Analysis and Design of Modular Multilevel Converters for Grid Connected Applications

This paper is a licentiate compilation-thesis that aims to bring clarity to the dimensioning aspects and limiting factors of modular multilevel converters. The dc-capacitor in each submodule is a driving factor for the size and weight of the converter. This means that it is important to distribute the stored energy evenly among all submodule capacitors. Typically, an increased switching frequency will improve the balancing process of the stored energy. As shown in this thesis, the lowest possible switching frequency at which the energy can be balanced over time is the fundamental frequency. This will, however, result in an increased voltage ripple in the submodule capacitors. In order to quantify the requirements on the dccapacitors a general analysis is provided in this thesis which is based on the assumption that the capacitor voltages are well balanced. It is found that for active power transfer, with a 50 Hz sinusoidal voltage reference, the capacitors must be rated for a combined energy storage of 21 kJ/MW if the capacitor voltages are allowed to increase by 10% above their nominal values. The operating region can be extended by injecting a second-order harmonic component in the circulating current. This will, however, increase the losses in the converter.