Combined cycle power generation plant and operating method thereof

A method of operating a combined cycle power generation plant which comprises a gas turbine plant including a high temperature section, a steam turbine plant including high, intermediate and low pressure turbines connected through a common shaft, and an exhaust heat recovery boiler including a reheater, and high, intermediate and low pressure superheaters and in which a turbine exhaust steam of the steam turbine plant is supplied to the high temperature section of the gas turbine plant and the turbine exhaust steam is recovered to the steam turbine plant after cooling the high temperature section of the gas turbine plant, said method comprising the steps of:

• supplying a steam from the exhaust heat recovery boiler to the high pressure turbine of the steam turbine plant during a start-up operation or a partial load operation;
• carrying out a constant pressure operation in the high pressure turbine while carrying out an inlet pressure control in a reheat steam system for connecting the reheater of the exhaust heat recovery boiler to the intermediate pressure turbine of the steam turbine plant;
• joining the turbine exhaust steam after being expanded in the high pressure turbine together with a steam generated from an intermediate pressure drum of the exhaust heat recovery boiler;
• supplying the joined steam to the high temperature section of the gas turbine plant so as to cool the high temperature section;
• and
• joining the joined steam, after cooling, together with a reheated steam from the reheater to form a secondary joined steam and supplying the secondary joined steam to the intermediate pressure turbine.