Converter Arm Energy Model and Submodule Capacitor Sizing for the Asymmetric Alternate Arm Converter Topology

This paper presents the converter arm energy model and submodule capacitor sizing procedure of the Asymmetric Alternate Arm Converter (AAAC) topology which was recently proposed for HVDC applications. The converter arm energy model is derived from its arm voltage and arm current profiles over one fundamental line cycle. Based on the derived arm energy model of the converter, the converter arm energy deviation is investigated for different modulation index and power factor load operation conditions. The arm energy investigation result shows that the converter arm energy deviation increases when the modulation index decreases. While, with regard to load power factor, the converter arm energy deviation has symmetrical relationship with the power factor axis and the maximum arm energy deviation occurs at a power factor of 0 or when the converter is processing full reactive power. Therefore, from the minimum possible modulation index value of the converter and a power factor of 0, the maximum arm energy deviation of the converter is determined and then the minimum submodule capacitance required for the converter is determined using the worst-case energy perturbation method from the maximum converter arm energy deviation over a fundamental line cycle, the number of submodules per converter arm and the maximum allowed submodule capacitor voltage deviation (ripple factor). The accuracy of the derived converter arm energy model is validated using both simulation and experimental results.