Methodology to evaluate contribution of thermal power plant flexibility to power system stability when increasing share of renewable energies: Classification and additional fuel cost of flexible operation

A methodology to evaluate the contribution of thermal power plant operational flexibility to the stable supply of electricity when increasing the share of intermittent renewable energy sources is developed. The operational flexibility required for load frequency control (LFC) unit of a secondary control reserve is classified with respect to the available output range (AOR), the minimum load (Pmin) versus AOR ratio (Lmin, =Pmin/AOR), and the maximum full activation time (Max. FAT). By applying the classification of the operational flexibility to a frequency analysis using the Automatic Generation Control 30 (AGC30) model, defined by the Institute of Electrical Engineering of Japan (IEEJ), we validated the effect of the classification on the stability of electricity supply. To estimate the contribution of the operational flexibility of coal-fired power plants to the demand–supply balance of a power system, the base and advanced operational flexibility performances of coal-fired power plants are assumed to operate as an LFC unit of the secondary control reserve. Since a coal mill needs a long time to change the flow rate of pulverized coal, the throttling valve control of the high-pressure steam turbine during partial load operation is needed to ensure a rapid response of load change of coal-fired power plants. The energy loss of coal-fired power plants during the high-pressure steam turbine throttling valve control is evaluated and the contribution of the LFC operation of coal-fired power plants to the demand–supply balance is estimated by unit commitment analysis. Throttling valve control at a low load is responsible for the increase in the equivalent forced outage factor (EFOF) of the coal-fired power plants; thus, the additional fuel cost of replacing the LFC unit with liquefied natural gas-fired steam cycle and gas turbine combined cycle power plants is estimated, which is required for the secure supply of electricity.