Method for operating a power plant

1. A method for operating a power plant having a gas turbine, a heat recovery steam generator connected downstream of the gas turbine, a steam turbine connected to the heat recovery steam generator, an auxiliary heat source and a control system, the method comprising:

controlling, via the control system, the power plant such that the heat recovery steam generator receives an input of heat from the gas turbine;

determining, by the control system, the gas turbine is operating at its maximum capacity or at an upper end of its control range and the power plant is operating at less than a target value for a power plant capacity;

in response to determining the gas turbine is operating at its maximum capacity or at the upper end of its control range and the power plant is operating at less than the target value for the power plant capacity, determining, by the control system, a target pressure value immediately upstream of the steam turbine, in order to achieve a predefined power plant capacity,

wherein the target pressure value is derived from a primary pressure for the steam turbine and a steam turbine capacity for the steam turbine;

wherein the target pressure value is a target value for the auxiliary heat source;

wherein the auxiliary heat source is a heat store arranged outside the heat recovery steam generator; and

based upon the target pressure value, controlling the heat store arranged outside the heat recovery steam generator to release heat into the heat recovery steam generator to achieve the predefined power plant capacity.

2. The method as claimed in claim 1 ,

wherein the target pressure value is deduced from a status of a primary control element.

3. The method as claimed in claim 2 ,

wherein the status of the primary control element is a capacity of the gas turbine.

4. The method as claimed in claim 2 , wherein, based on the status of the primary control element the control system determines a target pressure value variation, which is based on a linear relationship between the primary pressure and the steam turbine capacity for the steam turbine, wherein the variation is added to a current actual pressure upstream of the steam turbine; and wherein the target pressure value is set based on the target pressure value variation and the current actual pressure.

5. The method as claimed in claim 1 , wherein the auxiliary heat source is only operated in the event that the target value for the power plant capacity cannot be achieved or maintained by means of the gas turbine, within its control range, and the steam turbine alone.

6. The method as claimed in claim 1 , wherein the auxiliary heat source releases heat such that an actual pressure value upstream of the steam turbine is increased and the power plant achieves the predefined power plant capacity.

7. The method as claimed in claim 4 , wherein the status of the primary control element is based on a difference between an actual capacity of the gas turbine and one of a maximum capacity or a control range limit of the gas turbine.

8. The method as claimed in claim 7 , wherein the target pressure value for the auxiliary heat source is increased by a value in excess of the current actual pressure value upstream of the steam turbine when the actual capacity of the gas turbine is operating at the one of the maximum capacity or the control range limit and the power plant operating at less than the predefined power plant capacity.

9. The method as claimed in claim 4 , wherein the heat recovery steam generator receives a first heat input from the gas turbine and a second heat input from the auxiliary heat source and wherein the current actual pressure is based on the first heat input and the second heat input.

10. The method as claimed in claim 1 ,

wherein the target pressure value is derived from a linear relationship between a primary pressure for the steam turbine and steam turbine capacity for the steam turbine.

11. The method as claimed in claim 10 ,

wherein, based on the linear relationship, the actual pressure value from the heat recovery steam generator is increased to generate the target pressure value for the steam pressure upstream of the steam turbine, thereby increasing the steam turbine capacity to achieve a target value for power plant capacity.

12. A method for operating a power plant having a gas turbine, a heat recovery steam generator connected downstream of the gas turbine, a steam turbine connected to the heat recovery steam generator, and an auxiliary heat source, the method comprising:

operating the power plant such that the heat recovery steam generator receives an input of heat from the gas turbine;

generating an actual pressure value from the heat recovery steam generator;

determining the power plant is operating at less than a target value for a power plant capacity, and the actual capacity of the gas turbine is operating at a maximum capacity or a control range limit,

determining a target pressure value immediately upstream of the steam turbine for the auxiliary heat source;

wherein the target pressure value is derived from a linear relationship between a primary pressure for the steam turbine and a steam turbine capacity for the steam,

wherein the auxiliary heat source is a heat store arranged outside the heat recovery steam generator; and

based on the linear relationship, controlling the gas turbine and the auxiliary heat source such that the actual pressure value from the heat recovery steam generator is increased to generate the target pressure value for the steam pressure upstream of the steam turbine, thereby increasing the steam turbine capacity to achieve the target value for the power plant capacity.