Modular Multilevel Converter for Electric Motor Drive Applications
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- Institutt for elkraftteknikk 
In this master thesis the topic of Modular Multilevel Converters (MMC) has been studied. The working principle of the converter is presented with advantageous attributes such as a multilevel waveform, a modular realization and cost saving features. Vital control objectives are active and reactive power control, DC link voltage control, submodule capacitor voltage control and current control. A level-shifted pulse-width modulation (PWM) switching scheme was found to have relatively low total harmonic distortion (THD), thus used in the upcoming simulations. In order to ensure balancing of the converter capacitors, a voltage balancing algorithm was presented, sorting the capacitors based on their voltage level, and giving a state command accordingly. The thesis has examined the challenges of using MMC for electric motor drive applications. It has been found that the low frequency operation causes large voltage ripple in the capacitors, thus a large circulating current. Through a literature search, different measures where found in order to reduce the circulating current, including circulating current suppressing and manipulation. In addition an introduction of a common mode voltage was presented as a possible measure.After developing the one-phase model of the project thesis into a three-phase model, the circulating current suppressing controllers (CCSC) were tested, first at 50Hz, and then at 25Hz. At 50Hz, all three controllers worked as intended, reducing the circulating current by up to 72% and the voltage ripple was reduced from ∆vc = 10V to ∆vc = 6V . At 25Hz, all the controllers maintained their ability to reduce the circulating current. Nonetheless, it was concluded that further measures must be studied, as all controllers increased the capacitor voltage ripple atf =25Hz.