Combustion stability logic during off-load transients

1. A method for controlling a position of an inlet guide vane of a gas turbine engine, comprising:

a) determining an onset of an off-load transient;

b) determining a value representative of an amount of time elapsed since the onset of the off-load transient, and a value representative of a turbine temperature;

c) determining a time based provisional inlet guide vane command position based on the time elapsed since the onset of the off-load transient, and a temperature based provisional inlet guide vane command position based on the value representative of the turbine temperature;

d) determining which of the time based and temperature based provisional inlet guide vane command positions is associated with a highest relative value of an inlet guide vane angle and selecting the provisional inlet guide vane command position associated with the highest relative value of the inlet guide vane angle; and

e) adjusting the position of the inlet guide vane to match the selected inlet guide vane command position.

2. The method of claim 1 , further comprising

determining a value representative of a shaft speed;

determining a speed based provisional inlet guide vane command position based on the value representative of the shaft speed;

determining which of the time based, temperature based, and speed based provisional inlet guide vane command positions is associated with a highest relative value of the inlet guide vane angle and selecting the provisional inlet guide vane command position associated with the highest relative value of the inlet guide vane angle; and

adjusting the position of the inlet guide vane to match the selected inlet guide vane command position.

3. The method of claim 2 , wherein during an initial time period, the selected provisional inlet guide vane command position associated with the highest relative value is selected from the time based provisional inlet guide vane command position and the speed based provisional inlet guide vane command position.

4. The method of claim 1 , wherein the steps b) through e) are performed repeatedly until either an on-load transient is detected or the time elapsed since the onset of the off-load transient exceeds a maximum override time.

5. The method of claim 1 , wherein determining an onset of the off-load transient includes monitoring a load driven by the gas turbine engine, and determining that a power drain of the load has dropped more than a threshold amount within a specific time frame.

6. The method of claim 1 , wherein determining the time based provisional inlet guide vane command position based on the time elapsed since the onset of the off-load transient further includes

correlating the time elapsed with stored data that indicates provisional command positions as a function of time elapsed since onset of the off-load transient.

7. The method of claim 6 , wherein

when no time has elapsed since onset of the off-load transient, the time based provisional inlet guide vane command position is an angle limiting the air flow into a compressor section of the gas turbine engine;

when the time elapsed is between 0 seconds and a first time value, the time based provisional inlet guide vane command position remains constant from 0 seconds to the first time value; and

when the time elapsed is between the first time value and a maximum override time, the time based provisional inlet guide vane command position varies linearly from the first time value to the maximum override time value, in which at the maximum override time value, the time based provisional inlet guide vane command position is an angle allowing maximum possible air flow into the compressor section.

8. The method of claim 1 , wherein determining the temperature based provisional inlet guide vane command position based on the value representative of the turbine temperature further includes

correlating the value representative of the turbine temperature with stored data that indicates provisional command positions as a function of respective turbine temperatures.

9. The method of claim 8 , wherein

when the value representative of a turbine temperature is less than a first temperature value, the temperature based provisional inlet guide vane command position is an angle limiting the air flow into a compressor section of the gas turbine engine;

when the value representative of a turbine temperature is greater than a second temperature value, the temperature based provisional inlet guide vane command position is an angle allowing maximum possible air flow into the compressor section; and

when the value representative of a turbine temperature is between the first and second temperature values, the temperature based provisional inlet guide vane command position varies linearly from the first temperature value to the second temperature value.

10. The method of claim 2 , wherein determining the speed based provisional inlet guide vane command position based on the value representative of the shaft speed further includes

correlating the value representative of the shaft speed with stored data that indicates provisional command positions as a function of respective shaft speeds.

11. The method of claim 10 , wherein

when the value representative of a shaft speed is less than a first shaft speed, the speed based provisional inlet guide vane command position is an angle limiting the air flow into a compressor section of the gas turbine engine;

when the value representative of a shaft speed is greater than a second shaft speed, the speed based provisional inlet guide vane command position is an angle allowing maximum possible air flow into the compressor section; and

when the value representative of a shaft speed is between the first and second shaft speeds, the speed based provisional inlet guide vane command position varies linearly from the first shaft speed to the second shaft speed.

12. The method of claim 1 , wherein the step of d) further includes modifying the value of the selected inlet guide vane command position with a rate limiter.

13. A method for operating a gas turbine engine, comprising:

detecting an off-load transient of the gas turbine engine;

reducing a total flow of fuel to a combustor section of the gas turbine engine to a first reduced flow in response to detecting the off-load transient;

increasing a ratio of pilot fuel to total fuel in the first reduced flow of fuel in response to detecting the off-load transient;

adjusting a position of an inlet guide vane in response to detecting the off-load transient until either an on-load transient is detected or until an amount of time elapsed since detection of the off-load transient exceeds a maximum override time;

determining a value representative of at least two of the following: an amount of time elapsed since the onset of the off-load transient, a value representative of a turbine temperature, and a value representative of a shaft speed;

determining at least two of a time based provisional inlet guide vane command position based on the time elapsed since the onset of the off-load transient, a temperature based provisional inlet guide vane command position value based on the value representative of the turbine temperature, and a speed based provisional inlet guide vane command position based on the value representative of the shaft speed;

determining which of the time based, temperature based, or speed based provisional inlet guide vane command positions is associated with a highest relative value of an inlet guide vane angle and selecting the provisional inlet guide vane command position associated with the highest relative value of the inlet guide vane angle; and

adjusting the position of the inlet guide vane to match the selected inlet guide vane command position.

14. The method of claim 13 , wherein

determining the time based provisional inlet guide vane command position further includes correlating the time elapsed with stored data that indicates provisional command positions as a function of time elapsed since onset of the off-load transient; and

determining the temperature based provisional inlet guide vane command further includes correlating the value representative of the turbine temperature with stored data indicative of provisional command positions as a function of respective turbine temperatures; and

determining the speed based provisional inlet guide vane command further includes correlating the value representative of the shaft speed with stored data that indicates provisional command positions as a function of respective shaft speeds.

15. The method of claim 14 , wherein

when no time has elapsed since onset of the off-load transient, the time based provisional inlet guide vane command position is an angle limiting the air flow into a compressor section of the gas turbine engine;

when the time elapsed is between 0 seconds and a first time value, the time based provisional inlet guide vane command position remains constant from 0 seconds to the first time value; and

when the time elapsed is between the first time value and a maximum override time value, the time based provisional inlet guide vane command position varies linearly from the first time value to the maximum override time value, in which at the maximum override time value, the time based provisional inlet guide vane command position is an angle allowing maximum possible air flow into the compressor section.

16. The method of claim 14 , wherein

when the value representative of a turbine temperature is less than a first temperature value, the temperature based provisional inlet guide vane command position is an angle limiting the air flow into a compressor section of the gas turbine engine;

when the value representative of a turbine temperature is greater than a second temperature value, the temperature based provisional inlet guide vane command position is an angle allowing maximum possible air flow into the compressor section; and

when the value representative of a turbine temperature is between the first and second temperature values, the temperature based provisional inlet guide vane command position varies linearly from the first temperature value to the second temperature value.

17. The method of claim 14 , wherein

when the value representative of a shaft speed is less than a first shaft speed, the speed based provisional inlet guide vane command position is an angle limiting the air flow into a compressor section of the gas turbine engine;

when the value representative of a shaft speed is greater than a second shaft speed, the speed based provisional inlet guide vane command position is an angle allowing maximum possible air flow into the compressor section; and

when the value representative of a shaft speed is between the first and second shaft speeds, the speed based provisional inlet guide vane command position varies linearly from the first shaft speed to the second shaft speed.

18. A method for operating a gas turbine engine, comprising:

detecting an off-load transient of the gas turbine engine;

reducing a total flow of fuel to a combustor section of the gas turbine engine to a first reduced flow in response to detecting the off-load transient;

increasing a ratio of pilot fuel to total fuel in the first reduced flow of fuel in response to detecting the off-load transient;

adjusting a position of an inlet guide vane in response to detecting the off-load transient until either an on-load transient is detected or until an amount of time elapsed since detection of the off-load transient exceeds a maximum override time;

determining a value representative of an amount of time elapsed since the onset of the off-load transient, and a value representative of a turbine temperature;

determining a time based provisional inlet guide vane command position based on the time elapsed since the onset of the off-load transient, and a temperature based provisional inlet guide vane command position value based on the value representative of the turbine temperature;

determining which of the time based and temperature based provisional inlet guide vane command positions is associated with a highest relative value of an inlet guide vane angle and selecting the provisional inlet guide vane command position associated with the highest relative value of the inlet guide vane angle; and

adjusting the position of the inlet guide vane to match the selected inlet guide vane command position.