A power conversion system, comprising: a first power converter configured to convert an alternating-current power into a direct-current power or convert a DC power into an AC power; a second power converter configured to convert an AC power into a DC power or convert a DC power into an AC power; a first control device configured to control the first power converter; and a second control device configured to control the second power converter, wherein the first power converter is interconnectable to a first AC system via a first AC circuit breaker, the second power converter is interconnectable to a second AC system via a second AC circuit breaker, and a first DC terminal (2A, 3A) of the first power converter and a second DC terminal (2B, 3B) of the second power converter are connectable to each other, the first power converter and the second power converter each include a plurality of arms (5UA, 5VA, 5WA, 5UB, 5VB, 5WB) connected in parallel between a positive DC bus and a negative DC bus, the plurality of arms each include a positive arm (6UA, 6VA, 6WA, 6UB, 6VB, 6WB) and a negative arm (7UA, 7VA, 7WA, 7UB, 7VB, 7WB) which are connected in series, the positive arm and the negative arm each include one or more transducer cells (10A, 10B) which are connected in series, the transducer cells each include: a series body comprising a plurality of semiconductor switching elements (21, 22) connected in series; and a DC capacitor connected in parallel with the series body, the first power converter is configured to begin operation prior to the second power converter, characterised in that after the first control device controls the first power converter so that a voltage of the first DC terminal (2A, 3A) is at a level of DC voltage that can be output from the second power converter which is sent from the second control device, the second control device starts operation of the second power converter. a first power converter configured to convert an alternating-current power into a direct-current power or convert a DC power into an AC power; a second power converter configured to convert an AC power into a DC power or convert a DC power into an AC power; a first control device configured to control the first power converter; and a second control device configured to control the second power converter, wherein the first power converter is interconnectable to a first AC system via a first AC circuit breaker, the second power converter is interconnectable to a second AC system via a second AC circuit breaker, and a first DC terminal (2A, 3A) of the first power converter and a second DC terminal (2B, 3B) of the second power converter are connectable to each other, the first power converter and the second power converter each include a plurality of arms (5UA, 5VA, 5WA, 5UB, 5VB, 5WB) connected in parallel between a positive DC bus and a negative DC bus, the plurality of arms each include a positive arm (6UA, 6VA, 6WA, 6UB, 6VB, 6WB) and a negative arm (7UA, 7VA, 7WA, 7UB, 7VB, 7WB) which are connected in series, the positive arm and the negative arm each include one or more transducer cells (10A, 10B) which are connected in series, the transducer cells each include: a series body comprising a plurality of semiconductor switching elements (21, 22) connected in series; and a DC capacitor connected in parallel with the series body, the first power converter is configured to begin operation prior to the second power converter, characterised in that after the first control device controls the first power converter so that a voltage of the first DC terminal (2A, 3A) is at a level of DC voltage that can be output from the second power converter which is sent from the second control device, the second control device starts operation of the second power converter. a series body comprising a plurality of semiconductor switching elements (21, 22) connected in series; and a DC capacitor connected in parallel with the series body, the first power converter is configured to begin operation prior to the second power converter, characterised in that after the first control device controls the first power converter so that a voltage of the first DC terminal (2A, 3A) is at a level of DC voltage that can be output from the second power converter which is sent from the second control device, the second control device starts operation of the second power converter.