Steam cooling control for a combined cycle power plant

A method of controlling delivery of steam from a steam drum of a heat recovery steam generator in a combined cycle power plant, the method comprising:

• providing a cooling steam flow path between the steam drum and a downstream location passing through a heat exchange passage, the cooling steam flow path comprising a steam admission valve;
• providing a steam bypass flow path between the steam drum and the downstream location not passing through the heat exchange passage, the steam bypass flow path comprising a steam bypass valve;
• controlling flow rate of steam through the cooling steam flow path by partially closing the steam admission valve at power levels below a crossover power level;
• maintaining the steam admission valve in a full open position at power levels above the crossover power level;
• and
• controlling flow rate of steam through the cooling steam flow path by selectively positioning the steam bypass valve at power levels above the crossover power level.

A method for controlling steam flows in a combined cycle power plant comprising a heat recovery steam generator for generating steam, a component cooling circuit including a steam admission valve receiving a flow of cooling steam from the steam generator, and a steam bypass circuit including a steam bypass valve receiving a flow of bypass steam from the steam generator, the method comprising:

• selecting a cooling circuit demand pressure curve defining a relationship between power level and steam pressure in the steam generator necessary to produce a desired cooling steam flow rate in the component cooling circuit;
• selecting a steam bypass valve pressure curve that intersects the cooling circuit demand pressure curve at a crossover point below 100% power;
• controlling the steam admission valve in response to the cooling circuit demand pressure curve;
• controlling the steam bypass valve in response to the bypass valve pressure curve at power levels up to the crossover point, thus resulting in the steam admission valve being controlled to a full open position at the crossover point;
• and
• controlling the steam bypass valve in response to the cooling circuit demand pressure curve at power levels above the crossover point.

A method for controlling steam flows in a combined cycle power plant comprising a heat recovery steam generator, a component cooling circuit including a steam admission valve receiving a flow of cooling steam from the steam generator, and a steam bypass circuit including a steam bypass valve receiving a flow of bypass steam from the steam generator, the method comprising:

• selecting a demand value for the steam bypass valve necessary to control steam pressure in the steam generator to a predetermined value as a function of power level;
• selecting a demand value for the steam admission valve necessary to provide a required cooling steam flow as a function of power level;
• and
• if the required cooling steam flow is not achievable with the steam admission valve fully open, then determining a flow compensator value for combination with the demand value for the steam bypass valve that results in a steam generator pressure sufficient to produce the required cooling steam flow with the steam admission valve fully open.