HRSG and plant controls for combined cycle and gas power plants

The operation modes of a combined-cycle power plant vary from base-load operation with a constant power generation rate to the fast-cycling plant operation that follows a continuously changing load.

Valmet DNA covers the following operational modes for combined-cycle units

  • Base load mode, where the gas turbines run in the base load mode, practically at constant full load
  • Part load or load control mode, also called the pre-selected load control mode
  • Load control mode with remote set points from the load dispatcher
  • Load control mode based on district heat demand
  • Two-shifting mode, where the unit starts up and shuts down twice a day
  • Island mode

Depending on the type of power plant, the need for cycling the load differs. Industrial power plants follow the electricity and steam consumption of their processes or their heating customers. Utility power plants may follow load targets from remote load dispatchers and can participate in load stabilization and grid support by primary and secondary frequency control actions.

Due to the development of the energy markets, there are increasing demands for faster and more frequent startups of power plants. The so-called two-shifting operation mode means that the plant is started up in the morning, when the energy demand is greater, and then disconnected from the grid in the evening. The startup should be automatic, fast and take thermal stresses and fuel consumption into consideration.

Island mode occurs when the power plant, or part of it, is isolated from the national grid. In island mode, it is important to rapidly balance power consumption and power generation to avoid a forced shutdown due to a frequency deviation.

All these different possible operation modes and startup optimizations require a high level of automation and coordinated control for the efficient performance of the whole power plant.