Dynamic-characteristics analysis of an independent microgrid consisting of a SOFC triple combined cycle power generation system and large-scale photovoltaics

•An independent microgrid incorporating large-scale photovoltaics was proposed.•The dynamic characteristics of an SOFC triple combined cycle system were analyzed.•The relation between the inertial of turbines and power fluctuation was clarified.

The solid oxide fuel cell triple combined cycle (SOFC-TCC) power generation system considered in this study is of rated power 1.4 MW, and it consists of SOFC (542 kW), a gas turbine (G/T, 550 kW), and a steam turbine (S/T, 308 kW). The relation of the frequency deviation based on the difference between supply and demand of an independent microgrid that interconnects the SOFC-TCC system and large-scale photovoltaics was investigated through numerical analysis. Because the stabilization of the load fluctuations of the SOFC and S/T required 1.8 to 2 h, control of load fluctuations occurring over periods of 2 h or less was mainly determined by the governor-free control of the G/T. Furthermore, the power characteristics (frequency) owing to cyclic fluctuations (changes occurring over a period of several minutes or less) to sustained fluctuations (changes occurring over a period exceeding 20 min) of a microgrid with large-scale photovoltaics was found to be strongly influenced by the magnitude of the inertial force of the G/T and S/T. From the analysis results, long-term supply-and-demand fluctuations, such as changes occurring seasonally and down to a period of 1 day, are mainly controlled by output adjustment of the SOFC and S/T, whereas the operation controlled by the setting of the governor-free control of the G/T and the inertial system of rotary machines is appropriate for short-term power fluctuations.