Direct current systems that lack an inertial force, such as fuel cells or photovoltaics, are expected to be used for electrical power in advanced sustainable cities. A small-capacity transmission network (microgrid) without inertial force has difficulties for converging short-term electric power fluctuations. Consequently, the frequency of transmission on such networks frequently diverges. Therefore, the electric power fluctuation due to cyclic fluctuation of an interconnected system of clean and renewable power sources (namely a solid oxide fuel cell (SOFC)-hydraulic generator, photovoltaics, and a wind-power generator) is investigated by numerical analysis in this study, together with means of controlling it. The characteristics of this fluctuation are clarified, and the setting range required to control the cyclic fluctuation of the transmission network is illustrated based on the relation between the inertial force of the hydraulic generator and battery (lithium ion) capacity.