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This paper presents AC fault ride-through strategies for the "extended-overlap" operating mode of the Alternate Arm Converter (AAC), which is a type of modular multilevel Voltage Source Converter (VSC) that has been proposed for HVDC transmission applications. The AAC offers several benefits over the Half-Bridge (HB) Modular Multilevel Converter (MMC), such as requiring fewer Sub-Modules (SMs) with a smaller capacitance and providing DC fault ride-through capability. Novel symmetrical and asymmetrical AC fault ride-through strategies are described and these strategies are experimentally validated by using a Small Scale Prototype (SSP).

The Modular Multilevel Converter (MMC) is a subject of current research in power electronics. Though a number of control and modulation strategies have been proposed, still there is no general agreement on the methodology used for their classification, analysis and evaluation. This master thesis will analyze the operating principles of the MMC, with special focus on the different modulation strategies that can be applied. A new modulation approach will be developed , with the goal of reducing the size and cost of the passive components, such as the arm inductances and the cell capacitors. Control objectives as well as the evaluation and classification of the control strategies already proposed in the literature will also be covered. Finally, a new control strategy will be proposed. Simulation results will be provided in order to demonstrate the correct operation of the converter control as well as a brief comparison between different modulation strategies. This work has been supported by the Spanish Ministry of Economy and Competitivity, under research project ENE-2010-14941