This review article is mainly oriented to the control and applications of modular multilevel converters (MMC). The main topologies of the switching modules are presented, for normal operation and for the elimination of DC faults. Methods to keep the capacitor voltage balanced are included. The voltage and current modulators, that are the most internal loops of control, are detailed. Voltage control and current control schemes are included which regulate DC link voltage and reactive power. The cases of unbalanced and distorted networks are analyzed, and schemes are proposed so that MMC contribute to improve the quality of the grid in these situations. The main applications in high voltage direct current (HVDC) transmission along with other medium voltage (MV) and low voltage (LV) applications are included. Finally, the application to offshore wind farms is specifically analyzed.

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a b s t r a c tMulti-modular converters (MMC) are an emerging and promising option for high voltage direct current (HVDC) transmission, connection of offshore wind farms and FACTS. For such converters, two new strategies for current control are proposed, in which a band is defined around the reference current of the three phases, and modules to be turned ON are chosen to keep the three phase currents within the bands. In the first strategy, only the voltage levels adjacent to the grid voltage level are chosen; this is called "constant excitation" and it is the most appropriate when the number of modules per arm is small. The second strategy uses an excitation proportional to the current error, and it is the most appropriate when the number of modules per arm is great. The theoretical foundation of the strategies and the simulation results within an external active and reactive power control loop are presented. Finally, the current control strategies were applied to HVDC transmission from offshore wind farm to the onshore grid.

a b s t r a c tIn this paper, an implementation of the control and the synchronization algorithms for a Voltage Source Inverter used as the power conditioner for Photovoltaic renewable energy in a grid-connected structure is carried out. Its main purpose is to show, in a simple manner, the design and combined operation of the control and synchronization algorithms for attaining the proper behaviour of the Grid Inverter when the 3-phase utility grid is disturbed by voltage unbalances, frequency variations and harmonic distortions, according to power quality standards.In order to obtain a high efficiency of the system during perturbations, a Proportional Resonant controller with a Harmonic Compensator structure is designed for the control algorithm, whereas a Dual Second Order Generalized Integrator Frequency-Locked Loop (DSOGI-FLL) is used as the synchronization algorithm.In order to validate both the control and the synchronization algorithms, some simulations using MATLAB/SIMULINK from The MathWorks, Inc. are shown firstly, and secondly, some real-time digital simulations are carried out.

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