This work presents a comparison of three different modulation techniques applied to modular multilevel converters (MMCs). The three modulation strategies studied in this paper are the phase-shifted sinusoidal pulse width modulation (PS-SPWM), the space-vector modulation (SVM) and the nearest level modulation (NLM). This paper focuses on analysing the particularities and implementation of each modulation technique. The modulation technique largely defines the generated harmonic content, making this is a key point that must be studied in depth. The paper briefly describes the three modulation techniques and analyses the harmonics generated by each one of the methods. In addition, the paper presents and compares the digital implementation of the three modulation methods in a Field Programmable Gate Array (FPGA). The proposed approaches are validated using a real processing platform and experimentally evaluated in a real high-power six-level MMC.

In the next years, modular multilevel converters (MMCs) are going to be a next generation multilevel converters for medium to high voltage conversion applications, such as medium voltage motor drives, medium voltage flexible AC transmission systems (FACTS) and high voltage direct current transmission. They provide advantages such as high modularity, availability, low generation of harmonics, etc. However, the circulating current distorts the leg currents and increases the rated current of power devices, which further increases system cost. This paper focuses on analysis and suppression of these currents in a MMC using two algorithms for tracking of harmonics. For this work resonant controllers and repetitive controllers have been selected. Both controllers are analyzed and simulations results are presented. Moreover, the controllers have been tested and validated for a three phase MMC operating as an inverter using a real processing platform based on Zynq by Xilinx and designed to control large multilevel converters and in a real MMC prototype. These results are provided to demonstrate the feasibility of the proposed method.

The race to increase the efficiency and reduce the power losses in transmission systems has resulted in the substantial growth of High Voltage Direct Current (HVDC) transmission systems. Moreover, the interconnection of these transmission systems significantly increases their reliability. However, the control of these meshed grids is a key problem that usually is managed through the control of the VSC’s in those grids. But, the control of the VSC can be complemented with a reconfiguration algorithm. This paper proposes the use of the Particle Swarm Optimization (PSO) algorithm, in order to reconfigure meshed High Voltage Direct Current (HVDC) transmission systems and reduce losses. The proposed algorithm has been tested in the CIGRE benchmark grid, which comprises of several offshore wind farms that generate energy sent to the grid through several HVDC transmission lines. The results show that as the energy generation changes due to wind changes, the grid topology must be reconfigured in order to achieve the maximum efficiency. Doing this reconfiguration, power savings around 18-19% could be achieved.

One of the most up-and-coming multilevel converters are the Modular Multilevel Converters (MMCs) due to its simple structure which can be adapted to wide range of voltage levels. Nevertheless, one the main disadvantages of MMC is the presence of the circulating current which reduces the efficiency. With a view to overcome this problem, this paper proposes an approach to control and reduce the circulating current based on repetitive controllers. The repetitive controller is described and adapted to minimize the circulating current. In addition, experimental results in a MMC prototype have been carried out to validate the controller.