Multi-shaft combined cycle plant, and control device and operation method thereof

1. An operation method of a multi-shaft combined cycle plant including a gas turbine, an exhaust heat recovery boiler that generates steam using an exhaust gas from the gas turbine, and a steam turbine that is driven by the steam generated by the exhaust heat recovery boiler, in which a gas turbine rotor of the gas turbine and a steam turbine rotor of the steam turbine are not mechanically connected to each other, the method comprising:

switching, according to a demanded load, between a low-load mode in which an output of the multi-shaft combined cycle plant is adjusted by adjustment of only an output of the gas turbine and a high-load mode in which the output of the multi-shaft combined cycle plant is adjustable by adjustment of the output of the gas turbine and adjustment of an output of the steam turbine; and

supplying steam at a standby flow rate at which the steam turbine is capable of maintaining a predetermined initial load to the steam turbine, and applying the initial load to the steam turbine even in the low-load mode,

wherein, when the demanded load becomes low in the high-load mode and the high-load mode is switched to the low-load mode, the steam at the standby flow rate is supplied to the steam turbine,

wherein a load change time point which is a time point at which the demanded load after the switching to the low-load mode becomes a demanded load which is a high load that is not to be handled by adjustment of only the output of the gas turbine is recognized in advance, the steam at the standby flow rate is supplied to the steam turbine from the switching to the low-load mode to a steam supply stop time point which is a time point prior to the load change time point, the supply of the steam to the steam turbine is stopped from the steam supply stop time point to the load change time point, and the multi-shaft combined cycle plant transitions to the high-load mode from the load change time point, and

wherein the steam supply stop time point is a time point at which an internal temperature of the steam turbine is equal to or higher than a predetermined temperature when the load change time point is reached.

2. The operation method of a multi-shaft combined cycle plant according to claim 1 , wherein if a condition for supplying the steam from the exhaust heat recovery boiler to the steam turbine is satisfied after the gas turbine operation starts, the steam at the standby flow rate is supplied to the steam turbine regardless of the level of the demanded load.

3. The operation method of a multi-shaft combined cycle plant according to claim 1 ,

wherein the multi-shaft combined cycle plant comprises a steam condenser that returns steam from the steam turbine to water, a bypass steam valve that adjusts a flow rate of steam flowing to the steam condenser from the exhaust heat recovery boiler without passing through the steam turbine, and a main steam valve that adjusts a flow rate of steam supplied to the steam turbine from the exhaust heat recovery boiler, and

wherein when supplying the steam at the standby flow rate to the steam turbine, in a state where the bypass steam valve and the main steam valve are opened, an opening degree of at least one of the bypass steam valve and the main steam valve is controlled so that the steam at the standby flow rate is supplied to the steam turbine.

4. The operation method of a multi-shaft combined cycle plant according to claim 1 ,

wherein the multi-shaft combined cycle plant comprises an exhaust gas damper that adjusts a flow rate of the exhaust gas sent from the gas turbine to the exhaust heat recovery boiler, and

wherein when supplying the steam at the standby flow rate to the steam turbine, an opening degree of the exhaust gas damper is controlled so that the steam at the standby flow rate is supplied to the steam turbine.

5. A multi-shaft combined cycle plant comprising:

a gas turbine;

an exhaust heat recovery boiler that generates steam using an exhaust gas from the gas turbine;

a steam turbine that is driven by the steam generated by the exhaust heat recovery boiler, and includes a steam turbine rotor that is not mechanically connected to a gas turbine rotor of the gas turbine;

a control device that controls adjustment of an output of the gas turbine and an output of the steam turbine; and

a steam supply adjuster that adjusts a flow rate of steam supplied to the steam turbine from the exhaust heat recovery boiler,

wherein the control device is configured to switch, according to a demanded load, between a low-load mode in which an output of the multi-shaft combined cycle plant is adjusted by adjustment of only the output of the gas turbine and a high-load mode in which the output of the multi-shaft combined cycle plant is adjustable by adjustment of the output of the gas turbine and the output of the steam turbine,

wherein the control device is configured to control the steam supply adjuster so that, even in the low-load mode, steam at a standby flow rate at which the steam turbine is capable of maintaining a predetermined initial load is supplied to the steam turbine, and applies the initial load to the steam turbine,

wherein when the demanded load becomes low in the high-load mode and the high-load mode is switched to the low-load mode, the control device is configured to control the steam supply adjuster so that the steam at the standby flow rate is supplied to the steam turbine,

wherein the control device is configured to recognize in advance a load change time point which is a time point at which the demanded load after the switching to the low-load mode becomes a high-load demanded load that is not to be handled by adjustment of only the output of the gas turbine,

wherein the control device is configured to control the steam supply adjuster so that the steam at the standby flow rate is supplied to the steam turbine from the switching to the low-load mode to a steam supply stop time point which is a time point prior to the load change time point, and to stop the supply of the steam to the steam turbine from the steam supply stop time point to the load change time point, and to control the multi-shaft combined cycle plant to transition to the high-load mode from the load change time point, and

wherein the steam supply stop time point is a time point at which an internal temperature of the steam turbine is equal to or higher than a predetermined temperature when the load change time point is reached.

6. The multi-shaft combined cycle plant according to claim 5 , further comprising:

a steam condenser that returns steam from the steam turbine to water;

a bypass steam valve that adjusts a flow rate of steam supplied to the steam condenser from the exhaust heat recovery boiler without passing through the steam turbine; and

a main steam valve that adjusts a flow rate of steam supplied to the steam turbine from the exhaust heat recovery boiler,

wherein the steam supply adjuster includes the bypass steam valve and the main steam valve, and

wherein when supplying the steam at the standby flow rate to the steam turbine, in a state where the bypass steam valve and the main steam valve are opened, the control device is configured to control an opening degree of at least one of the bypass steam valve and the main steam valve so that the steam at the standby flow rate is supplied to the steam turbine.

7. The multi-shaft combined cycle plant according to claim 5 , further comprising:

an exhaust gas damper that adjusts a flow rate of the exhaust gas supplied from the gas turbine to the exhaust heat recovery boiler,

wherein the steam supply adjuster includes the exhaust gas damper, and

wherein, when supplying the steam at the standby flow rate to the steam turbine, the control device is configured to control an opening degree of the exhaust gas damper so that the steam at the standby flow rate is supplied to the steam turbine.

8. A control device of a multi-shaft combined cycle plant comprising a gas turbine, an exhaust heat recovery boiler that generates steam using an exhaust gas from the gas turbine, a steam turbine that is driven by the steam generated by the exhaust heat recovery boiler, and a steam supply adjuster that adjusts a flow rate of steam supplied to the steam turbine from the exhaust heat recovery boiler, in which a gas turbine rotor of the gas turbine and a steam turbine rotor of the steam turbine are not mechanically connected to each other, wherein the control device is configured to:

switch, according to a demanded load, between a low-load mode in which an output of the multi-shaft combined cycle plant is adjusted by adjustment of only an output of the gas turbine and a high-load mode in which the output of the multi-shaft combined cycle plant is adjustable by adjustment of the output of the gas turbine and an output of the steam turbine, and

control the steam supply adjuster so that, even in the low-load mode, steam at a standby flow rate at which the steam turbine is capable of maintaining a predetermined initial load is supplied to the steam turbine, and applies the initial load to the steam turbine,

wherein when the demanded load becomes low in the high-load mode and the high-load mode is switched to the low-load mode, the control device is configured to control the steam supply adjuster so that the steam at the standby flow rate is supplied to the steam turbine,

wherein the control device is configured to recognize in advance a load change time point which is a time point at which the demanded load after the switching to the low-load mode becomes a demanded load which is a high load that is not to be handled by adjustment of only the output of the gas turbine,

wherein the control device is configured to control the steam supply adjuster so that the steam at the standby flow rate is supplied to the steam turbine from the switching to the low-load mode to a steam supply stop time point which is a time point prior to the load change time point, and to stop the supply of the steam to the steam turbine from the steam supply stop time point to the load change time point, and to control the multi-shaft combined cycle plant to transition to the high-load mode from the load change time point, and

wherein the steam supply stop time point is a time point at which an internal temperature of the steam turbine is equal to or higher than a predetermined temperature when the load change time point is reached.