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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]

Load demand in small autonomous island systems is typically covered by Diesel Units (DU). Although a favourable renewable energy source (RES) potential might exist, the technical constraints introduced by the conventional generators result in relatively low RES penetration levels, typically up to 15%–20% of the annual energy demand. To overcome such limitations, introduction of energy storage is necessary. In the case of very small islands (less than 1 MW peak load), lead-acid battery energy storage systems (BESS) constitute a technically mature solution with considerable application potential. In this paper, the potential for achieving very high RES penetration levels with the introduction of BESS in an existing small island system is investigated. An operating policy is first introduced for the overall system, including conventional generators, RES (wind and photovoltaic) stations and storage system. Simulation results are then presented to quantify the expected energy benefits in terms of RES energy penetration and the impact on the economics of the island system. The sizing of the hybrid system components is then investigated by conducting a parametric analysis and then optimized by applying genetic algorithms. [ABSTRACT FROM PUBLISHER]

Harmonic emissions of variable speed wind turbines (WTs) are a well-known issue, but little information that is based on field measurements on actual machines has been published on the subject. In this paper, the harmonic behavior of modem WTs using extensive measurements performed on commercially avail- able machines is discussed. Specific issues addressed include the shape and the frequency range of the harmonic current spectrum, the variation of the harmonics with the WT operating point, the statistical characteristics of their magnitude and phase angle, the effect of grouping and time-averaging, as well as their symmetrical component characteristics. The objective of the paper is to outline important general characteristics of the harmonic behavior of WTs, rather than to analyze specific machines or compare alternative WT designs. [ABSTRACT FROM AUTHOR]

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]

Variable-speed wind turbines inject harmonic currents in the network, which may potentially create voltage distortion problems. In this paper, a case study is presented for a 10-MW wind farm, intended to be connected to a network with extended high-voltage submarine cable lines. First, the system modeling approach and the harmonic load-flow calculation is described. Then, the harmonic impedance of the system is calculated for a variety of configurations and operating conditions, and its main characteristics are discussed. Harmonic load-flow calculations are provided to indicate potential voltage distortion problems. A simplified methodology, suggested in relevant IEC publications, is applied to the system and its results are compared to those of the harmonic load flow. A discussion on the summation of harmonic currents within a wind farm is also included. [ABSTRACT FROM AUTHOR]

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]