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 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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This article deals with the vector control in dq axes of a three-phase grid-connected photovoltaic system with single-stage topology and low-voltage-ride-through capability. The photovoltaic generator is built using an array of several series-parallel Suntech PV modules and is modeled as a Lookup Table (two-dimensional; 2-D). The requirements adopted when grid voltage sags occur are based in both the IEC 61400-21 European normative and the allowed amount of reactive power to be delivered according to the Spanish grid code, which avoids the disconnection of the inverter under grid faults by a limitation in the magnitude of the three-phase output inverter currents. For this, the calculation of the positive- and negative-sequences of the grid voltages is made and a conventional three-phase Phase-Locked Loop is used for the inverter-grid synchronization, allowing the control of the active and reactive powers solely with the dq components of the inverter currents. A detailed enhanced flowchart of the control algorithm with low-voltage-ride-through capability is presented and several simulations and experiments using Matlab/SIMULINK and the Controller Hardware-in-the-Loop simulation technique, respectively, are run for several types of one- and three-phase voltage sags in order to validate its behavior.

The quality of power and current control are the greatest challenges of grid-connected wind farms during abnormal conditions. The negative-and positive-sequence components of the grid currents may be injected into a wind generation system during grid faults, which can affect the power stability and damage the wind system. The proposed work assures a low-voltage ride through capability of doubly-fed induction generator-based wind turbines under the grid voltage sag. A new technique to protect the wind system and to recompense the reactive power during failures of the utility grid according to the Spanish grid code is proposed. The control design is implemented to the power converters, and the grid current regulation is developed by using proportional-resonant regulators in a stationary two-phase (αβ) reference frame. The control performance is significantly validated by applying the real-time simulation for the rotor-side converter and the hardware in the loop simulation technique for the experiment of the generator's grid-side converter control.

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