Method for controlling the load variations in a gas turbine

1. A method for avoiding malfunctions during load reductions in a double shaft gas turbine of the type comprising at least one compressor, capable of compressing the air introduced into it through an inlet duct, at least one combustor, in which said compressed air is mixed and combusted with a gaseous fuel coming from a feeding duct, and at least one turbine, capable of transforming the energy of the combusted gas coming from said combustor into work energy which can be exploited for activating one or more operating machines, wherein said combustor is equipped with a plurality of burners and wherein said compressor is equipped with a plurality of adjustable stator vanes, one or more anti-surge valves and one or more overboard bleeds, the method comprising performing the following steps in sequence:

(a) reducing a flow of the gaseous fuel entering said combustor to a predefined minimum value, if an increase is observed in the rotation regime of said turbine above a predefined maximum value and a total reduction in the load;

(b) modifying the angulation of said plurality of adjustable stator vanes in order to reduce the speed rate of said compressor;

(c) opening said one or more anti-surge valves and said one or more overboard bleeds in order to reduce an air flow at an inlet of said combustor; and

(d) supplying a maximum quantity possible of the gaseous fuel to one or more burners in order to cover an overall demand for the gaseous fuel of said turbine and to avoid a flame loss if said turbine is operating in premixed flame mode.

2. The method according to claim 1 , further comprising deactivating, through an electronic control system of said turbine, the possibility of switching a working mode of said turbine from premixed flame mode to diffusive mode or vice versa.

3. The method according to claim 1 , further comprising supplying a predefined quantity of gaseous fuel to a first burner during said selective feeding sequence of said burners.

4. The method according to claim 3 , further comprising supplying a predefined quantity of gaseous fuel to a second burner if the overall demand for gaseous fuel on the part of said turbine is greater than the maximum quantity possible of gaseous fuel supplied to said first burner.

5. The method according to claim 4 , further comprising supplying a predefined quantity of gaseous fuel to at least a third burner if the overall demand for gaseous fuel on the part of said turbine is greater than the maximum quantity possible of gaseous fuel supplied to said first burner and said second burner.

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

managing the flow of said gaseous fuel with an electronic control system of said turbine once the rotation regime of said turbine has been restored to below said predefined maximum value;

supplying gaseous fuel only to a first burner if said turbine is operating in a premixed flame mode; and

activating said plurality of adjustable stator vanes, said one or more anti-surge valves and said one or more overboard bleeds according to a standard logic used in the premixed flame mode of said turbine.

7. The method according to claim 6 , further comprising maintaining the supply of said predefined quantity of gaseous fuel to said first burner, a second burner and at least a third burner if said turbine is operating in diffusive flame mode.

8. The method according to claim 6 , further comprising:

effecting a temperature control inside said combustor after the rotation regime of said turbine has dropped substantially to its nominal or operating value; and

activating the transfer from the functioning mode of said turbine from premixed flame to diffusive flame if said temperature control is successful and if said turbine is operating in premixed flame mode.