Method of operating a gas-turbine group

A method of operating a gas-turbine group for loads less than a rated load, the gas-turbine group comprising a compressor unit, a first combustion chamber which is connected downstream of the compressor unit and connected to deliver heated gases to a first turbine, a second combustion chamber which is connected downstream of the first turbine and connected to deliver heated gases to a second turbine, and at least one generator driven by the turbines, the method comprising the steps of:

• reducing a fuel flow amount to the first combustion chamber and a fuel flow amount to the second combustion chamber to lower inlet temperatures at the two turbines for a 5% reduction in the rated load,
• adjusting an angle of compressor guide blades to reduce the mass flow until the load drops below 50% of the rated load, wherein the fuel flow amount to the first combustion chamber is regulated according to the change in mass flow to maintain substantially constant the inlet temperature at the first turbine and the fuel flow amount to the second combustion chamber is regulated according to the mass flow to lower the inlet temperature at the second turbine so that an outlet temperature of the second turbine does not exceed a rated nominal value,
• further reducing the fuel flow amount to the second combustion chamber to further lower the inlet temperature at the second turbine after the adjustment of the compressor guide blades is complete and thereafter,
• further reducing the fuel flow amount to the first combustion chamber to further lower the inlet temperature at the first turbine.