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.