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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 investigates grid export constraints applicable to photovoltaic (PV) and wind farm (WF) installations, both with and without behind-the-meter storage, aimed at enhancing grid hosting capacity. The study focuses on static output power limitations, i.e., simple export constraints that are preventively imposed on renewable energy source (RES) plants regardless of the prevailing network congestion conditions. These constraints are easy to apply, implemented ex ante and out of market, and do not require additional investments in energy storage or advanced devices. They also yield a measurable increase in hosting capacity and grid utilization, with their impact on RES plant operation and return of investment straightforwardly calculable. Analysis defines the level and shape of these constraints assuming an indicative acceptable curtailment level of 5% for each RES technology attributed to the preventing action of the imposed limitations, while the respective implications for RES energy yield and investment viability are explored. The findings indicate that an export power limitation of ca. 68% is effective for stand-alone PVs, while a stepwise static limitation is necessary for stand-alone WFs and WFs with integrated storage to manage midday solar generation peaks. PV plants tightly coupled with storage facilities can handle static limitations as low as 35% with minimal impact on the economic feasibility of the investments. [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]

This paper presents a quantitative assessment of the value of hydroelectric power plants (HPPs) in power systems with a significant penetration of variable renewable energy sources (VRESs). Through a capacity expansion planning (CEP) model that incorporates a detailed representation of HPP operating principles, the study investigates the construction and application of HPP rule curves essential for seasonal operation. A comparative analysis is also conducted between the proposed rule curve formulation and alternative modeling techniques from the literature. The CEP model optimizes installed capacities per technology to achieve predefined VRES penetration targets, considering hourly granularity and separate rule curves for each HPP. A case study involving twelve reservoir hydropower stations and two open-loop pumped hydro stations is examined, accounting for standalone plants and cascaded hydro systems across six river basins. The study evaluates the additional generation and storage required to replace the hydropower fleet under high VRES penetration levels, assessing the resulting increases in total system cost emanating from introducing such new investments. Furthermore, the study approximates the storage capabilities of HPPs and investigates the impact of simplified HPP modeling on system operation and investment decisions. Overall, the findings underscore the importance of reevaluating hydro rule curves for future high VRES penetration conditions and highlight the significance of HPPs in the energy transition towards carbon neutrality. [ABSTRACT FROM AUTHOR]

The challenges of modern power systems will inevitably impose increased ancillary service requirements to photovoltaic (PV) plants in the future, including operating reserves. Recent studies have investigated methods to maintain active power reserves without energy storage in the standard case of uniform illumination. In this paper, this functionality is extended to partial shading conditions, often encountered in PV systems. A new control scheme is proposed that permits operation at a reduced power level, estimating at the same time the shading conditions and maximum available power. This is achieved by applying a least-squares curve fitting algorithm on voltage and current measurements, without relying on any irradiance or temperature sensors. To the best of our knowledge, this is the first power reserves control scheme for PV systems under partial shading presented in the literature. The robustness and effectiveness of the proposed method are validated under rapidly changing shading conditions through simulations and experimental tests on a 2-kW PV system prototype. [ABSTRACT FROM AUTHOR]

The main scope of this paper is to propose a suitable frequency control scheme for high-voltage dc-links (HVDC) based on voltage source converters (VSCs), operating in island systems with on-site conventional generation and external ac interconnectors. The proposed droop-type and inertia emulator is built upon the power synchronization control (PSC) concept, where grid synchronization is achieved without the need of a dedicated synchronization unit. The dynamics of the proposed PSC-based scheme are assessed both in frequency and time domain, utilizing a realistic study-case system, which corresponds to the current planning for the interconnection of Crete to the Greek mainland system, involving ac and dc interconnectors operating in parallel with conventional local thermal units. To demonstrate the benefits offered by the proposed controller in the context of frequency response, a detailed average value model is developed for the VSC-HVDC link in MATLAB/Simulink, where severe contingencies are simulated, such as the sudden loss of the external ac interconnector or local generation, leading from mainland grid-connected to islanded operation. [ABSTRACT FROM PUBLISHER]

This paper presents an alternative control scheme for offshore wind turbines (WTs) connected to the mainland grid through high voltage dc (HVDC) links based on voltage source converters (VSCs). The proposed WT controller inherently provides rapid droop-type frequency response, without the need to implement an additional WT frequency controller. A coordinated control approach is proposed between the WT and HVDC controllers in order to achieve fault ride-through (FRT) and frequency response capabilities, relying solely on offshore frequency modulation. To demonstrate the FRT capability of the VSC-HVDC link, a detailed EMT-type model is first developed in Matlab/Simulink, where onshore grid faults are tested. Then, the offshore wind farm is connected to a study case system in order to assess the contribution of the VSC-HVDC link to frequency regulation, by exploiting the proposed WT controller along with the communication capability between the two HVDC VSCs. [ABSTRACT FROM PUBLISHER]

In this paper, the response of the new generation silicon carbide (SiC) power transistors under short-circuit faults is examined. The study is focused on real life applications, where a large stray inductance is typically part of the main power loop. Three types of short-circuits are experimentally investigated and the effect of the stray inductance in each case is recorded. A comparative analysis regarding the performance of the SiC junction field effect transistors (JFETs) and SiC metal oxide field effect transistors (MOSFETs), along with the latest technology silicon (Si) MOSFETs is carried out. Maximum instantaneous power, critical energy and saturation current are the main criteria for evaluating the robustness of each power device. A destructive test is performed in order to obtain the short-circuit withstand capability and the failure mechanism of each semiconductor. [ABSTRACT FROM AUTHOR]

Increasing penetration of Renewable Energy Sources (RES) to the power systems leads to the continuous evolution of grid interconnection requirements, which now include Low Voltage Ride Through (LVRT) practically for all types of generating units connected to the MV or HV networks, including photovoltaics (PV). It is essential that the Distributed Generation (DG) models used in power system studies properly represent the enhanced response characteristics of the stations. In this paper, a model is presented for a PV system with grid-code compatibility, which is suitable for power system dynamic and transient analysis studies. For this purpose, an overall control strategy is introduced to achieve LVRT capability, while elements essential for the correct representation of the dynamic performance of the system, such as the Phase Locked Loop (PLL) and the output LCL filters, are properly accounted for. Time domain simulations are provided for balanced and unbalanced voltage dips, in order to demonstrate the performance of the PV system and evaluate the accuracy of the proposed simplified model. The response of the detailed switching model is also included in the simulation results. [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]

Photovoltaic (PV) system performance is influenced by several factors, including irradiance, temperature, shading, degradation, mismatch losses, soiling, etc. Shading of a PV array, in particular, either complete or partial, can have a significant impact on its power output and energy yield, depending on array configuration, shading pattern, and the bypass diodes incorporated in the PV modules. In this paper, the effect of partial shading on multicrystalline silicon (mc-Si) PV modules is investigated. A PV module simulation model implemented in P-Spice is first employed to quantify the effect of partial shading on the I–V curve and the maximum power point (MPP) voltage and power. Then, generalized formulae are derived, which permit accurate enough evaluation of MPP voltage and power of mc-Si PV modules, without the need to resort to detailed modeling and simulation. The equations derived are validated via experimental results. [ABSTRACT FROM PUBLISHER]

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]

Abstract: Fault level considerations may be an inhibiting factor for the interconnection of distributed generation (DG) to the network, particularly at the medium voltage level. In this paper, the latest edition of the IEC 60909 Standard is applied for the calculation of the resulting fault level in medium and low voltage distribution networks with DG. First, an outline of the IEC calculation methodology is presented, including all relevant equations for DG cases studies. Then the short-circuit contribution of the various DG types is extensively discussed. The application of the methodology is demonstrated on a study case medium voltage distribution network, which includes all representative types of DG sources. Emphasis is placed on the contribution of the upstream system, which is the dominant source of short-circuit current. A discussion is also included on potential measures for fault level reduction. [Copyright &y& Elsevier]

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: Technical advances and institutional changes in the electric power industry have resulted in a constantly increasing penetration of distributed generation (DG) resources in the grids. For the connection of new DG installations to the network a variety of factors are taken into account, including technical requirements imposed by utilities to ensure that the DG station does not adversely affect the operation and safety of the network. In this paper, fundamental issues related to the interconnection of DG installations to the grid are discussed and evaluation rules are presented, which address power quality considerations and are suitable for application by electric utility and DG engineers. The attention is focused on the steady-state and fast voltage variations, flicker and harmonic emissions. The simplified evaluation procedures of the paper are largely based on the relevant IEC publications and reflect the current practice of several European utilities. A discussion of the interconnection protection requirements is also included in the paper. [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]

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]

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]

Autonomous power systems, such as those existing in small and medium-size islands, typically consist of medium voltage networks fed by conventional power stations and present characteristics not typical for large interconnected grids. In this paper, results are presented and analyzed from a power-quality measurement campaign, conducted in a diesel-powered island system with significant wind penetration. Emphasis is placed on the voltage flicker, which was found quite increased under all operating conditions. It is shown that the diesel engines of the conventional power station are the main source of flicker in the system, due to the pulsed variation of their torque. The operation of the wind farms, on the other hand, does not result in any notable deterioration of the voltage quality level in the system. [ABSTRACT FROM AUTHOR]

Analyzes the effect of faults and other disturbances, common in rural medium voltage (MV) distribution systems for a typical fixed speed wind turbine (WT), equipped with an induction generator. Evaluation of resulting torque transients; Type of electrical faults to be considered in the design and certification of WT.

• A new UC method for autonomous power systems is presented, employing dual minimum loading levels for thermal units. • Method enhances systems flexibility by operating inflexible units below their minimum loading level in a structured manner. • Application of the proposed unit commitment approach reduces the generation cost of the system and RES curtailments. This paper investigates the benefits from operating thermal units in isolated grids below their technical minimum loading (TML) levels for short time periods, as a means of providing increased flexibility to the system. Such an operating practice is primarily dictated by the fact that small island systems exhibit very high seasonal variability of their load demand. The base load generating units of island systems are usually internal combustion engines (ICE) running on heavy fuel oil, with relatively large capacity and high TML levels. The inflexibility of these units renders in several occasions necessary the dispatch of flexible but costly peaking units, even in low demand periods, to ensure the feasibility of the unit commitment and economic dispatch (UC-ED) problem. Further, renewable energy penetration is severely hampered by the high TML of the base load units. To address such issues, a two-level TML technique is proposed for ICE units of isolated power systems, incorporated in a UC-ED problem based on the Mixed Integer Linear Programming approach. The benefits from this method are evaluated in terms of conventional generation management and cost, as well as regarding the achieved improvement in the penetration of renewables, examining the operation of two indicative island systems. [ABSTRACT FROM AUTHOR]

The generation management concept for non-interconnected island (NII) systems is traditionally based on simple, semi-empirical operating rules dating back to the era before the massive deployment of renewable energy sources (RES), which do not achieve maximum RES penetration, optimal dispatch of thermal units and satisfaction of system security criteria. Nowadays, more advanced unit commitment (UC) and economic-dispatch (ED) approaches based on optimization techniques are gradually introduced to safeguard system operation against severe disturbances, to prioritize RES participation and to optimize dispatch of the thermal generation fleet. The main objective of this paper is to comparatively assess the traditionally applied priority listing (PL) UC method and a more sophisticated mixed integer linear programming (MILP) UC optimization approach, dedicated to NII power systems. Additionally, to facilitate the comparison of the UC approaches and quantify their impact on systems security, a first attempt is made to relate the primary reserves capability of each unit to the maximum acceptable frequency deviation at steady state conditions after a severe disturbance and the droop characteristic of the unit's speed governor. The fundamental differences between the two approaches are presented and discussed, while daily and annual simulations are performed and the results obtained are further analyzed. [ABSTRACT FROM AUTHOR]

This paper presents a new multi-objective optimization technique for multi-terminal DC (MTDC) networks to enhance power dispatch control under stochastic operating conditions. The proposed method utilizes the DC voltage droop control concept to achieve distributed DC voltage control of the MTDC grid, while a centralized power dispatch controller monitors system operation and updates the voltage droop settings of the onshore converters in order for them to comply with scheduled power set-points dispatched by the transmission system operator, maintaining at the same time low system losses. The overall optimization problem is formulated and its effectiveness is assessed applying the Monte-Carlo simulation method. A five-terminal MTDC grid is selected as a study-case system, which corresponds to the planning for the future interconnection of Aegean Sea islands with significant wind potential to the mainland grid. [ABSTRACT FROM AUTHOR]

Photovoltaic (PV) stations are increasingly becoming subject to grid code requirements that include frequency response and active power control capability. The main goal of this paper is to propose a generic model for a two-stage grid-connected PV system with frequency response capability, suitable for power system studies. The proposed model includes a suitable control scheme, which provides both droop and inertial response, as well as the ability to operate at a scheduled active power reserve, enabling thus the provision of under-frequency response. A linearized small-signal model is developed to assess the stability of the proposed PV power control loop when the PV generator provides frequency response, whereas time-domain simulations are performed in order to quantify the benefits achieved by droop-type and inertia frequency controllers, including a discussion on the selection of their parameters. The analysis demonstrates the satisfactory performance of the proposed PV system model, which provides all functionality required by grid codes in the context of active power control. [ABSTRACT FROM AUTHOR]

Offshore wind farms connected to the mainland through High Voltage DC links based on Voltage Source Converters (VSC–HVDC) are subject to Grid Code (GC) requirements, such as frequency response, Fault Ride Through (FRT) capability, and active and reactive power control. Due to the presence of HVDC links, GC compatibility becomes a challenging task from a control and regulation perspective, since the offshore network AC voltage and frequency are decoupled from the onshore grid voltage and frequency and hence grid-side disturbances are not directly experienced by the Wind Turbines (WTs). To address this challenge of modulating the response of offshore WTs according to the onshore grid conditions, without resorting to dedicated communication schemes, which may introduce delays and reliability concerns, three alternative control strategies are presented and evaluated for the offshore HVDC VSC, which are based only on local measurements. Each strategy can cope with onshore frequency response, FRT and power control requirements at the same time. A coordinated control approach between the WT and HVDC converters is also proposed by integrating a voltage dependent current modulation strategy into the default WT current controllers. Time domain simulations are carried out in DIgSILENT PowerFactory in order to verify the effectiveness of the proposed control schemes. [ABSTRACT FROM AUTHOR]

In this paper, new expressions are introduced for the determination of the maximum power point (MPP) of photovoltaic (PV) systems as explicit functions of the five parameters of the single-diode model employing the Lambert W function. These equations provide the voltage and current at MPP in a direct and straightforward manner, thus dispensing with any need for iterative solution. They are initially derived for a PV system operating under uniform conditions, and subsequently extended for mismatched conditions at the PV string level. The novelty of these formulae lies in their solid theoretical foundation, which supports their validity in the general case and offers a well-founded symbolic formulation for the MPP evaluation problem. Extended simulations and experimental validation are performed to verify the accuracy and computational efficiency of the proposed equations compared with other methods available in the literature. [ABSTRACT FROM AUTHOR]

• Alternative BES management concepts are analyzed to support high RES penetrations. • Suitable sizing of BES components benefits system costs and increase RES penetration. • Central dispatch concept is more cost-effective in supporting high RES penetrations. This paper presents a comparative evaluation of central and self-dispatch management concepts for battery energy storage (BES) facilities in island power systems with a high renewable energy source (RES) penetration. BES facilities deployed to support the integration of additional wind capacity can be either centrally dispatched by the island System Operator or they can be self-dispatched within a Virtual Power Plant entity comprising renewables and storage, called a Hybrid Power Station (HPS). To explore the anticipated benefits of each BES management paradigm, annual simulations are performed for an example island system, employing a three-layer mixed integer linear programming (MILP) method to simulate the unit commitment and economic dispatch processes. The levelized cost of energy (LCOE) of combined BES and renewables investments is calculated and the achieved RES penetration levels and island system generation cost are evaluated, allowing the identification of Pareto optimal storage configurations, leading to lowest LCOE for a given RES penetration target. Overall, the centrally dispatched BES systems prove to be substantially more cost-effective, compared to the self-dispatched alternative, for achieving similar RES penetration levels. [ABSTRACT FROM AUTHOR]

The objective of this paper is to evaluate the contribution of energy storage systems to resource adequacy of power systems experiencing increased levels of renewables penetration. To this end, a coherent methodology for the assessment of system capacity adequacy and the calculation of energy storage capacity value is presented, utilizing the Monte Carlo technique. The main focus is on short-duration storage, mainly battery energy storage systems (BESS), whose capacity values are determined for different power and energy configurations. Alternative operating policies (OPs) are implemented, prioritizing system cost or reliability, to demonstrate the significant effect storage management may have on its contribution to system adequacy. A medium-sized island system is used as a study case, applying a mixed integer linear programming (MILP) generation scheduling model to simulate BESS and system operation under each OP, in order to determine capacity contribution and overall performance in terms of renewable energy sources (RES) penetration, system operating cost and BESS lifetime expectancy. This study reveals that BESS contribution to system adequacy can be significant (capacity credit values up to ~85%), with energy capacity proving to be the most significant parameter. Energy storage may at the same time enhance system reliability, reduce generation cost and support RES integration, provided that it is appropriately managed; a combined reliability-oriented and cost-driven management approach is shown to yield optimal results. [ABSTRACT FROM AUTHOR]

• New MILP-based UC model with embedded ac power flow constraints. • Piecewise linear ac power flow equations introduced, compatible with MILP models. • UC model perfectly applicable to small power systems containing tens of system buses. This paper presents a new network-constrained unit commitment (UC) model, which is formulated as a mixed-integer linear programming (MILP) optimization problem with embedded ac power flow (AC-PF) constraints. Piecewise linear AC-PF equations are formulated, which constrain the UC dispatch schedule within permissible transmission capacity and voltage deviation limits, solely by executing the main optimization problem. The proposed model is tested on a six-bus and the IEEE RTS-79 test systems, using GAMS software, whereas the obtained AC-PF results are validated against DIgSILENT/Powerfactory package. The UC schedules are comparatively assessed with detailed results presented in the relevant literature for the same test systems, employing different UC+AC-PF models. [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]

The main objective of this special issue is to collect and disseminate publications that highlight recent advances and breakthroughs in the area of renewable energy resources. The use of these resources for production of electricity is increasing rapidly worldwide. As of 2015, a majority of countries have set renewable electricity targets in the 10%?40% range to be achieved by 2020?2030, with a few notable exceptions aiming for 100% generation by renewables. We are experiencing a truly unprecedented transition away from fossil fuels, driven by environmental, energy security, and socio-economic factors.Electric machines can be found in a wide range of renewable energy applications, such as wind turbines, hydropower and hydrokinetic systems, flywheel energy storage devices, and low-power energy harvesting systems. Hence, the design of reliable, efficient, cost-effective, and controllable electric machines is crucial in enabling even higher penetrations of renewable energy systems in the smart grid of the future. In addition, power electronic converter design and control is critical, as they provide essential controllability, flexibility, grid interface, and integration functions. [ABSTRACT FROM AUTHOR]