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This paper presents the power flow modeling of droop-controlled distributed generation units with secondary frequency and voltage restoration control for hierarchically controlled islanded microgrids. These models are incorporated in the conventional Newton-Raphson power flow method as a new bus, without the necessity of a slack bus, and include the gains of the control systems that influence the steady-state solution. Two case studies are addressed. In the first case study, comparisons of the proposed models against the steady-state solutions obtained with PSCAD and Simscape Power System of Simulink, where the closed-loop controls are explicitly modeled, are presented. In the second case study, the proposed method is contrasted against the droop-based approach. The results obtained exhibit low computing effort, reliability, and effectiveness of the proposed models since quadratic convergence behavior is maintained independently of the size and topology of the microgrid. Besides, it is demonstrated that hierarchical and droop controllers lead to different solutions, which confirms the necessity of including the hierarchical control in the power flow model.

This paper presents the application of a hierarchical control scheme for islanded AC microgrids with a primary droop control and a centralized extended optimal power flow control. The centralized control is responsible for computing and sending, in an online manner, the control references to the primary controls in order to achieve three operational goals, i.e., improvement of the global efficiency, voltage regulation through reactive power management and compliance of the restrictions regarding the generation units capacities. Two case studies are defined and online tested in a laboratory-scaled microgrid implemented in the Microgrid Laboratory at Aalborg University. The primary controllers are included in a real-time simulation platform (dSPACE 1006), while the extended optimal power flow is conducted in a central controller by using a Smart Meter and LabVIEW for data acquisition and MATLAB for its implementation, taking into account load and capacity profiles. The obtained results show the reliability of the proposed scheme in a real system and its advantages over the conventional droop control.

Abstract-This paper presents the application of Newton-based methods in the time-domain for the computation of the periodic steady state solutions of microgrids with multiple distributed generation units, harmonic stability and power quality analysis. Explicit representation of the commutation process of the power electronic converters and closed-loop power management strategies are fully considered. Case studies under different operating scenarios are presented: grid-connected mode, islanded mode, variations in the Thevenin equivalent of the grid and the loads. Besides, the close relation between the harmonic distortion, steady state performance of the control systems, asymptotic stability and power quality is analyzed in order to evaluate the importance and necessity of using full models in stressed and harmonic distorted scenarios.

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