Abstract: A combined cycle power plant includes a gas turbine, a steam turbine and a heat recovery steam generator having a duct burner for supplementary firing. In the unfired mode, gas turbine exhaust is used in the heat recovery steam generator for generating steam for the steam turbine. The power plant includes a main steam output line from the heat recovery steam generator connected to the first stage inlet of the high pressure section of the steam turbine. A bypass loop is connected either to an intermediate pressure connection of the HRSG or from the high pressure line to an admission point downstream of the high pressure inlet nozzle, at a lower pressure than the nominal inlet pressure. By operating the bypass system in conjunction with the duct burning process in the HRSG, the steam inlet pressure to the steam turbine can be maintained at the optimal level during both fired and unfired operation of the HRSG.