Exhaust temperature based mode control method for gas turbine and gas turbine

1. A method for controlling an operating point of a gas turbine that includes a compressor, a combustor and at least a turbine, the method comprising:

determining an exhaust pressure drop at an exhaust of the turbine;

measuring a compressor pressure discharge at the compressor;

determining a turbine pressure ratio based on the exhaust pressure drop and the compressor pressure discharge;

calculating a primary to lean-lean mode transfer threshold reference curve as a function of the turbine pressure ratio, wherein the primary to lean-lean mode transfer threshold curve includes operating points of the gas turbine at which an operation of the gas turbine is changed between a primary mode and a lean-lean mode;

determining, at a first time, when an exhaust temperature associated with at least one of the operating points is higher than an exhaust temperature associated with the primary to lean-lean mode transfer threshold reference curve for the same turbine pressure ratio; and

after a predetermined time after the first time, changing a split fuel quantity from a first value to a second value if the exhaust temperature associated with the operating point remains higher than the exhaust temperature of the primary to lean-lean mode transfer threshold reference curve;

wherein the primary mode is defined as providing more than half of the fuel to primary burners of the combustor and igniting the provided fuel in a primary region of the combustor only, the primary region being adjacent to a secondary region;

wherein the lean-lean mode is defined as providing fuel to both the primary burners and the secondary burner and burning the provided fuel both in the primary region and the secondary region; and

wherein the split fuel quantity describes in percentages a first amount of the total fuel that is received by the primary burner and a second amount of the total fuel that is received by the secondary burners.

2. The method of claim 1 , wherein the second value of the split fuel quantity is smaller than the first value, the primary mode is characterized by the first value, and a lean-lean pre-fill mode is characterized by the second value, which is part of the lean-lean mode.

3. The method of claim 2 , further comprising:

changing, at a predetermined second time, the second value of the split fuel quantity to a third value, by which a lean-lean transient mode is characterized, which is part of the lean-lean mode.

4. The method of claim 3 , further comprising:

changing, at a predetermined third time, the third value of the split fuel quantity to a fourth value, by which a lean-lean steady state mode is characterized, which is part of the lean-lean mode.

5. The method of claim 4 , wherein the third value of the split fuel quantity is lower than the second value and wherein the fourth value is between the second value and the third value.

6. The method of claim 4 , further comprising:

prior to operating the gas turbine, calculating lasting time intervals for the lean-lean pre-fill mode and the lean-lean transient mode while the lean-lean steady state mode has no precalculated lasting time interval.

7. The method of claim 4 , further comprising:

determining at a fifth time that the exhaust temperature associated with the operating point is lower than a difference between (i) the exhaust temperature of the primary to lean-lean mode transfer threshold reference curve and (ii) a predetermined exhaust temperature value, both of which are taken for the same turbine pressure ratio; and

changing the split fuel quantity from the fourth value to the first value.

8. The method of claim 4 , further comprising:

aborting the lean-lean transient mode and the lean-lean steady mode at any time at which the exhaust temperature associated with the operating point is lower than a difference between (i) the exhaust temperature of the primary to lean-lean mode transfer threshold reference curve and (ii) a predetermined exhaust temperature value, both of which are taken for the same turbine pressure ratio.

9. The method of claim 1 , wherein calculating the primary to lean-lean mode transfer threshold reference curve comprises:

using a parameter indicative of a characteristic of the fuel to determine the primary to lean-lean mode transfer threshold reference curve, wherein the parameter is one of a lower heating value of the fuel, a NOx factor of the fuel, an upper to lower flammability ratio of the fuel, or a combination thereof.

10. A controller for controlling an operating point of a gas turbine that includes a compressor, a combustor and at least a turbine, the controller comprising:

a pressure sensor configured to measure a compressor pressure discharge at the compressor; and

a processor connected to the pressure sensor, the processor programmed to:

determine an exhaust pressure drop at an exhaust of the turbine;

determine a turbine pressure ratio based on the exhaust pressure drop and the compressor pressure discharge;

calculate a primary to lean-lean mode transfer threshold reference curve as a function of the turbine pressure ratio, wherein the primary to lean-lean mode transfer threshold curve includes operating points of the gas turbine at which an operation of the gas turbine is changed between a primary mode and a lean-lean mode;

determine, at a first time, when an exhaust temperature associated with at least one of the operating points is higher than an exhaust temperature associated with the primary to lean-lean mode transfer threshold reference curve for the same turbine pressure ratio; and

after a predetermined time after the first time, change a split fuel quantity from a first value to a second value if the exhaust temperature associated with the operating point remains higher than the exhaust temperature of the primary to lean-lean mode transfer threshold reference curve;

wherein the primary mode is defined as providing more than half of the fuel to primary burners of the combustor and igniting the provided fuel in a primary region of the combustor only, the primary region being adjacent to a secondary region;

wherein the lean-lean mode is defined as providing fuel to both the primary burners and the secondary burner and burning the provided fuel both in the primary region and the secondary region; and

wherein the split fuel quantity describes in percentages a first amount of the total fuel that is received by the primary burner and a second amount of the total fuel that is received by the secondary burners.

11. The controller of claim 10 , wherein the second value of the split fuel quantity is smaller than the first value, the primary mode is characterized by the first value, and a lean-lean pre-fill mode is characterized by the second value, which is part of the lean-lean mode.

12. The controller of claim 10 , wherein the processor is further programmed to:

change, at a predetermined second time, the second value of the split fuel quantity to a third value, by which a lean-lean transient mode is characterized, which is part of the lean-lean mode.

13. The controller of claim 10 , wherein the processor is further programmed to:

change, at a predetermined third time, the third value of the split fuel quantity to a fourth value, by which a lean-lean steady state mode is characterized, which is part of the lean-lean mode.

14. The controller of claim 10 , wherein the third value of the split fuel quantity is lower than the second value and wherein the fourth value is between the second value and the third value.

15. The controller of claim 10 , wherein the processor is further programmed to:

prior to operating the gas turbine, calculate lasting time intervals for the lean-lean pre-fill mode and the lean-lean transient mode while the lean-lean steady state mode has no precalculated lasting time interval.

16. The controller of claim 14 , wherein the processor is further programmed to:

determine at a fifth time that the exhaust temperature associated with the operating point is lower than a difference between (i) the exhaust temperature of the primary to lean-lean mode transfer threshold reference curve and (ii) a predetermined exhaust temperature value, both of which are taken for the same turbine pressure ratio; and

change the split fuel quantity from the fourth value to the first value.

17. The controller of claim 14 , wherein the processor is further programmed to:

abort the lean-lean transient mode and the lean-lean steady mode at any time at which the exhaust temperature associated with the operating point is lower than a difference between (i) the exhaust temperature of the primary to lean-lean mode transfer threshold reference curve and (ii) a predetermined exhaust temperature value, both of which are taken for the same turbine pressure ratio.

18. The controller of claim 10 , wherein to calculate the primary to lean-lean mode transfer threshold reference curve, the processor is further programmed to:

use a parameter indicative of a characteristic of the fuel to determine the primary to lean-lean mode transfer threshold reference curve, wherein the parameter is one of a lower heating value of the fuel, a NOx factor of the fuel, an upper to lower flammability ratio of the fuel, or a combination thereof.