A gas turbine plant including a gas turbine and a compressor is provided with a steam turbine plant including a steam turbine and a condenser, and, an exhaust heat recovery boiler. Steam from the exhaust heat recovery boiler is directly flown to the condenser of the steam turbine plant through a bypass control valve. A pressure sensor detects pressure in a turbine bypass system. A controller outputs, based on a set value from an input device and a process value from the pressure sensor, an open level instruction value to the control value so as to make the process value consistent with the set value in a predetermined sampling cycle. A corrector corrects the set value from the input device in a direction in which the open level instruction value decreases when the open level instruction value from the controller becomes a value that substantially fully opens the control valve.

The purpose of the present invention is to decrease the loss of energy of flue gas when superheated steam is cooled and prevent heat efficiency from being decreased. A boiler is provided with: economizers (a medium-pressure economizer (13) and a high-pressure secondary economizer (18)) which heat water supplied by water supply pumps (a medium-pressure water supply pump (27) and a high-pressure water supply pump (28)); evaporators (a medium-pressure evaporator (16) and a high-pressure evaporator (21)) which evaporate the water that is heated by the economizers; and cooling devices (a medium-pressure system and a high-pressure system) which mix water, as a coolant, which has passed through the economizers via the water supply pumps with steam.

A gas turbine plant including a gas turbine and a compressor is provided with a steam turbine plant including a steam turbine and a condenser, and, an exhaust heat recovery boiler. Steam from the exhaust heat recovery boiler is directly flown to the condenser of the steam turbine plant through a bypass control valve. A pressure sensor detects pressure in a turbine bypass system. A controller outputs, based on a set value from an input device and a process value from the pressure sensor, an open level instruction value to the control value so as to make the process value consistent with the set value in a predetermined sampling cycle. A corrector corrects the set value from the input device in a direction in which the open level instruction value decreases when the open level instruction value from the controller becomes a value that substantially fully opens the control valve.

There is provided a combined cycle power plant in which a high-pressure steam turbine and an intermediate-pressure steam turbine can operate in a state where amounts of thermal effect thereof are close to a limit value, and capable of reducing start-up time. A combined cycle power plant includes: an exhaust heat recovery boiler that includes a high-pressure superheater which superheats steam for a high-pressure steam turbine, and a reheater which reheats steam for an intermediate-pressure steam turbine; bypass pipes through which steam bypasses the high-pressure superheater and the reheater; bypass valves that regulate flow rates of steam which flows through the bypass pipes; and a bypass controller that controls the bypass valves such that a difference between thermal effect-amount margins of the turbines is decreased.