The Modular Multilevel Converter (MMC) is a most promising converter technology for the High Voltage DC application. The complex topology of the MMC requires several additional controllers to balance the energy in the capacitors which are distributed all over the converter. Typically, there is a requirement of two controls; one is the regulation of the totalenergy in each leg, and the other is the distribution of the energy between the upper and the lower arms. This paper presents control strategies for the latter one being capable of distributing the energy only by internal power flow, so that undesired interference with the associated grids can be completely avoided. The proposed controls are achieved by forcing the common mode currents to be balanced while keeping the classic cascaded control structure as much as possible. The effectiveness and advantage of the proposed solutions are demonstrated by simulations.
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