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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]

This paper analyzes the operating and economic benefits anticipated from the introduction of centrally managed battery energy storage systems (BESSs) in noninterconnected island (NII) grids with high renewable energy source (RES) penetration. A mixed integer linear programming based generation management model is developed to simulate the operation of NII systems with and without BESS facilities. The impact of BESS energy and power capacity sizing is discussed, showing that the main contribution of storage lies in the provision of fast response reserves that facilitate the integration of increased intermittent RES generation. A feasibility analysis is then performed, considering BESS facilities either as independent storage investments or as facilities being developed in conjunction with new wind power capacity. The levelized cost of energy or reserves is calculated to evaluate the required remuneration to ensure investment viability. [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]

In order for a photovoltaic (PV) system to provide a full range of ancillary services to the gird, including frequency response, it has to maintain active power reserves. In this paper, a new control scheme for the dc/dc converter of a two-stage PV system is introduced, which permits operation at a reduced power level, estimating the available power (maximum power point—MPP) at the same time. This control scheme is capable of regulating the output power to any given reference, from near-zero to 100% of the available power. The proposed MPP estimation algorithm applies curve fitting on voltage and current measurements obtained during operation to determine the MPP in real time. This is the first method in the literature to use the nonsimplified single-diode model for the determination of the MPP and the five model parameters while operating at a curtailed power level. The developed estimation technique exhibits very good accuracy and robustness in the presence of noise and rapidly changing environmental conditions. The effectiveness of the control scheme is validated through simulation and experimental tests using a 2-kW PV array and a dc/dc converter prototype at constant and varying irradiance conditions. [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]

Determination of PV model parameters usually requires time consuming iterative procedures, prone to initialization and convergence difficulties. In this paper, a set of analytical expressions is introduced to determine the five parameters of the single-diode model for crystalline PV modules at any operating conditions, in a simple and straightforward manner. The derivation of these equations is based on a newly found relation between the diode ideality factor and the open circuit voltage, which is explicitly formulated using the temperature coefficients. The proposed extraction method is robust, cost-efficient, and easy-to-implement, as it relies only on datasheet information, while it is based on a solid theoretical background. Its accuracy and computational efficiency is verified and compared to other methods available in the literature through both simulation and outdoor measurements. [ABSTRACT FROM PUBLISHER]

In this paper, the electrical response of a partially shaded photovoltaic (PV) array, comprising several strings connected in parallel, is investigated. The PV array is simulated by employing an enhanced version of the widely used single-diode model, reformulated in an explicit manner employing the Lambert \bmW function. The multiple maximum power points (MPPs) that appear on the \bmP-\bmV characteristic of the array in partial shading conditions are analyzed, in terms of their number and properties. Simplified empirical expressions are then derived to calculate the voltage, current, and power for each local MPP, at any irradiance level and temperature, using only datasheet information, in a most simple and straightforward manner, without resorting to detailed modeling and simulations. The derived formulae are validated using both simulation and experimental results. [ABSTRACT FROM PUBLISHER]

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]

In this paper, a reformulation of the widely used one-diode model of the photovoltaic (PV) cell is introduced, employing the Lambert W function. This leads to an efficient PV string model, where the terminal voltage is expressed as an explicit function of the current, resulting in significantly reduced calculation times and improved robustness of simulation. The model is experimentally validated and then used for studying the operation of PV strings under partial shading conditions. Various shading patterns are investigated to outline the effect on the string I-V and P-V characteristics. Simplified formulae are then derived to calculate the maximum power points of a PV string operating under any number of irradiance levels, without resorting to detailed modeling and simulation. Both the explicit model and the simplified expressions are intended for application in shading loss and energy yield calculations. [ABSTRACT FROM PUBLISHER]

This paper presents an investigation on wind turbine (WT) contribution to the frequency control of noninterconnected island systems. The capability of WTs to participate in the primary frequency control and offer primary reserve is discussed. The investigation includes both transient frequency support (inertial response) and permanent frequency response (droop characteristic), as well as the combined application of these concepts. A quantitative analysis is presented for the expected benefits and drawbacks of each method, including the appropriate selection of their parameters. The power system of Rhodes Island has been selected as a study case, which includes different types of conventional generation and the three basic WT types, based on Active-Stall Induction Generator (ASIG), Doubly Fed Induction Generator (DFIG), and Permanent Magnet Synchronous Generator (PMSG). [ABSTRACT FROM PUBLISHER]

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]

Pumped storage is generally viewed as the most promising technology to increase renewable energy source penetration levels in power systems and particularly in small autonomous island grids, where technical limitations are imposed by the conventional generating units. The Hybrid Power Station (HPS) of Ikaria Island, Greece, which is currently in the construction stage, will be one of the first wind-hydro-pumped-storage hybrid stations in the world. In this paper, the HPS is described and a suitable operating policy is introduced, both for the HPS and the overall generation system of Ikaria, taking into consideration the existing regulatory framework for HPS in isolated island grids. Based on the proposed policy, the operation of the Ikaria Island system is simulated on an annual basis, in order to quantify the expected benefits from the operation of the HPS. [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]

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]

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.

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]