Modular multilevel converter pre-charging

A controller for a Modular Multilevel Converter (MMC) including a plurality of cells connected in series between a positive rail and a negative rail and wherein each cell of the plurality of cells includes a bypass switch, an insert switch, and a cell capacitor, wherein the plurality of cells is connected in parallel to a Direct Current (DC) link capacitor, the controller configured to perform an operation comprising:

• driving a bypass current from an auxiliary power source through the bypass switch of each of the plurality of cells;
• in response to a summed voltage across the cell capacitors in the plurality of cells satisfying a drive threshold, driving an insert current from the cell capacitors through the insert switch of each of the plurality of cells;
• and
• in response to a voltage across the DC link capacitor satisfying a pre- charge threshold, opening a circuit breaker connecting the auxiliary power source to a rectifier and connecting a wind turbine generator, via the positive rail and negative rail, to an external power line,
• wherein driving the bypass current includes tuning a rate at which individual bypass switches switch to equalize charges in the plurality of cell capacitors as the cell capacitors charge;
• and wherein driving the insert current includes tuning a rate at which individual insert switches switch to equalize charges in the plurality of cell capacitors as the cell capacitors discharge, and
• wherein the operation further comprises while driving at least one of the bypass current and the insert current, measuring a difference in voltage level of each of the cell capacitors;
• and in response to the difference exceeding a difference threshold, stopping the auxiliary power source from charging the cell capacitors and the cell capacitors from charging the DC link capacitor.

A Modular Multilevel Converter (MMC), comprising:

• a positive rail;
• a negative rail;
• a Direct Current (DC) link, including a DC link capacitor, connected between the positive rail and the negative rail;
• a rectifier, connected between the positive rail and the negative rail, and selectively connected to an auxiliary power source in a wind turbine generator (WTG), wherein the rectifier supplies DC power from auxiliary power source of the WTG on the positive rail and the negative rail having a rectified peak voltage below a pre-charge threshold for the DC link capacitor;
• a plurality of cells, connected in series between the positive rail and the negative rail, each cell of the plurality of cells comprising:
• a cell capacitor, connected between a first node and a second node;
• an insert switch, connected to the first node and a third node;
• a bypass switch, connected to the third node and the second node;
• a first flow diode, configured to allow current flow from the third node to the first node and prevent current flow from the first node to the third node;
• a second flow diode, configured to allow current flow from the second node to the third node and prevent current flow from the third node to the second node;
• and a gate controller, configured to:
• control a voltage across the DC link capacitor and the cell capacitor by switching open and closed the insert switch and the bypass switch of respective ones of the plurality of cells to charge the cell capacitors in the plurality of cells beyond a nominal voltage and then switching open and closed the insert switch and the bypass switch to discharge the cell capacitors to charge the DC link capacitors to the pre- charge threshold, and to drive a bypass switch current includes tuning a rate at which individual bypass switches switch to equalize charges in the cell capacitors as the cell capacitors charge;
• and to drive an insert switch current includes tuning a rate at which individual insert switches switch to equalize charges in the of cell capacitors as the cell capacitors discharge.