Method for pre-charging a cascade converter and cascade converter

A cascade converter, comprising:

• a main transformer, comprising a primary winding and multiple secondary windings;
• multiple power units, connected with the multiple secondary windings in a one-to-one correspondence;
• wherein each of the power units comprises a rectifier circuit, a bus capacitor, and an inverter circuit, and the bus capacitor is connected between the rectifier circuit and the inverter circuit;
• a precharge unit, connected with at least one power unit;
• wherein the precharge unit comprises a low-voltage AC power supply and a first switch set, and the low-voltage AC power supply is configured to provide power to pre-charge the converter;
• wherein the at least one power unit connected with the precharge unit is a first power unit group, and power units not connected with the precharge unit are a second power unit group;
• and
• a control unit, configured to control the first switch set to be turned on, so that the low-voltage AC power supply performs pre-charging for a corresponding bus capacitor through the inverter circuit in the first power unit group, and control the rectifier circuit in the first power unit group to work to magnetize the main transformer when a voltage of the bus capacitor in the first power unit group reaches a first voltage threshold, thereby performing pre-charging for bus capacitors in the second power unit group.