Low load turndown for combined cycle power plants

A method of operating a heat recovery steam generator for a combined cycle power unit, the combined cycle power unit including a gas turbine, the heat recovery steam generator (HRSG) to generate steam by recovering thermal energy of exhaust gas from the gas turbine, and a steam turbine driven by the steam generated from the HRSG, the method including:

• operating the combined cycle power unit in a base load mode of operation wherein superheated steam is provided from the HRSG to the steam turbine at a base load operating temperature and at a base load operating pressure;
• and
• subsequently, operating the combined cycle power unit in a low load mode of operation wherein superheated steam is provided from the EMS G to the steam turbine at the base load operating temperature and at a reduced operating pressure that is less than the base load operating pressure,
• wherein the HRSG includes at least an upstream superheater stage and a downstream superheater stage that supply the superheated steam to the steam turbine, and including throttling a steam flow from the upstream superheater stage to the downstream superheater stage during the low load mode of operation.

A combined cycle power plant comprising:

• a gas turbine operable to generate power and producing an exhaust output;
• a heat recovery steam generator (HRSG) receiving the exhaust output from the gas turbine and producing superheated steam;
• and
• wherein the HRSG includes:
• an evaporator providing a source of saturated steam;
• an upstream superheater stage connected to an output from the evaporator for receiving steam from the evaporator;
• a downstream superheater for receiving steam from the upstream superheater via a steam conduit;
• and
• a throttling valve located in the steam conduit, the throttling valve controlling steam flow from the upstream superheater stage to the downstream superheater stage between a base mode of operation and a low load mode of operation of the combined cycle power plant:
• a steam turbine receiving the superheated steam from the HRSG to generate power;
• and
• a control unit operatively connected to the throttling valve, and configured to:
• actuate the throttling valve to provide to the steam turbine, the superheated steam at a base load operating temperature and at a base load operating pressure during the base load mode of operation;
• and
• actuate the throttling valve to provide to the steam turbine, the superheated steam at the base load operating temperature and at a reduced operating pressure that is less than the base load operating pressure during the low load mode of operation.