Energy Balancing in the Modular Multilevel Converter under Unbalanced Grid Conditions

A comprehensive energy balancing concept for the Modular Multilevel Converter (MMC) under unbalanced grid conditions was proposed. Grid supporting converters need to be able to fed unbalanced grid currents; grid failures lead to unbalanced grid voltages. Both unbalanced grid conditions generate energy unbalance inside the MMC. In order to handle these critical operating points, an analytical description of the energy distribution inside the MMC - depending on the grid currents and voltages and based on a double synchronous reference frame (DSRF) approach - was derived. The obtained mathematical relations were presented in a compact representation using matrix notations and an energy balancing concept was deduced. An implementation to a hardware setup in power electronics laboratory has been performed. The proposed concept has been proven and verified with measurement results.