Device which transforms a first DC voltage (U DC1) into a higher second DC voltage (U DC2), or vice versa, having an overall converter (GMMC) which has a first and a second modular multilevel converter (11, 12) connected in series on the DC side, wherein the first modular multilevel converter is connected to the second modular multilevel converter on the DC side, wherein the first DC voltage (U DC1) is applied to the first multilevel converter, and having an actuation unit which generates actuation signals for controlling the multilevel converters (11, 12), wherein the first multilevel converter converts the first DC voltage into a three-phase first AC voltage, wherein the overall converter (GMMC) has a first transformer having a first winding and a second winding, characterized in that the first transformer transforms the three-phase first AC voltage applied to the first winding into a single-phase second AC voltage applied to the second winding, in that the first multilevel converter is connected to the first AC voltage and the second multilevel converter is connected to the second AC voltage, in that the series connection comprising the multilevel converters has a third multilevel converter which is connected to the second multilevel converter on the DC side, wherein the second DC voltage (U DC2) is applied to the series connection comprising the multilevel converters (11, 12, 13), and in that the first transformer has a third winding which transforms the three-phase first AC voltage into a single-phase third AC voltage to which the third multilevel converter is connected, or in that the overall converter (GMMC) has a second transformer which has a first winding and a second winding and which transforms the three-phase first AC voltage applied to the first winding into a single-phase second AC voltage which is applied to the second winding and to which the third multilevel converter is connected. having an overall converter (GMMC) which has a first and a second modular multilevel converter (11, 12) connected in series on the DC side, wherein the first modular multilevel converter is connected to the second modular multilevel converter on the DC side, wherein the first DC voltage (U DC1) is applied to the first multilevel converter, and having an actuation unit which generates actuation signals for controlling the multilevel converters (11, 12), wherein the first multilevel converter converts the first DC voltage into a three-phase first AC voltage, wherein the overall converter (GMMC) has a first transformer having a first winding and a second winding, characterized in that the first transformer transforms the three-phase first AC voltage applied to the first winding into a single-phase second AC voltage applied to the second winding, in that the first multilevel converter is connected to the first AC voltage and the second multilevel converter is connected to the second AC voltage, in that the series connection comprising the multilevel converters has a third multilevel converter which is connected to the second multilevel converter on the DC side, wherein the second DC voltage (U DC2) is applied to the series connection comprising the multilevel converters (11, 12, 13), and in that the first transformer has a third winding which transforms the three-phase first AC voltage into a single-phase third AC voltage to which the third multilevel converter is connected, or in that the overall converter (GMMC) has a second transformer which has a first winding and a second winding and which transforms the three-phase first AC voltage applied to the first winding into a single-phase second AC voltage which is applied to the second winding and to which the third multilevel converter is connected.