Modular multi-level converter

The converter (10) has converter branches (20a, 20b) for phases (16a-16c) of an alternating voltage-grid (12), where converter cells (26) are connected in rows and connect the phases with respective direct voltage outputs (22, 24). The cells contain unipolar converter cells (26b) and bipolar converter cells (26a). A control (30) controls semiconductor switches of the converter cells so that a direct voltage between the outputs is adjustable between a minimum voltage and a maximum voltage. The minimum voltage is smaller than 70 percentage of the maximum voltage. Independent claims are also included for the following: (1) a voltage supply system comprising a direct voltage device (2) a converter system for connecting an alternating voltage source with an alternating voltage grid (3) a grid for transmission of medium voltage and high voltage (4) a method for rectifying two phases of an alternating voltage into direct voltage or for inversion of direct voltage into two phases of an alternating voltage.