This document is a summary of a report prepared by the IEEE PES Task Force (TF) on Microgrid Stability Definitions, Analysis, and Modeling \cite{task}, which defines concepts and identifies relevant issues related to stability in microgrids. In this paper, definitions and classification of microgrid stability are presented and discussed, considering pertinent microgrid features such as voltage-frequency dependency, unbalancing, low inertia, and generation intermittency. A few examples are also presented, highlighting some of the stability classes defined in the paper. Further examples, along with discussions on microgrid components modeling and stability analysis tools can be found in the TF report.

The replacement of directly connected synchronous generators with power electronics interfaced generation has led to a decrease in system's inertia posing a significant challenge on frequency dynamics. In isolated systems with reduced inertia predefined limits for renewable penetration and primary reserves are frequently set for dynamic security purposes. This approach might not ensure dynamic security or can prove conservative in certain conditions. Furthermore, these approaches rarely consider the capabilities of inverter based renewable generation to provide frequency services. In this paper, a data driven approach, based on optimal classification trees is proposed to extract, from a detailed dynamic model of the system, the constraints for a frequency dynamic unit commitment formulation. Hence, both dynamic security and optimal exploitation of renewable and conventional units for power production and frequency support can be achieved. The advantages of the proposed method compared to conventional and state of the art approaches in frequency security are validated through dynamic simulations on a realistic model of Rhodes island and IEEE 118. Uncertainties in load demand and renewable generation are dealt by a robust optimization method. Its economic performance, computational overhead and modelling complexity is compared to a stochastic approach.

Microgrids (MGs), as novel paradigms of active Distribution Networks, have been gaining increasing interest by the research community in the last 20 years. Currently, they are considered as key components in power system decentralization, providing viable solutions for rural electrification, enhancing resilience and supporting local energy communities. Their main characteristic is the coordinated control of the interconnected distributed energy resources (DER), which can be realized by various methods, ranging from decentralized communication-free approaches to centralized ones, where decisions are taken at a central point. This paper provides an overview of this development focusing on the technical control solutions proposed by reseachers for the various levels of MG organization hierarchy. A critical assessment of selected, popular technologies is provided and open research questions regarding the trend to more decentralized power systems are discussed.

The role of flexible alternating current transmission systems (FACTSs) in the provision of grid services is becoming increasingly important, due to the massive integration of intermittent renewable energy sources, energy storage systems, and the decommissioning of thermal plants. A comprehensive literature review of grid services offered by FACTS is performed, focusing on the different grid services that they can provide, such as power flow control, reactive power control, voltage control, power quality improvement, harmonic mitigation, improvement of transient stability, and damping of inter-area and intra-area oscillations. These grid services need to be realistically and economically validated in suitable testing environments. A review of relevant standards, guides, and the literature is performed, which covers the entire range from functional specification and factory testing up to the field testing of FACTS. Advanced industry practices, such as controller hardware in the loop (CHIL) testing of FACTS controllers by the manufacturer, and recent trends, such as CHIL testing of replica controllers by the owner, are underlined. Limitations of conventional testing and CHIL testing are explained and the use of power hardware in the loop (PHIL) simulation for FACTS testing is discussed. CHIL and scaled-down PHIL tests on a transmission static synchronous compensator (STATCOM) are performed and a comparison of the results is presented.

In this paper, a highly reliable back-to-back (BTB) power converter is proposed for doubly-fed induction generator-based wind turbines (DFIG-WTs). When a power switch open-circuit fault is encountered in either the grid-side converter (GSC) or rotor-side converter (RSC), a four-switch three-phase (FSTP) topology is formed to avoid using redundant bridge arms, which reduces the power circuit complexity and minimizes the conduction and switching losses. A simplified space vector pulse width modulation (SVPWM) technique is used to eliminate sector identification and complex trigonometric calculations. In addition, the influence of DC-bus capacitor voltage unbalance on the electromagnetic torque is analyzed in detail. The offset current components are calculated, and they are deducted from the reference values in the modified control strategies to suppress the DC-bus voltage deviation. Moreover, the power loss model of BTB converter is analyzed in detail, and the efficiency study is performed in various post-fault situations. Simulations in Matlab/Simulink are carried out to verify the performance of a DFIG-WT based on FSTP BTB converter. Furthermore, the control hardware-in-the-loop (CHIL) setup with RSC and GSC separately simulated in a digital real-time simulator (DRTS) is applied for experimental verification of the proposed control strategy.

The noninterconnected island of Kythnos (100 km2 with 1,600 inhabitants) belongs to the complex of the Western Cyclades islands in Greece and is located in the Aegean Sea, 104 km from Athens. The island has a rich history in the adoption of sustainable energy applications, starting from the installation of the first wind farm in Europe [5 ? 20-kW wind turbines (WTs)] by the Public Power Corporation (PPC) in 1982, followed in 1983 by the first hybrid station comprising a 100-kW photovoltaic (PV) system coupled with a 400-kWh battery storage, also by PPC. In 1989, the WTs were replaced by 5 ? 33 kW (Figure 1), and in 1992, the PV inverters were upgraded.