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This paper presents a novel approach of employing hierarchical control to optimize the operation of islanded AC microgrids. The proposed method is an offline, centralized, power-flow-based energy management scheme which includes primary and secondary control dynamics in a modified power-flow formulation. The inner power-flow level maintains bus voltages and system frequency within the desired range and ensures power balance in the network. The outer optimization level ensures that the various components remain within their operational constraints, while optimizing a system-level objective. Two case studies are explored using a modified 14-bus medium voltage (MV) CIGRE benchmark microgrid to validate the proposed energy management algorithm. The first case includes minimization of conventional generator operating cost and renewable energy curtailment, both with linear and non-linear loads, and the second case includes minimization of conventional generator operating cost with load shedding. The results obtained from the case studies show the efficiency of the proposed energy management algorithm, and evidence its reliability for the optimal operation of islanded AC microgrids with multiple renewable energy sources.

The secondary control is applied in islanded Microgrids (MGs), after the primary control, in order to restore the buses voltage and frequency to specified values. The existing power flow methods can accurately calculate the power flow for droop-controlled islanded MGs, but in many cases, they cannot calculate the steady-state solution of the MG after the action of secondary controllers. The main challenge in the steady-state modelling of the secondary layer lies in that the secondary controllers consist of integral parts, which can integrate functions with different integral histories, and therefore, under certain circumstances, can imply inaccurate power sharing between the distributed generation units (DGs). This phenomenon is most pronounced under communication failures, as will be shown in the simulations. In this way, this paper proposes a power flow method for calculating, accurately, the steady-state solution of hierarchically controlled islanded AC MGs, including droop-based primary control and secondary control. The paper includes four main features: a) generalized implementation in several communication strategies e.g., centralized, decentralized, consensus, distributed averaging, b) precise simulation of communication links and integral parts of secondary controllers, c) low computation time, and d) accurate 4-wire network representation. Simulations were executed to validate the proposed method against Simulink and to highlight the importance of an accurate modelling of secondary control in the power flow method for islanded MGs

Wind turbine converters (WTCs) with grid forming (GFM) control can contribute to voltage and frequency support in the power system, by emulating synchronous machine (SM) dynamics, provided that there are sufficient buffers. Over the past few years, several control algorithms have been developed for the GFM operation of WTCs. This article investigates the state-of-the-art in the field of GFM control algorithms for WTCs, where the principle of each control algorithm is discussed in detail, and corresponding characteristics are summarized. Comparison results with respect to voltage build-up strategies, the synchronization of GFM WTCs, the interaction attenuation, the virtual inertia realization and the complexity of individual GFM algorithm are given in a table, which leads to the conclusion that mitigation of oscillations among WTCs, appropriate inertia emulation, and coordination with energy storage systems need to be further investigated in the future GFM WTCs.

The proliferation of renewable energy sources in autonomous microgrids has led to the deterioration of system inertia. In the past decade, the decrease in system inertia was addressed through numerous virtual synchronous generator (VSG) topologies. Synchronverter is one such control technique exhibiting promising performance as that of synchronous machines (SMs). Recently, myriad other modifications were carried out to enhance the capability of the synchronverter for better dynamic response. However, the limitations of fixed virtual inertia and damping coefficient were left untouched, which can be optimized for better frequency regulation of microgrids. Hence, this paper proposes an adaptive synchronverter (ASV) by optimizing OSV parameters through fuzzy logic. Subsequently, the proposed ASV was employed to control a novel pico hydel and battery hybrid energy storage for frequency regulation of the microgrid. The small-signal model of parallel operating ASVs is presented along with the eigenvalue analysis to prove the system stability under parameter variation. The MATLAB/Simulink simulation results revealed that the proposed ASV exhibited a lower rate of change of frequency and frequency nadir compared to the original synchronverter (OSV) and vector control (VC).

This paper presents the power flow modelling for AC/DC hybrid islanded microgrids including droop-controlled distributed generation units, secondary frequency and voltage restoration control for the AC side of the microgrid, and secondary voltage restoration control for the DC side of the microgrid. The interlink converter between the AC and DC microgrids includes a frequency-voltage droop control, and considers the effect of the secondary control for the AC microgrid side. Two case studies are presented for the power flow model evaluation, in the first case a microgrid with linear loads and equal droop characteristic for the distributed generation units are used; in the second case, voltage dependent loads for both AC and DC microgrids are included, and different droop characteristic are chosen for each distributed generation unit. Comparisons between the power flow solutions through the proposed modelling and the professional simulator MATLAB/Simulink are presented. Additionally, the computational speed and convergence rate of the power flow method are shown. The obtained results corroborate the reliability and effectiveness of the proposed power flow modeling to represent the controlled AC/DC hybrid microgrid including hierarchical controllers.

This document presents a performance assessment of the three-phase two-level voltage source converter (VSC) model of the real-time digital simulator RTDS/RSCAD. This evaluation is conducted through two case studies. One case is a simple two-bus power electronic-based system, and the second case is a three-bus system with a frequency-dependent transmission line model and a closed-loop controlled VSC with communication delay. The results obtained reveal that the RTDS/RSCAD VSC model presents some issues on stability, harmonic spectrum, ripple, and power quality in general, as compared with the PSCAD VSC solutions. These results also suggest the need for better power electronic components models.