A power conversion system includes: a self-excited converter connected between an AC power system, and a first DC main line and a DC return line; a separately-excited converter connected between the AC power system, and the DC return line and a second DC main line; and a control device. When activating the self-excited converter and the separately-excited converter, the control device activates the self-excited converter, and after completion of activation of the self-excited converter, activates the separately-excited converter. The self-excited converter can be activated first to take advantage of a function of the self-excited converter that is not included in the separately-excited converter.