A power conversion system (1, 1A to 1T) for transferring power between a first connection target (17, 28) and a second connection target (17, 27, 28, 29) in at least one direction, the system comprising: a first external connector connectable to the first connection target (17, 28); a second external connector connectable to the second connection target (17, 27, 28, 29); a transformer circuit unit including a primary winding connected to the first external connector, a secondary winding magnetically coupled with the primary winding, a first converter unit connected between the first external connector and the primary winding, and a second converter unit connected to the secondary winding; a third converter unit connected to the second external connector; a connector including a first connection terminal and a second connection terminal interconnecting the second converter unit and the third converter unit; and a control unit configured to control at least one of the first converter unit and the second converter unit so that positive and negative voltages are applied across the primary winding alternately and a voltage of the first connection terminal with respect to the second connection terminal is positive, characterized in that the control unit is further configured to control the third converter unit not to cause transfer of power between the transformer circuit unit and the third converter unit in a first time period including a reversal time period in which a reversal of polarity of the voltage across the primary winding occurs, by forming a circulation path (71, 73, 83, 89) in the third converter unit, and control the third converter unit to cause transfer of power in a first direction from the transformer circuit unit to the third converter unit or a second direction opposite to the first direction in a second time period different from the first time period. a first external connector connectable to the first connection target (17, 28); a second external connector connectable to the second connection target (17, 27, 28, 29); a transformer circuit unit including a primary winding connected to the first external connector, a secondary winding magnetically coupled with the primary winding, a first converter unit connected between the first external connector and the primary winding, and a second converter unit connected to the secondary winding; a third converter unit connected to the second external connector; a connector including a first connection terminal and a second connection terminal interconnecting the second converter unit and the third converter unit; and a control unit configured to control at least one of the first converter unit and the second converter unit so that positive and negative voltages are applied across the primary winding alternately and a voltage of the first connection terminal with respect to the second connection terminal is positive, characterized in that the control unit is further configured to control the third converter unit not to cause transfer of power between the transformer circuit unit and the third converter unit in a first time period including a reversal time period in which a reversal of polarity of the voltage across the primary winding occurs, by forming a circulation path (71, 73, 83, 89) in the third converter unit, and control the third converter unit to cause transfer of power in a first direction from the transformer circuit unit to the third converter unit or a second direction opposite to the first direction in a second time period different from the first time period. control the third converter unit not to cause transfer of power between the transformer circuit unit and the third converter unit in a first time period including a reversal time period in which a reversal of polarity of the voltage across the primary winding occurs, by forming a circulation path (71, 73, 83, 89) in the third converter unit, and control the third converter unit to cause transfer of power in a first direction from the transformer circuit unit to the third converter unit or a second direction opposite to the first direction in a second time period different from the first time period.