Method and apparatus for controlling voltages over series-connected dc power supplies

An apparatus for controlling voltages over series-connected DC power supplies (1.1 - 1. m ), comprising an inverter (2) comprising a positive input, a negative input, and means for controlling a voltage between the inputs. The apparatus also comprises a first series connection of two or more capacitors (3.1 - 3. m ) between the inverter (2) inputs. The apparatus further comprises one or more balancing units (5.1 - 5. m -1) connected between the inverter (2) inputs, comprising means for controlling ratios between first series connection capacitor voltages.