A system includes the HRSG having an economizer disposed along a fluid flow path, and a drum disposed along the fluid flow path downstream of the economizer. The HRSG also includes a drum level control module configured to modulate an amount of the fluid provided to the drum along the fluid flow path and a supplemental control module configured to control an amount of the fluid in a different manner than the drum level control module. The heat recovery steam generator also includes a drum level event controller configured to monitor a rate of change of a level of the fluid in the drum. If the rate of change is over a threshold value, a signal goes to the supplemental control. If the rate of change is less than or equal to the threshold value, the signal goes to the drum level control module.

To provide a condensate flow rate control device and a control method for a power plant which improve responsiveness to frequency fluctuations or requested load changes and can suppress frequency fluctuations with precision or improve precision with which output power conforms to requested load instructions. A power plant to which a condensate flow rate control device is adapted includes a deaerator to which condensate generated by a condenser is supplied via a deaerator water level adjustment valve and into which bleed steam from a steam turbine is introduced. The condensate flow rate control device has a water level adjustment unit for executing condensate flow rate control, wherein the water level adjustment unit adjusts pressure in a condensate flow path extending from the deaerator water level adjustment valve to the deaerator so that inputted frequency fluctuations are suppressed or an output value of a generator conforms to inputted requested load changes.