A method is configured for operating a converter (10) which is implemented as a modular-multilevel converter and comprises a control arrangement (38) and a number M of phase-legs (21 to 29). The method comprises detecting whether the converter (10) has to be set into one mode of a group comprising a static synchronous compensator mode or a grid unbalance mode, generating mode control signals (MCS) depending on the detected mode, generating balance voltage reference signals (ubal,ref) depending on a first side frequency (ωg), a second side frequency (ωm), second side current reference signals (im,ref) and the mode control signals (MCS), generating a phase-leg control signal (uref), generating cell control signals (51 to S4) and providing the cell control signals (51 to S4) to semiconductor switches (41 to 44) of cells (31) of the phase-legs (21 to 29).