Control and modulation of the modular multilevel converter under unbalanced cell power conditions

This thesis presents a control and modulation scheme for the Modular Multilevel Converter under unbalanced power conditions. The modular multilevel converter is a relatively new converter topology and since its introduction has garnered interest in both literature and industry for its applications in High Voltage DC transmission. Whilst the majority of the published work has been for traditional applications for the converter, there has been recent interest in new applications for the converter. This includes the use of the converter as a photovoltaic interface, where power sources are directly connected to each converter cell. To maintain operation under a variety of operating conditions, new control and modulation schemes may be required. The thesis presents an energy balancing model based on the production of a number of controlled circulating current components in the converter. This energy model allows for the decoupling of the power generation in each arm, resulting in independent control of the arm voltages, and the compensation for large unbalanced power production across the converter. A model based deadbeat current controller is developed to control the converters output and circulating currents. A new heuristic model predictive control based modulation scheme is proposed to allow for the operation of each converter cell at separate references while generating the desired arm voltages. This is achieved while providing a balance between switching transitions and the spread of cell voltages from their references in each converter arm. Simulation and experimental results are presented to validate the performance and functionality of the proposed control and modulation scheme. It was found that the control scheme provides for the ability to maintain stable converter operation with large varied powers across all converter cells while maintaining balanced output powers and low device switching frequencies.