of operation without any need to reconfigure the control system and without islanding detection. To improve MG stability, the proposed control structure is realized via two steps. First, an emulated inertia and damping functions are adopted. Second, to guarantee stability and high performance of the MG system during sudden harsh transients such as islanding, grid reconnection, and large load power changes, a nonlinear MG stabilizer is proposed. An augmented converter model is developed and used to design the MG stabilizer via the adaptive backstepping (AB) technique to guarantee large-angle stability and robustness against unmodeled dynamics. Theoretical analysis and evaluation results are presented to show the effectiveness of the proposed control scheme in achieving stable and smooth operation of a MG system in grid-connected, islanding, and transition modes. Index Terms-Grid-connected, islanded microgrid, microgrid, power stability, supplementary control. NOMENCLATURE Voltage frequency. Virtual rotor flux. Voltage angle. Current frequency. Power angle. Virtual electrical torque. Current angle.Virtual rotor momentum of inertia.