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Abstract: This paper proposes an improved control strategy for a three phase, four leg inverter used for the simultaneous supply of three phase and single phase AC loads in a transformerless hybrid power system (HPS) application. First, the mathematical model of the inverter is obtained by using the average technique, which allows converter systems to be rapidly and accurately simulated. Then, two control strategies are described. The first one uses the conventional dq0 frame rotating with the angular frequency ω (ω =314rad/s) and PI (proportional integral) controllers to ensure voltage and current regulation. The second one is an improved control strategy based on the decomposition of the supply three phase voltage and current into instantaneous positive, negative and homopolar sequence components using phasor representation. These three sequences are controlled independently in their own reference frames as DC signals. The positive sequence is regulated by the PI controllers in a positive reference frame, which rotates counterclockwise, while the negative sequence is regulated by the PI controllers in a negative reference frame, which rotates clockwise at the same angular frequency. Since the proposed strategy uses phasor representation, the actually superposed phasors of the homopolar sequence are spatially displaced at 120°. Then, using a negative reference frame system, the obtained DC signals are regulated by the PI controllers. Simulation results show the validity of the innovative control strategy. [Copyright &y& Elsevier]

Abstract: Wind energy technology has experienced important improvements this last decade. The transition from fixed speed to variable speed wind turbines has been a significant element of these improvements. It has allowed adapting the turbine rotational speed to the wind speed variations with the aim of optimizing the aerodynamic efficiency. A classic controller that has slow dynamics relative to the mechanical dynamics of the drive train is implemented in commercial wind turbines. The objective of the work related in this paper has been to evaluate the implementation, on a test bench, of a controller whose dynamics can be adjusted to be faster and to compare in particular its aerodynamic efficiency with the conventional controller. In theory, the higher dynamics of the non-classic controller has to lead to a better efficiency. A 180kW wind turbine whose simulation model has been validated with field data is emulated on an 18kW test bench. The emulator has also been validated. Test bench trials are a very useful step between numerical simulation and trials on the real system because they allow analyzing some phenomena that may not appear in simulations without endangering the real system. The trials on the test bench show that the non-conventional controller leads to a higher aerodynamic efficiency and that this is offset by higher mechanical torque and electric power fluctuations. Nevertheless, the amplitudes of these fluctuations are relatively low compared to their rated values. [Copyright &y& Elsevier]

Highlights: [•] The control of a hybrid storage system using a Three Level NPC converter is analysed. [•] A sinusoidal PWM with an offset injection is used to control the storage system. [•] The operation of the selected converter is analysed in its entire operation range. [•] The operational limits of the Three Level NPC converter are defined. [Copyright &y& Elsevier]

Abstract: A high penetration rate of the Renewable Energy Sources (RES) can create stability, reliability and power quality problems in the main electrical grid. The microgrid should be a feasible alternative to solve these issues. As it is a weak electrical grid, the microgrid is very sensitive to any variation and consequently an Energy Storage System (ESS) is necessary to reduce their effect. A microgrid needs an ESS with high energy and power densities, but none of the currently available storage technologies satisfies both requirements at the same time. Consequently, it is necessary to associate more than one storage technology creating a Hybrid Energy Storage System (HESS). The objective of this work is to compare by means of simulations three different topologies that are used to control a HESS formed by a SuperCapacitor (SC) bank and a Vanadium Redox Battery (VRB). The compared topologies are the parallel active topology, the floating topology and the Three-Level Neutral Point Clamped (3LNPC) converter topology. The analysed microgrid is formed by the mentioned HESS and different renewable energy sources. This microgrid is interconnected to a weak grid based on a Diesel Generator (DG). Two case studies have been analysed: the effect of the RES while the microgrid is connected to the weak grid and the effect of the disconnection of the microgrid from the weak grid. The different topologies are compared analysing their capability of facing the mentioned variations, their power losses and their Total Harmonic Distortion (THD). The Matlab/Simulink software and the SimPowerSystems toolbox have been used to carry out the simulations. [Copyright &y& Elsevier]