Abstract
Modular multilevel converters are playing a key role in the present and future development of topologies for medium–to–high–power applications. Among this category of power converters, there is a direct AC–AC modular multilevel converter called “Hexverter”, which is well suited to connect three–phase AC systems operating at different frequencies. This topology is the subject of study in this manuscript. The complete Hexverter system is composed of an Hexverter power converter and several control layers, namely, a “virtual VC2 controller”, a branch current controller in a two–frequency dq reference frame, a modulator, and a voltage balancing algorithm. The paper presents a thorough description and analysis of the entire Hexverter system, providing research contributions in three key aspects: (i) modeling and control in a unified two–frequency dq framework; (ii) developing a “virtual VC2 controller” to dynamically account for Hexverter’s active power losses allowing to achieve active power balance on the fly; and (iii) a comparative evaluation of modulation strategies (nearest level control and phase disposition–sinusoidal pulse width modulation). To this end, a detailed switched simulation was implemented in the PSCAD/EMTDC software platform. The proposed “virtual VC2 controller” is evaluated through the measurement of its settling time and calculation of active power losses. Each modulation technique is assessed through total harmonic distortion and frequency spectrum of the synthesized three–phase voltages and currents. The results obtained suggest that the control scheme is able to properly regulate the Hexverter system under both modulation strategies. Furthermore, the “virtual VC2 controller” is able to accurately determine the active power loss, which allows the assessment of the efficiency of the modulation strategies. The nearest level control technique yielded superior efficiency.
Highlights
Modular multilevel converters (MMCs) have been during the last years, and will continue to be in the near future, a trending research topic
Despite the fact that some modulation techniques, such as nearest level control (NLC) and phase disposition–sinusoidal pulse width modulation (PD–SPWM), have been implemented and investigated for some multilevel topologies, there is still room to investigate and present a detailed assessment of modulation strategies when these are implemented for the direct AC–AC modular multilevel topology called, in short, “Hexverter”
Vbs1 corresponds to synthesized branch voltage utilizing NLC modulation technique
Summary
Modular multilevel converters (MMCs) have been during the last years, and will continue to be in the near future, a trending research topic. Hexverter modeling and control in a unified two–frequency dq framework; The proposal and evaluation of a “virtual VC2 controller” to dynamically account for Hexverter’s active power losses, allowing one to achieve active power balance on the fly; Detailed assessment of modulation strategies through total harmonic distortion of synthesized voltages and currents.
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