Combined cycle plant, method for reducing minimum output thereof, and control device therefor

1. A method for operating a combined cycle plant including a gas turbine that is driven using combustion gas, an exhaust heat recovery boiler that generates steam using heat of exhaust gas discharged from the gas turbine, and a steam turbine that is driven using the steam, the method comprising:

a reception step of a control device receiving a load schedule indicating a load of the combined cycle plant;

a decrease time-setting step of the control device recognizing a load decrease time at which the load is to be decreased with reference to the load schedule received in the reception step and setting a time prior to the load decrease time as a temperature decrease time at which a temperature of the steam supplied to the steam turbine is to be decreased;

a temperature-decreasing step of decreasing the temperature of the steam supplied to the steam turbine at the temperature decrease time by spraying water into the steam; and

a gas turbine output-decreasing step of decreasing an amount of fuel supplied to the gas turbine at the load decrease time.

2. The method according to claim 1 , wherein the decrease time-setting step includes setting a time prior to the load decrease time by a predetermined fixed time as the temperature decrease time.

3. The method according to claim 1 , wherein the decrease time-setting step includes setting a time prior to the load decrease time by a time corresponding to a decrease in load indicated by the load schedule as the temperature decrease time.

4. The method according to claim 1 , wherein the temperature-decreasing step includes decreasing the temperature of the steam such that a rate of change which is a change per unit time of the temperature of the steam is within a predetermined range.

5. The method according to claim 1 , wherein the temperature-decreasing step includes decreasing an amount of the water sprayed.

6. The method according to claim 5 , wherein the temperature-decreasing step includes decreasing the amount of the water sprayed at a constant rate.

7. A control device for a combined cycle plant including a gas turbine configured to be driven using combustion gas, a fuel regulator configured to regulate a flow rate of fuel supplied to the gas turbine, an exhaust heat recovery boiler configured to generate steam using heat of exhaust gas discharged from the gas turbine, a steam turbine configured to be driven using the steam, and a steam temperature regulator configured to regulate a temperature of the steam, the control device comprising:

a reception unit configured to receive a load schedule indicating a load of the combined cycle plant;

a steam temperature control unit configured to output a command indicating an amount of water sprayed into the steam for decreasing the temperature of the steam to the steam temperature regulator prior to a load decrease time at which the load is to be decreased in the load schedule; and

a fuel control unit configured to output a command indicating an amount of operation for reducing an amount of supplied fuel at the load decrease time to the fuel regulator.

8. The control device according to claim 7 , further comprising a temperature decrease time-instructing unit configured to recognize the load decrease time received by the reception unit, set a time prior to the load decrease time as a temperature decrease time at which the temperature of the steam is to be decreased, and cause the steam temperature control unit to output the command to the steam temperature regulator at the temperature decrease time.

9. The control device according to claim 8 , wherein the temperature decrease time-instructing unit is configured to store a predetermined time and set a time prior to the load decrease time by the predetermined time as the temperature decrease time.

10. The control device according to claim 8 , wherein the temperature decrease time-instructing unit is configured to store a relationship between a decrease in load and a lead time, set the lead time corresponding to the decrease in load indicated by the load schedule using the relationship, and set a time prior to the load decrease time by the lead time as the temperature decrease time.

11. The control device according to claim 8 , wherein the steam temperature control unit is configured to set the amount of water sprayed by the steam temperature regulator such that a rate of change which is a change per unit time of the temperature of the steam is within a predetermined range.

12. The control device according to claim 8 , wherein the steam temperature control unit is configured to decrease the amount of water sprayed by the steam temperature regulator.

13. The control device according to claim 12 , wherein the steam temperature control unit is configured to decrease the amount of water sprayed by the steam temperature regulator at a constant rate.

14. The control device according to claim 7 , further comprising:

an output unit configured to output the load schedule received by the reception unit; and

an operation terminal configured to issue an instruction for the amount of water sprayed by the steam temperature regulator to the steam temperature control unit.

15. A combined cycle plant comprising:

the control device according to claim 7 ;

the gas turbine;

the fuel regulator;

the exhaust heat recovery boiler;

the steam turbine; and

the steam temperature regulator.

16. The combined cycle plant according to claim 15 , further comprising a steam line in which the steam supplied to the steam turbine is configured to flow,

wherein the steam temperature regulator includes a nozzle configured to spray the water into the steam line and a spray water regulator configured to regulate a flow rate of the water supplied to the nozzle.

17. The combined cycle plant according to claim 16 ,

wherein the exhaust heat recovery boiler includes a plurality of overheaters configured to overheat steam,

wherein the plurality of overheaters are connected to each other via a first steam line, and

wherein the nozzle is in the first steam line or a second steam line connected to one of the plurality of overheaters which is furthest downstream in a steam flow to the steam turbine.

18. The combined cycle plant according to claim 16 ,

wherein the exhaust heat recovery boiler includes a plurality of overheaters configured to overheat steam, and

wherein the nozzle is in a steam line connecting a first of the plurality of overheaters which is furthest downstream in a steam flow to the steam turbine to a second of the plurality of overheaters which is next upstream from the first of the plurality of overheaters.