The concept of the modular multilevel converter (MLC) has been raising interest in research in order to improve their performance and applicability. The potential of an MLC is enormous, with a great focus on medium- and high-voltage applications, such as solar photovoltaic and wind farms, electrified railway systems, or power distribution systems. This concept makes it possible to overcome the limitation of the semiconductors blocking voltages, presenting advantageous characteristics. However, the complexity of implementation and control presents added challenges. Thus, this paper aims to contribute with a critical and comparative analysis of the state-of-the-art aspects of this concept in order to maximize its potential. In this paper, different power electronics converter topologies that can be integrated into the MLC concept are presented, highlighting the advantages and disadvantages of each topology. Nevertheless, different modulation techniques used in an MLC are also presented and analyzed. Computational simulations of all the modulation techniques under analysis were developed, based on four cascaded full-bridge topologies. Considering the simulation results, a comparative analysis was possible to make regarding the symmetry of the synthesized waveforms, the harmonic content, and the power distribution in each submodule constituting the MLC.

The extensive use of grid-connected power electronics converters in the most different conditions, e.g., grid voltage quality, power level and grid codes, requires a robust synchronization method in order to properly feed other converter controllers with reliable and exact information, namely the phase-angle and the magnitude of the grid voltage. The grid phase-angle is a vital variable used in almost any single- or three-phase converter. The paper proposes a synchronizer based on the Least Squares approach: using the sampled grid voltages to fit a straight line corresponding to the grid phase-angle estimation. An extensive set of simulation results, including comparative results with three other methods, covering steady state and transient conditions, is provided and demonstrates the feasibility of the proposed method. Also, a set of experimental results, obtained with a field programmable gate array platform, complements the validation of the synchronizer and its very good performance in all conditions.

Electric locomotives in AC traction power systems represent a huge single-phase non-linear load and, detrimentally, affect the power quality and the efficiency of the three-phase power grid. Nevertheless, along the last decades, power electronics are being used to mitigate power quality problems in the three-phase power grid. In particular, Rail Power Conditioner (RPC) helps to increase the loading capacity of traction substations and improve the power quality of three-phase power grids. As the main characteristics, an RPC can supply reactive power, suppress current harmonics and overcome currents imbalance of the three-phase power grid. On the other hand, the traction substations may be constituted by different types of power transformers. For instance, single-phase power transformers and open-delta (V/V) power transformers are widely used, while Scott power transformers are less frequently used, since they are more complex and expensive. In this framework, this work presents a review study of RPC topologies, including their operation modes, and a comprehensive comparison between the characteristics of the RPC topologies when using different types of AC traction substations and power transformers. This helps to ensure the correct selection of the RPC topology for a specific application, according to the main structure of the traction substation. Consequently, and based on the established review, it is possible to sort and allocate each RPC topology for limited or wider applications.

The doubly fed induction generator is particularly appropriate to be used in wind power generation systems. However, in weak grids its stator connection to unbalanced or distorted voltages imposes low frequency and high amplitude rotor current harmonics. Also, the grid side converter must deal with the same voltage conditions. The paper addresses the first issue presenting a voltage harmonics controller in the dq reference frame to be included in the high dynamics rotor current controller. Simulation and experimental results demonstrate the performance of the proposed controller imposing a sinusoidal input current.