Combined gas turbine and steam turbine power plant

A combined cycle steam and gas turbine power plant for generating electrical power to satisfy an electrical load which varies from a nominal load set point during normal operation of said power plant after power plant start-up, said power plant comprising:

• at least one gas turbine having a compressor, a combustor and a turbine arranged in serial flow relationship, said combustor adapted to receive compressed air from said compressor and fuel and generate a hot gas stream for driving said turbine, said turbine arranged in driving connection with said compressor and an electrical generator;
• a heat recovery steam generator adapted to receive the hot gas stream exhausted by said gas turbine and a supply of water and generate steam;
• a steam turbine arranged in driving connection with an electrical generator and having an inlet for receiving steam for driving said steam turbine and an outlet for discharging steam;
• a first conduit for communicating steam generated by said heat recovery steam generator with said steam turbine inlet;
• a second conduit for bypassing steam generated by said heat recovery steam generator from said steam turbine inlet;
• a control valve disposed in said first conduit for adjusting steam flow to said steam turbine inlet;
• a bypass valve disposed in said second conduit for adjusting steam flow through said second conduit;
• means for sensing variations from said nominal load set point in the electrical power load to be satisfied by said power plant and for generating a load signal which is representative of said load variations;
• a power plant control for controlling the electrical power generated by said power plant, said power plant control adapted to supply fuel to said gas turbine at a generally constant rate during normal operation of said power plant after power plant startup and including means, operative during normal operation of said power plant after power plant startup and responsive to said load signal, for increasing and decreasing the opening of said control valve in response, respectively, to increases and decreases in said power plant electrical load from said nominal load set point, said increase in the opening of said control valve, in the case of increases in power plant load from said nominal load set point, being sized to provide an increase in steam flow to said steam turbine which is sufficient to cause said steam turbine power output to be increased in the amount of said increase in power plant electrical load from said nominal load set point, said decrease in the opening of said control valve, in the case of decreased power plant load, being sized to provide a decrease in steam flow to the steam turbine which is sufficient to cause said steam turbine power output to be decreased in the amount of said decrease in power plant electrical load from said nominal load set point, and means for proportionately decreasing the opening of said bypass valve in response to increases in the opening of said control valve and for proportionately increasing the opening of said bypass valve in response to decreases in the opening of said control valve so that total steam flow from said heat recovery steam generator remains generally constant, whereby variations in the electrical power loading on said power plant from said nominal load set point are carried by said steam turbine and whereby the gas turbine, the heat recovery steam generator and the steam turbine are not subjected to thermal cycling as a result of such load variations.