HRSG and plant controls for combined cycle power plants

The need for cycling the load differs

The operation modes of a combined cycle power plant vary from base load operation with constant rate power generation to the fast-cycling plant operation that follows the continuously changing load. Industrial power plants follow the electricity and steam consumption of the processes or their heat customers. Utility power plants may follow load targets from some remote load dispatcher and can participate in load stabilization and grid support with primary and secondary frequency control actions.

Due to the development of energy markets, there are increasing demands for faster and more frequent power plant startups. The so-called two-sifting operation mode means that the plant is started up in the morning when the energy demand is higher and disconnected from the grid in the evening. The startup should be automatic and as fast as possible, while considering thermal stresses and fuel consumption.

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

All different operation modes and optimized startups require a high automation level and coordinated control of the whole power plant.

Typical operation modes

• Base load – gas turbines are running in base load and practically in constant full load
• Part load or load control mode – also sometimes called the pre-selected load control
• Load control with a remote setpoint from a load dispatcher
• Load control based on district heat demand
• Two-sifting – unit startups on a daily basis
• Participation in the grid frequency control
• Island mode operation

The CCPP main controls

• Automatic generation control (AGC) takes care of the load control of one or several units based on a local or remote load target
• Unit startup sequence control and related sub-process startup sequences
• Gas turbine controls and protection
• Supplementary firing controls and burner management system (BMS)
• HRSG controls consisting of drum level and/or feedwater controls, steam temperature and pressure controls
• HRSG boiler protection system
• Steam turbine controls and protection
• Balance of Plant controls, including steam turbine bypass controls and other steam network controls, feedwater and condensate controls, cooling water and condenser controls
• Grid frequency support (primary and secondary frequency controls) and island mode operation
• Electrical power distribution control and monitoring, including the fast load shedding application

Combined cycle power plant layout