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Non-interconnected island systems around the globe present a great variety and substantial particularities regarding the methods applied in generation scheduling, primarily under high renewable energy sources penetration levels. Under such conditions, one of the main challenges in island systems is the coherent treatment of intermittent renewable generation and the implementation of robust unit commitment and economic dispatch methodologies, seeking to achieve high RES absorption rates, least cost scheduling, and system security. In this paper, an extensive and in-depth literature review is undertaken to identify the generation scheduling principles and methods applied in island systems experiencing high renewable penetrations. Principal features of electricity markets in islands are highlighted, state-of-the-art unit commitment and economic dispatch models suitable for such applications are identified, and their advantages and limitations are discussed. The effect of renewables uncertainty on the islands' scheduling is analyzed, especially regarding the classification and quantification of system operating reserves to enhance operational security and system stability. Given the isolated nature and limited inertia of islands, very high RES penetration targets may impact their ability to survive severe contingencies, a topic also addressed in this paper, extending the review to include literature on frequency-related dynamic phenomena for isolated systems. • An overview of generation scheduling principles for island systems is presented. • Techniques available to deal with intermittent RES uncertainty are discussed. • A taxonomy of operating reserves adopted in the literature is performed. • A survey on island systems dynamics under high RES penetrations has been conducted. [ABSTRACT FROM AUTHOR]

This paper deals with the internal dispatch policy for Hybrid Power Stations (HPS) consisting of renewable energy source (RES) based generation and storage facilities, operating in isolated island power systems in a coordinated manner to provide dispatchable power. Objective of the proposed dispatch method is the maximization of HPS revenues during real time operation, while avoiding deviations from the dispatch orders (DOs) issued by the System Operator (SO). DOs can be met either by RES production directly injected to the grid or by the storage facilities of the station. A Model Predictive Control (MPC) method is implemented to regularly update the internal dispatch decisions for HPS components, relying on an optimization problem built upon the Mixed Integer Linear Programming (MILP). In the proposed formulation the technical and operating constraints of the examined HPS configuration are taken into consideration, while a variable time resolution technique is also adopted to reduce computational complexity and achieve feasible solutions within acceptable time limits. • An internal dispatch method for HPSs contributes to their optimal management. • The variable time resolution approach achieves optimal solutions in reasonable time. • Exploitation of imbalance tolerances may increase revenues for the station. • BESS of high rated power and limited energy capacity improve operation of the HPS. • SoC variability penalization for BESS greatly improves battery life expectancy. [ABSTRACT FROM AUTHOR]

Abstract: Pumped storage is generally viewed as the most promising technology to increase renewable energy source (RES) penetration levels in power systems and particularly in small autonomous island grids. Combined wind and pumped-storage “virtual power plants”, called hybrid power stations (HPS), constitute a realistic and feasible option to achieve high penetrations, provided that their components are properly sized. In this paper, the optimum sizing is investigated for a pumped storage HPS operating in an island system. The analysis addresses the sizing of the main HPS components (hydro turbines, pumps, wind farm, reservoirs), adopting either the investor’s perspective, where the objective is to maximize the return on the HPS investment, or a system perspective, where the optimization target is the maximization of RES penetration, along with maintaining the lowest possible generation cost in the system. Genetic Algorithms (GAs) are applied for the optimization and a real isolated island power system is used as a study case. The adopted operating policy and pricing principles, which critically affect the optimal sizing of an HPS project, are based on the existing regulatory framework for storage stations in Greek islands. [Copyright &y& Elsevier]

Abstract: In order for a wind generator (WG) to participate in load-frequency control (LFC) – either primary or secondary – some de-loading of its turbine is required. This paper discusses and rectifies important drawbacks of methods previously suggested on the two main de-loading techniques, i.e. over-speeding and pitch-controlled de-loading. In the same scope, the inertial support offered by WGs is also discussed. Two enhanced control strategies emerge from the above analysis: the first is based on over-speeding de-loading, which, for wind speed close to or above nominal, avoids overloading the converter of WGs equipped with Doubly Fed Induction Generator (DFIG); the second is a method of pitch-controlled de-loading combined with inertial support for fast LFC action. Simulations presented show both the positive performance of the two proposed strategies and their favorable results versus previous cited methodologies. In MATLAB® Simulink, a two-bus power system consisting of a varying load, a diesel-synchronous generator and WG topologies with either a DFIG or a Permanent Magnet Synchronous Generator (PMSG) are considered. [Copyright &y& Elsevier]

Abstract: Objective of this paper is to provide insight in the component selection criteria of an autonomous wind-driven desalination plant. For this purpose, a suitable logistic model of such a system is developed, which simulates its steady-state operation, taking into account the power and energy equilibrium in the system. The simulation of the system operation is performed employing two alternative control strategies and a variety of different configurations with respect to the size of its main components (wind turbine, desalination plant and batteries). For each case, the annual water production is calculated and an economic assessment is performed to estimate the expected water production cost, which is the ultimate measure of the feasibility of the stand-alone system. Other important factors, such as the desalination unit start/stop operations are also calculated. Based on the simulation results, conclusions are drawn regarding the optimal sizing of the system components and its recommended operating strategy. [Copyright &y& Elsevier]

Abstract: Wind farms operating in island systems are subject to output power limitations, related with technical constraints of the conventional generating units, namely the minimum loading levels of the thermal units (technical minima) and a dynamic penetration limit, applied for stability purposes. Evaluation of the expected wind energy yield in isolated island systems requires, therefore, proper consideration, not only of the prevailing wind conditions at the installation site, but also of the power limitations imposed by the system, which ultimately depend on the total load demand. In the paper, a methodology is presented for the evaluation of the wind energy absorption capability of island systems and the expected energy yield of wind farms operating therein. This methodology is currently applied in Greece for the determination of the power restrictions applied to existing and new wind farms, to schedule new wind capacity tenders and for the evaluation of the expected energy yield of new investments for permitting and financing purposes. [Copyright &y& Elsevier]

In this paper, the generation scheduling problem for autonomous island systems with significant penetration of non-dispatchable Renewable Energy Source (RES) generation is investigated, proposing a unit commitment-economic dispatch (UC-ED) model which is fully compatible with the provisions of the regulatory framework currently applicable to the Greek non-interconnected island (NII) systems. Two UC-ED model variants are presented and comparatively assessed, in order to illustrate the impact of various system parameters (technical characteristics of the thermal units, reserve requirements, etc.) on the expected operating profile of the NII system. A detailed UC-ED model is first proposed, which takes into account the reserve requirements at different time scales (primary, secondary and tertiary reserves) and respective capabilities of generating units. An alternative, simplified UC-ED model is also presented, formulated on the basis of aggregate spinning reserves, to cater for uncertainties in the actual reserve capabilities of existing thermal units. The developed UC-ED models are applied to an existing NII system to evaluate their performance and results obtained on an annual basis. [ABSTRACT FROM AUTHOR]

Combined wind and pumped-storage virtual power plants, called hybrid power stations (HPS), constitute a realistic and feasible option to achieve high renewable energy source (RES) penetration levels in power systems and particularly in autonomous island grids. Technical issues arising from the integration of HPS in islands have not been sufficiently investigated yet. In this paper, the dynamic behaviour of an island system with an HPS is analysed, the effects of the various HPS operating modes on the transient behaviour and stability of the system are investigated, constraints regarding the operation of the HPS are identified and potential solutions are proposed. [ABSTRACT FROM AUTHOR]