Modularized multi-level voltage regulating device of direct-current power transmission system transformer, control method and control system

The invention discloses a modularized multi-level voltage regulating device of a direct-current power transmission system transformer, a control method and a control system. The device comprises a connecting switch, a connecting transformer, a voltage regulating transformer, a connecting valve and a voltage regulating valve, the voltage regulating valve and the connecting valve are both of an H-bridge structure composed of bridge arms, the direct-current side of the voltage regulating valve is connected with the direct-current side of the connecting valve in parallel, two midpoints of an H bridge of the voltage regulating valve are connected to a primary winding of the voltage regulating transformer, and a secondary winding of the voltage regulating transformer is connected between the low-voltage side of the primary winding of the direct-current power transmission system transformer and the ground in series. Two midpoints of an H bridge of the connecting valve are connected to a primary winding of the connec