A power supply system offshore plant comprises: a generator; an AC/DC converter converting, AC generated from the generator into DC to supply the direct current to a DC bus; a power load connected to the DC bus to generate regenerative power; a first power storage unit for storing power when a voltage of the DC bus is maintained at a level of a first threshold value or higher for a first time period, and supplying the stored power to the DC bus when the voltage of the DC bus is maintained at a level of a second threshold value or lower for a second time period; and a first resistance unit for consuming power when the voltage of the DC bus is maintained at the level of the first threshold value or higher for a third time period. The third time period is longer than the first time period.