Gas turbine fuel supply method and arrangement

1. A method of controlling a supply of a fuel to a combustor of a gas turbine comprising a compressor upstream of the combustor, the method comprising:

supplying the fuel to the combustor;

obtaining an inlet air pressure (PT 7 ) at a compressor inlet;

obtaining an inlet air temperature (Tinlet) at the compressor inlet;

obtaining an outlet air pressure (PT 8 ) at a compressor outlet;

obtaining an air bleed signal (THBOV) indicative of an amount of air not supplied to the combustor;

estimating a heat input (HIengmodel, HIexpected) of the fuel supplied to the combustor based on the inlet air pressure (PT 7 ), the inlet air temperature (Tinlet), the outlet air pressure (PT 8 ) and the air bleed signal (THBOV);

comparing the estimated heat input (HIengmodel, HIexpected) with a demanded heat input (FFDEM) to derive an error signal; and

controlling a fuel valve regulating the supply of the fuel to the combustor based on the error signal.

2. The method according to claim 1 ,

wherein the air bleed signal (THBOV) comprises a bleed valve setting.

3. The method according to claim 1 ,

wherein estimating the heat input comprises:

forming a pressure ratio (PRC 87 ) between the outlet air pressure (PT 8 ) and the inlet air pressure (PT 7 ) and

estimating the heat input (HIengmodel, HIexpected) based on the pressure ratio.

4. The method according to claim 3 ,

wherein estimating the heat input (HIengmodel, HIexpected) based on the pressure ratio (PRC 87 ) comprises using at least one curve fit based on plural measurements performed on at least one gas turbines, the curve fit relating measured pressure ratio to measured heat input.

5. The method according to claim 4 ,

wherein the curved fit is in particular described by a polynomial of second order having a positive first coefficient and a positive second coefficient.

6. The method according to claim 4 ,

wherein a representation of the curve fit is stored in an electronic storage.

7. The method according to claim 4 ,

wherein the at least one curve fit comprises:

a first curve fit obtained based on measuring data, where no air was bled upstream of the combustor,

a second curve fit obtained based on measuring data, where a portion of air was bled upstream of the combustor,

wherein based on the first curve fit a first heat input (HIisonobleed) is derived and

wherein based on the second curve fit a second heat input (HIiso10% bleed) is derived,

wherein based on the first heat input and the second heat input the estimated heat input (HIengmodel, HIexpected) is derived.

8. The method according to claim 7 ,

wherein the portion is between 5% and 20% of air bled upstream of the combustor.

9. The method according to claim 7 , further comprising:

deriving a corrected heat input (HIiso) using a first function comprising the first heat input (HIisonobleed) and the second heat input (HIiso10% bleed), if the pressure ratio (PRC 87 ) exceeds a threshold;

deriving the corrected heat input (HIiso) using a second function comprising at least the first heat input, if the pressure ratio (PRC 87 ) does not exceed the threshold;

wherein estimating the heat input (HIengmodel, HIexpected) is further based on the corrected heat input (HIiso).

10. The method according to claim 9 , further comprising:

calculating the estimated heat input (HIengmodel, HIexpected) from the corrected heat input (HIiso), the inlet air pressure (PT 7 ) and the inlet air temperature (Tinlet).

11. The method according to claim 9 ,

wherein the threshold is between 4 and 8.

12. The method according to claim 9 , further comprising

estimating a portion (bleed %) of air bled upstream of the combustor based on a bleed valve setting (THBOV),

wherein the first function comprises the estimated portion of bled air.

13. The method according to claim 12 ,

wherein the bleed valve setting (THBOV) is estimated using a bleed valve characteristic.

14. The method according to claim 1 ,

wherein the fuel valve is controlled using a Pi-controller to which the error signal is supplied as an input,

the method further comprising:

deriving a volume specific heat input (WIengcal) for the fuel based on the error signal; and

transmitting the derived volume specific heat input (WIengcal) to the fuel valve, wherein based on the derived volume specific heat input the fuel valve is adjusted for controlling the supply of fuel.

15. The method according to claim 1 , further comprising:

setting the valve to a flow rate of the fuel such that the error signal is zero;

the method afterwards comprising:

supplying a second fuel to the combustor, the second fuel being different from the fuel;

obtaining a second inlet air pressure (PT 7 ) at the compressor inlet;

obtaining a second inlet air temperature (Tinlet) at the compressor inlet;

obtaining a second outlet air pressure (PT 8 ) at the compressor outlet;

estimating a second heat input (HIengmodel) of the second fuel supplied to the combustor based on the second inlet air pressure; the second inlet air temperature and the second outlet air pressure;

comparing the second estimated heat input with the demanded heat input (FFDEM) to derive a second error signal; and

controlling the valve based on the second error signal such that the valve is set to a second flow rate of the second fuel different from the flow rate of the fuel previously applied.

16. The method according to claim 15 ,

wherein the second flow rate is higher than the flow rate, if the second estimated volume specific heat input is lower than the estimated volume specific heat input.

17. The method according to claim 15 ,

wherein the fuel has a composition that is different from a composition of the second fuel.

18. An arrangement for controlling a supply of a fuel to a combustor of a gas turbine comprising a compressor upstream of the combustor, the arrangement comprising:

a fuel valve configured to supply the fuel to the combustor;

a processor configured

to obtain an inlet air pressure (PT 7 ) at a compressor inlet;

to obtain an inlet air temperature (Tinlet) at the compressor inlet;

to obtain an outlet air pressure (P 18 ) at a compressor outlet;

to obtain an air bleed signal (THBOV) indicative of an amount of air not supplied to the combustor;

to estimate a heat input (HIengmodel) of the fuel supplied to the combustor based on the inlet air pressure; the inlet air temperature, the outlet air pressure and the air bleed signal (THBOV);

to compare the estimated heat input (HIengmodel) with a demanded heat input (FFDEM) to derive an error signal; and

to control the fuel valve regulating the supply of the fuel to the combustor based on the error signal.