The invention relates to a combined cycle power plant including a gas turbine the exhaust gas outlet of which is connected to a heat recovery steam generator, which is part of a water/steam cycle, whereby, for having a large power reserve and at the same time a higher design performance when operated at base load, the gas turbine is designed with a steam injection capability for power augmentation. For having a large power reserve at improved and optimized design performance when the plant is being operated at base load, the gas turbine includes at least one combustor, and a compressor for providing cooling air for that gas turbine, which is extracted from the compressor and cooled down in at least one cooling air cooler. The steam for steam injection is generated in said cooling air cooler, whereby said steam is injected into an air side inlet or outlet of said cooling air cooler and/or directly into said at least one combustor. The heat recovery steam generator is equipped with a supplementary firing, which is at least a single stage supplementary firing to increase the high pressure steam production and providing augmentation power as power reserve to a grid when required.