Buck Boost Converter Cell for MMC

A converter cell for a modular multilevel converter, the cell consisting of:

• a primary energy storage;
• an inductor;
• a secondary energy storage, the primary energy storage, the inductor and the secondary energy storage all connected in parallel with each other between two terminals of the cell;
• a first converter valve comprising a first semiconductor switch and a first antiparallel diode, wherein the secondary energy storage is connected in series with the first converter valve, and together with the first converter valve in parallel with the inductor;
• and
• a second converter valve comprising a second semiconductor switch and a second antiparallel diode, the second semiconductor switch able to conduct electrical current in the same direction as the first semiconductor switch when switched to a conducting state, wherein the primary energy storage is connected in series with the second converter valve, and together with the second converter valve in parallel with the inductor;
• wherein the cell is configured such that, when power is flowing into the cell:
• when the first semiconductor switch is switched to conducting and the second semiconductor switch is switched to non-conducting, a current is allowed to flow from the secondary energy storage to the inductor, charging the inductor, via the first semiconductor switch, after which;
• when both the first semiconductor switch and the second semiconductor switch are switched to non-conducting, a current is allowed to flow from the charged inductor to the primary energy storage, charging the primary energy storage, via the second antiparallel diode;
• and
• wherein the cell is configured such that, when power is flowing out of the cell:
• when the first semiconductor switch is switched to non-conducting and the second semiconductor switch is switched to conducting, a current is allowed to flow from the primary energy storage to the inductor, charging the inductor, via the second semiconductor switch, after which;
• when both the first semiconductor switch and the second semiconductor switch are switched to non-conducting, a current is allowed to flow from the charged inductor to the secondary energy storage, charging the secondary energy storage, via the first antiparallel diode.

A method performed by a control arrangement for controlling a converter cell in a Modular Multilevel Converter (MMC),

• wherein the converter cell comprises:
• a primary energy storage;
• an inductor;
• a secondary energy storage, the primary energy storage, the inductor and the secondary energy storage all connected in parallel with each other between two terminals of the cell;
• a first converter valve comprising a first semiconductor switch and a first antiparallel diode, wherein the secondary energy storage is connected in series with the first converter valve, and together with the first converter valve in parallel with the inductor;
• and
• a second converter valve comprising a second semiconductor switch and a second antiparallel diode, the second semiconductor switch able to conduct electrical current in the same direction as the first semiconductor switch when switched to a conducting state, wherein the primary energy storage is connected in series with the second converter valve, and together with the second converter valve in parallel with the inductor;
• and
• wherein the method comprises:
• when power is flowing into the cell:
• switching the first semiconductor switch to conducting while the second semiconductor switch is switched to non-conducting, allowing a current to flow within the cell from the secondary energy storage to the inductor, charging the inductor, via the first semiconductor switch, and
• switching the first semiconductor switch to non-conducting while the second semiconductor switch remains switched to non-conducting, allowing a current to flow within the cell from the charged inductor to the primary energy storage, charging the primary energy storage, via the second antiparallel diode which is antiparallel to the second semiconductor switch;
• and
• when power is flowing out of the cell:
• switching the second semiconductor switch to conducting while the first semiconductor switch is switched to non-conducting, allowing a current to flow within the cell from the primary energy storage to the inductor, charging the inductor, via the second semiconductor switch;
• and
• switching the second semiconductor switch to non-conducting while the first semiconductor switch remains switched to non-conducting, allowing a current to flow within the cell from the charged inductor to the secondary energy storage, charging the secondary energy storage, via the first antiparallel diode which is antiparallel to the first semiconductor switch.