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

This paper presents the arm energy model and submodule capacitance sizing of the asymmetric alternate arm converter (AAAC) topology that has been recently proposed for high voltage DC (HVDC) applications. A step-by-step derivation of the converter arm energy model is presented to arrive at a final arm energy expression that aids to determine the maximum arm energy deviation of the converter that is required to determine the minimum submodule capacitance of the converter. A simplified mathematical expression is also derived to help determine the required submodule capacitance for the converter from the maximum arm energy deviation, the number of submodules per converter arm and the maximum allowed submodule capacitor voltage deviation (ripple factor). The derived arm energy expression is validated using both simulation and experimental results. The calculated, the simulated, and the experimentally measured converter arm energy values have a good match with each other verifying the accuracy of the derived converter arm energy model. In addition, comparison of the energy storage requirements of the AAAC topology with other similar converter topologies such as the conventional (symmetric) alternate arm converter topology and the modular multilevel converter topology is provided to further highlight the significance of the derived converter arm energy and submodule capacitance sizing expressions.