Medium voltage converters have acquired a high relevance for high power industrial and traction applications as well as regenerative energy sources. This thesis provides an outcome of different kinds of medium voltage converters, focusing on the modular multilevel converter (MMC) due to its advantages with respect to other converters. In particular, due to its usefulness in different kinds of medium voltage applications, the indirect MMC topology has been selected. Therefore, the operating principle, modulations, control techniques, balancing algorithms and component sizing of indirect MMCs are studied.

The performance of modular multilevel converters (MMCs) in medium voltage (MV) applications, where the number of required sub-modules (SMs) is not high, can be improved utilizing low switching frequency modulations such as selective harmonic elimination-pulse width modulation (SHE-PWM), which provides tight control of lower order harmonics and low switching losses. Two main challenges must be met to utilize SHE-PWM with indirect MMCs. Firstly, a high number of harmonics must be controlled throughout the modulation index range. In this way, a wide range of fundamental frequencies, using similar switching frequency and different modulation index values can be employed. Therefore, a new method which simplifies the process to calculate a high number of firing angles would be desirable. Secondly, the circulating current must be controlled with the aim of reducing the SM capacitor voltage ripple and the MMC losses.

This thesis proposes a calculation method, which is based on a novel formulation, to solve the SHE-PWM problem with quarter wave (QW) and half wave (HW) symmetries. In particular, MMCs with (N+1) and (2N+1) phase output voltage levels are considered, obtaining (N+1) and (2N+1) SHE-PWM waveforms, respectively, where N is the number of SMs at each arm. This method utilizes a unique system of equations which is valid for any possible waveform. Therefore, it is able to calculate simultaneously, without predefined waveforms, both the switching patterns and the associated firing angles that solve the SHE-PWM problem. Consequently, the search process is simplified and optimized. Furthermore, this thesis also proposes two circulating current control techniques which can be applied along with (N+1) SHE-PWM and (2N+1) SHE-PWM, respectively. The proposed control techniques do not disturb the phase output voltage.

Finally, simulation results have validated the proposed SHE-PWM formulations, with QW and HW symmetries, besides the circulating current control techniques proposed for (N+1) SHE-PWM and (2N+1) SHE-PWM. In addition, experimental results, obtained from a prototype of single-phase MMC, have validated the formulation with QW symmetry and the circulating current control proposed for (2N+1) SHE-PWM.