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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]

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 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]

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]

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]