Compensation of Circulating Currents in Modular Multilevel Converters through Stored Energy in the Submodules

Abstract

The integration of offshore wind farms can be done using direct current transmission systems based on modular multilevel converters. However, the use of this converter is accompanied by challenges that involve equalizing the voltages of the submodules and controlling the internal currents that flow through the converter arms. Therefore, this work investigates the energy-based indirect control strategy for suppressing circulating currents in the converter arms. Additionally, the operation of the MMC connected to a balanced network is studied, the analysis of the behavior of internal currents in the MMC arms, the implementation of submodule voltage balancing algorithms and the modeling of control loops to synthesize the output AC currents. Results obtained through digital simulations in the PSCAD/EMTDC software are used to validate the theoretical analyses.