An interconnected system that supplies power to a power system, and includes a power generator that changes in output, and an electric power storage-compensation device. The interconnected system is controlled so that operation of a sodium-sulfur battery that is included in the electric power storage-compensation device and compensates for a change in output of the power generator is stopped, or the interconnected system is set in a non-power transmission mode in which a planned power generation value of a power generation schedule is set to the sum of power generated by the power generator and local load power of the system, when the planned power generation value is 0 kW or less, and the sum of the power generated by the power generator and the local load power of the system has reached a value at which power is supplied to the interconnected system from the power system.