Suppressing circulating current in the modular multilevel converter using line-to-line voltage correction modules

The minimization of circulating currents is a major technical challenge in the modular multilevel converter (MMC) topology. Circulating currents are an inherent property of the MMC that originate from the voltage difference between the phases. These currents have a great impact on the submodule (SM) capacitor voltage ripple, voltage/current stress on the power switching devices and the power losses. Therefore, minimization of circulating currents is imperative for the smooth operation of the MMC. In order to achieve this goal, this paper proposes a modified MMC topology which makes use of additional line-to-line connected modules termed as voltage correction modules. These modules compensate for the voltage difference between the phases of the MMC and suppress the circulating currents. This solution offers a decoupled control between the circulating currents and the power stage and is independent of the number of SMs used in the MMC. Mathematical modeling along with the control strategy is explained throughout the paper and two test cases are conducted via detailed PSCAD/EMTDC simulations to validate this approach.