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In the last decade, there have been several initiatives for the deployment of cross-Mediterranean HVDC (High Voltage Direct Current) links to enable the transmission of electrical power from renewable energy sources between North Africa and Europe. These initiatives were mainly driven by the potential economic, environmental and technical benefits of these HVDC interconnections. In previous studies on these projects, some technical aspects of critical importance have not been addressed or studied in sufficient detail. One of these key aspects relates to the impact and possible benefit of these HVDC links on the dynamic performance of the European system which is the major focus of this paper. Several issues relating to the dynamic performance of the system are addressed here. Based on the experience gained from existing AC/DC projects around the world, this paper shows that the HVDC links between North Africa and Europe can greatly improve the dynamic performance of the European system especially in the southern regions. In addition, some challenges on the operation and control of these HVDC links are highlighted and solutions to overcome these challenges are proposed. This review paper, therefore, serves as a preliminary study for further detailed investigation of specific impacts or benefits of these interconnections on the overall performance of the European system.

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The integration of wind generation to the grid is growing rapidly across the world. As a result, grid operators have introduced the so-called grid codes (GC), which nowadays include a range of technical conditions and requirements, which wind generators must fulfill. Among these, the low voltage ride through (LVRT) is a requirement for wind turbines to stay connected to the grid and continue to operate during the disturbance. In this study, a control structure, combining inertial kinetic energy storage with a crowbar circuit, is proposed to enhance the ride-through capability of a wind turbine generator (WTG) based on a wound-field synchronous generator (WFSG) under unsymmetrical voltage dips. For the grid-side converter (GSC), a decoupled double synchronous reference frame (DDSRF) d-q current controller is used. Furthermore, a Second-Order Sliding Mode Controller (SOSMC) with Super-twisting (ST) algorithm is proposed for the GSC and the machine-side converter (MSC) to improve the response speed and achieve an accurate regulation of the dq-axis current components simultaneously. The main objectives of the GSC are to achieve a balanced, sinusoidal current and smooth the real and reactive powers to reduce the influence of the negative sequence voltage. A series of simulations are presented to demonstrate the effectiveness of the proposed control scheme in improving the LVRT capability of the WFSG-driven wind turbine and the power quality of the system under unbalanced grid voltage conditions.

A comparative control study for a maximum power tracking strategy of variable speed wind turbine is provided. The system consists of a direct drive permanent magnet synchronous generator (PMSG) and an uncontrolled rectifier followed by a DC/DC switch-mode step down converter connected to a DC load. The buck converter is used to catch the maximum power from the wind by generating an efficient duty cycle. Distinct Maximum Power Point Tracking (MPPT) algorithms are analyzed and compared: a classical Proportional-Integral controller (PI) and two based Fuzzy Logic Controllers (FLC), including a conventional Fuzzy-PI and an Adaptive FLC-PI. The main aim of the presented study is to develop an advanced control scheme for wind generators to ensure a high level operating of the system and a maximum power extraction from the wind. This is achieved by analyzing the behavior of the system under fluctuating wind conditions employing Matlab Simpower Systems tool.Simulation results confirm that the Adaptive FLC-PI controller algorithm has better performances in terms of dynamic response and efficiency especially in comparison with the ones of a PI controller under variable wind speed. KEYWORDS DC-DC buck converter, fuzzy logic controller (FLC), maximum power point tracking (MPPT), permanent magnet synchronous generator (PMSG), proportional integral (PI) controller, wind turbine

The paper proposes a new sliding mode power control strategy for a wound-field synchronous generator-based variable speed wind energy conversion systems to maximize the power extracted from the wind turbine. The proposed controller can handle the inherent nonlinearities in wind energy conversion systems and the randomness of the wind speed as well as the uncertainties of the model and external disturbances. To reduce the chattering phenomenon that characterizes conventional sliding mode control, a sigmoid function with a variable boundary layer is proposed. The adaptive switching gains are adjusted on-line by using a fuzzy logic-based technique. Several simulation scenarios were performed to evaluate the performance of the proposed control scheme. The results demonstrate that this controller provides excellent response characteristics, is robust against parameter variations, and free from chattering phenomenon as compared with the conventional sliding mode control.