Power transmission method and power transmission apparatus

A method of controllably transmitting power by an apparatus between a first electricity network and a second electricity network, the first electricity network being either a single-phase DC-network or a polyphase AC-network, and the second electricity network being either a polyphase AC-network or a polyphase electric machine, the apparatus comprising a plurality of low-voltage power cells arranged to connect the first electricity network to the second electricity network, wherein each low-voltage power cell of the plurality of low-voltage power cells includes a single-phase input connection (IN) and a single-phase output connection (OUT), wherein the plurality of low-voltage power cells are arranged into one or more groups, wherein each group of the one or more groups includes at least one low-voltage power cell per each phase of the first electricity network, at least one low-voltage power cell per each phase of the second electricity network, controllable semiconductor switches connected to the input connections (IN) of each of the low-voltage power cells of the group, and a group-specific transformer, wherein, inside each group of the one or more groups, a single-phase input connection (IN) of a first low-voltage power cell is connected to a first winding of the group-specific transformer, the first winding being galvanically isolated from other windings of the group-specific transformer that are connected to single-phase input connections (IN) of other low-voltage power cells of the corresponding group, the method comprising:

• controlling, inside each group of the one or more groups, the controllable semiconductor switches so that voltage waveforms of all input connections (IN) of the corresponding group are cophasal with a 50% pulse ratio.