A method for controlling a steam generator of a once-through steam generator type, which is integrated in a combined cycle power plant with a steam turbine and a gas turbine, the steam generator comprising a high pressure economizer, a high pressure evaporator, and at least one superheater,
- wherein opening of a valve for regulating flow volume in a feedwater supply line into the high-pressure evaporator is controlled in dependence upon a temperature set-point value for steam for the steam turbine and also upon a measured steam temperature at the outlet of the at least one superheater and by means of a plurality of control steps in series,
- wherein the measured steam temperature at the outlet of the superheater, a degree of superheating in consideration of a prevailing saturation temperature at the outlet of the high-pressure evaporator and a degree of subcooling of the feedwater in consideration of a prevailing saturation temperature at the inlet into the high-pressure evaporator are integrated in the control steps,
- wherein the method comprises the steps:
- measuring steam temperature at the outlet of the superheater,
- in a first control step, comparing the steam temperature at the outlet of the superheater with a set-point value for turbine steam temperature at the outlet of the superheater according to an operating state of the steam turbine, and generating a set-point value for the degree of superheating at the outlet of the high-pressure evaporator based on the difference between the measured value and the set-point value of the steam temperature at the outlet of the superheater,
- measuring the current degree of superheating at the outlet of the high-pressure evaporator,
- in a second control step, generating a set-point value for the degree of subcooling of the feedwater at the inlet of the high-pressure evaporator based on the difference between the set-point value and current measured value of the degree of superheating at the outlet of the high-pressure evaporator,
- measuring the current degree of subcooling of the feedwater at the inlet into the high-pressure evaporator,
- in a third control step, generating a set-point value for the flow volume for the feedwater upstream of the high-pressure evaporator based on the difference between the set-point value and the current measured value for the degree of subcooling of the feedwater at the inlet into the high-pressure evaporator,
- based on the set-point value for the flow volume of the feedwater, generating a control signal for the opening of a valve in the feedwater supply line and transmitting the control signal to the valve,
- wherein the set-point value for the degree of superheating at the outlet of the high-pressure evaporator is determined between a maximum value (Max Sh) and a minimum value (Min Sh), said maximum value (Max Sh) being a value established to prevent an exceeding of the steam temperature beyond a permissible material temperature at the outlet of the high pressure evaporator and said minimum value (Min Sh) being a value established to prevent a saturation state at the outlet of the high-pressure evaporator.