Method for operating a gas turbine plant

A method, for operating a gas turbine plant haying a compressor, a first combustion chamber followed by a high pressure turbine, a second combustion chamber, and a low pressure turbine, the method comprising:

• injecting and combusting a first gaseous fuel in the first combustion chamber, generating first hot gases;
• at least partly expanding the first hot gases in the high pressure turbine;
• injecting and combusting a second fuel in the at least partially expanded first hot gases in the second combustion chamber, generating second hot gases;
• expanding the second hot gases in the low pressure turbine;
• injecting water at least in the first combustion chamber to increase the gas turbine plant power output, wherein injecting water comprises injecting in the first combustion chamber at a water flow rate such that flame stability in the first combustion chamber does not pass a predetermined flame stability limit;
• regulating flame temperature in the second combustion chamber to burn non-combusted products generated in the first combustion chamber to bring the amount of non-combusted products below a predetermined emission limit, and without exceeding a predetermined NOx emission limit, and wherein regulating comprises regulating the flame temperature in the second combustion chamber by regulating a water flow rate injected into the second combustion chamber;
• and
• wherein regulating the water flow rate comprises:
• in a first period (I), injecting water in both the first and second combustion chambers;
• in a second period (II) after said first period, injecting water in the first combustion chamber at a flow rate up to the stability limit of the first combustion chamber, and in the second combustion chamber at a constant flow rate, and
• in a third period (III) after said first and second periods, when the stability limit for the first combustion chamber has been reached, injecting water at a constant flow rate, and injecting water at an increased flow rate in the second combustion chamber.