A power conversion device that includes a unit between a first terminal and a second terminal, and is configured to transmit a power between the first terminal and the second terminal, the unit including a plurality of DC-to-DC converter cells and a cell count M, the first terminal being located on a first side, the second terminal being located on a second side, the first side being one of an input and an output, the second side being the other one of the input and the output, the power conversion device comprising: a short-circuit device that short-circuits a failed cell which is a failed DC-to-DC converter cell among the plurality of DC-to-DC converter cells; and a control unit being configured to sense a failed cell count m and to control the plurality of DC-to-DC converter cells based on the sensed failed cell count m, wherein the failed cell count m is a natural number less than the cell count M; wherein the unit has a first unit terminal on the first side of the unit and a second unit terminal on the second side of the unit, each DC-to-DC converter cell has first cell terminals connected in parallel to the first unit terminal of the unit, and second cell terminals connected in series to the second unit terminal of the unit, the first cell terminals being on the first side, the second cell terminals being on the second side, wherein the M cells are connected in series before a cell failure, while M-m cells are connected in series after the cell failure, wherein in normal operation the control unit is configured to calculate a control command value, such that a voltage detection value of the second cell terminal is close to a target voltage, and in case of a cell failure the control unit is configured to calculate the control command value, based on the target voltage and the failed cell count m; characterized in that the control unit is configured to perform the control in order to protect the first cell terminal and the second cell terminals of the cells from overvoltage and over current when the failed cell count m is increased; wherein the control unit includes a lookup table storing a correspondence between the control command value and the failed cell count m, and is configured to use the lookup table to calculate the control command value corresponding to the failed cell count m, and wherein the control unit is configured to previously derive a current Root Mean Square I Lrms and a control coefficient k 1 that yields a minimum current Root Mean Square I Lrms as a function of an output voltage scale factor k 1 that yields a minimum current Root Mean Square I Lrms for each failed cell count m, to store a derivation in a storage unit in a form of the lookup table, and to use the lookup table to select the output voltage scale factor k 1 as a function of the failed cell count m. a short-circuit device that short-circuits a failed cell which is a failed DC-to-DC converter cell among the plurality of DC-to-DC converter cells; and a control unit being configured to sense a failed cell count m and to control the plurality of DC-to-DC converter cells based on the sensed failed cell count m, wherein the failed cell count m is a natural number less than the cell count M; wherein the unit has a first unit terminal on the first side of the unit and a second unit terminal on the second side of the unit, each DC-to-DC converter cell has first cell terminals connected in parallel to the first unit terminal of the unit, and second cell terminals connected in series to the second unit terminal of the unit, the first cell terminals being on the first side, the second cell terminals being on the second side, wherein the M cells are connected in series before a cell failure, while M-m cells are connected in series after the cell failure, wherein in normal operation the control unit is configured to calculate a control command value, such that a voltage detection value of the second cell terminal is close to a target voltage, and in case of a cell failure the control unit is configured to calculate the control command value, based on the target voltage and the failed cell count m; characterized in that the control unit is configured to perform the control in order to protect the first cell terminal and the second cell terminals of the cells from overvoltage and over current when the failed cell count m is increased; wherein the control unit includes a lookup table storing a correspondence between the control command value and the failed cell count m, and is configured to use the lookup table to calculate the control command value corresponding to the failed cell count m, and wherein the control unit is configured to previously derive a current Root Mean Square I Lrms and a control coefficient k 1 that yields a minimum current Root Mean Square I Lrms as a function of an output voltage scale factor k 1 that yields a minimum current Root Mean Square I Lrms for each failed cell count m, to store a derivation in a storage unit in a form of the lookup table, and to use the lookup table to select the output voltage scale factor k 1 as a function of the failed cell count m.