Combined cycle power plant and cooling steam supply method for gas turbine therein

A method of supplying cooling steam in a combined cycle power plant comprising a combination of a gas turbine plant including a gas turbine, a waste heat recovery boiler and a steam turbine plant including a high pressure turbine. A high temperature portion of the gas turbine to be cooled is supplied with auxiliary steam prior to start of the gas turbine so that the gas turbine is warmed up, the auxiliary steam being of pressure of combustion gas pressure or more of starting time of the gas turbine. The gas turbine is then started in that state to do a holding operation for a predetermined time at a predetermined load. When steam from the waste heat recovery boiler comes to a condition of the auxiliary steam, supply of the auxiliary steam is stopped and steam coming from a high pressure superheater of the waste heat recovery boiler, by-passing the high pressure turbine, is supplied into the high temperature portion of the gas turbine to be cooled. When output of the waste heat recovery boiler comes to a rating thereof, the by-passing is closed and exhaust steam from the high pressure turbine is supplied into the high temperature portion of the gas turbine to be cooled so as to switch to a rated operation. Thereby, a safe and reliable starting procedure of the plant is provided without occurrence of thermal shock.