Combined cycle power plant

A combined cycle power plant, comprising:

• a gas turbine plant including a gas turbine provided with a high temperature cooled portion;
• a heat recovery steam generator to generate steam by recovering thermal energy of exhaust gas from said gas turbine plant;
• a steam turbine plant driven by said steam generated from said heat recovery steam generator;
• and
• a cooling steam system to flow said steam from said heat recovery steam generator to said high temperature cooled portion of said gas turbine for cooling;
• said heat recovery steam generator including,
• a main stream side steam piping,
• a bypass side steam piping,
• a steam branching portion connected to said main stream side steam piping and said bypass side steam piping to branch steam flowing from a former stage in said heat recovery steam generator into two steams, one as main stream side steam flowing through said main stream side steam piping and another as de-superheating steam flowing through said bypass side steam piping,
• a high pressure superheater connected to said main stream side steam piping for superheating said main stream side steam, and
• a steam merging portion to merge said main stream side steam superheated by said high pressure superheater and said de-superheating steam passed through said bypass side steam piping,
• steam merged in said steam merging portion being supplied to a next stage in said heat recovery steam generator;
• and
• said heat recovery steam generator being provided with, at one of said steam branching portion and said steam merging portion, means to prevent closing of said main stream side steam piping and said bypass side steam piping.