A power conversion device (1, 1A, 1B) comprising: a cluster (12uv, 12vw, 12wu) in which one or more unit cells, each comprising power storage means and switching means for outputting the power of the power storage means, are connected in series; and a three-phase transformer that has at least three legs (113uv, 113vw, 113wu) on each of which a primary winding (111uv, 111vw, 111wu) and a secondary winding (112uv, 112vw, and 112wu) are wound for magnetic coupling wherein both ends of the secondary winding (112uv, 112vw, 112wu) are connected respectively to both ends of the cluster (12uv, 12vw, 12wu), wherein a control unit turns on or off the switching means included in the unit cell for controlling the cluster voltages (Vuv, Vvw, Vwu), so that each cluster voltage becomes a desired voltage value on the basis of the line-to-line voltages (Vsuv, Vsvw and Vswu), characterized in that each unit cell further comprises a first voltmeter which is connected to both ends of the power storage means to measure a voltage vckj and outputs the information on the measured voltage vckj to a control unit, said control unit is further connected to second voltmeters (14uv, 14vw, 14wu) measuring each the line-to-line voltages (Vsuv, Vsvw, Vswu) between the respective phases of an electric power system, linked to the power conversion device, the control unit detects the voltage of the power storage means of each unit cell included in the clusters (12uv, 12vw, 12wu) and controls the cluster voltages (Vuv, Vvw, Vwu) by a zero-phase voltage Vo, which is the sum of the cluster voltages (Vuv, Vvw, Vwu), to limit the zero-phase current I 0, which is the current that circulates through all clusters and flows into the three-phase transformer. a cluster (12uv, 12vw, 12wu) in which one or more unit cells, each comprising power storage means and switching means for outputting the power of the power storage means, are connected in series; and a three-phase transformer that has at least three legs (113uv, 113vw, 113wu) on each of which a primary winding (111uv, 111vw, 111wu) and a secondary winding (112uv, 112vw, and 112wu) are wound for magnetic coupling wherein both ends of the secondary winding (112uv, 112vw, 112wu) are connected respectively to both ends of the cluster (12uv, 12vw, 12wu), wherein a control unit turns on or off the switching means included in the unit cell for controlling the cluster voltages (Vuv, Vvw, Vwu), so that each cluster voltage becomes a desired voltage value on the basis of the line-to-line voltages (Vsuv, Vsvw and Vswu), characterized in that each unit cell further comprises a first voltmeter which is connected to both ends of the power storage means to measure a voltage vckj and outputs the information on the measured voltage vckj to a control unit, said control unit is further connected to second voltmeters (14uv, 14vw, 14wu) measuring each the line-to-line voltages (Vsuv, Vsvw, Vswu) between the respective phases of an electric power system, linked to the power conversion device, the control unit detects the voltage of the power storage means of each unit cell included in the clusters (12uv, 12vw, 12wu) and controls the cluster voltages (Vuv, Vvw, Vwu) by a zero-phase voltage Vo, which is the sum of the cluster voltages (Vuv, Vvw, Vwu), to limit the zero-phase current I 0, which is the current that circulates through all clusters and flows into the three-phase transformer.