The purpose of the invention is to provide a guideline for, when adding a new power cell to a power network, solving the problem of how to set an operation mode of a power conversion leg. Assuming that a power conversion leg (JRL) of the power router (JR) of a power cell (J) and a power conversion leg (KRL) of the power router (KR) of a power cell (K) are connected to each other and power is transferred from the power cell (J) to the power cell (K) when the utility grid breaks down, the leg operation modes of the leg (JRL) of the power router (JR) and leg (KRL) of the power router (KR) are set as follows:the power conversion leg (JRL) of the power router (JR) is set to a stand-alone mode; and the power conversion leg (KRL) of the power router (KR) is set to a designate power transmission/reception mode when the utility grid does not break down and to a master mode from the designate power transmission/reception mode when the utility grid breaks down.