Flexible generating units help provide stability to the electric grid by ramping output up or down as demand and system loads fluctuate. Because solar and wind generation can change within minutes, electric grid operators rely on power plants that can provide additional load (or curtail load) on the same timescale as variations in renewable output. The increase or reduction in output per minute in spinning mode is called the ramp rate and is usually expressed either as % per minute or MW per minute.
Ramp rates from spinning mode of most industrial frame gas turbine models are around 20% / minute and around 50% / minute for aeroderivative gas turbines. For combined cycle gas turbines, typical ramp rates are around 10 % / minute. Alternatively, ramp rates are sometimes expressed as MW / minute. Ramp rate depends on generating unit capacity, operating conditions (whether unit is just starting up or operating at a minimum load hold point) and optional technologies for reducing startup time and increasing ramp rate. The ramp rate of a power plant also depends on the number of units and configuration. For example, a ramp rate of 110 MW/min is based on multi-turbine plant designs with large unit capacity, such as a 2x1 combined cycle gas turbine (CCGT) plant (net power output of 880 MW) where each gas turbine is rated to ramp at 55 MW/min. While ramp rate in MW/minute is a valuable metric, it is important to understand the operating conditions under which advertised ramp rates can be achieved.
Wärtsilä power plants are perfectly suited to cycling, as the ability to quickly ramp up and down in load does not affect the maintenance schedule. In addition to a fast startup time, Wärtsilä engines can stop within one minute and have lower emissions due to lean-burn technology. Even in combined cycle, Wärtsilä engines retain their loading responsiveness because the steam portion of the power plant is designed for low gas temperatures and pressures which can be maintained with only a small number of engines running. Due to the modular design of Wärtsilä power plants, the engines can be loaded and unloaded individually, providing the high plant efficiency even at part load.
Once running and at nominal operating temperatures, Wärtsilä power plants can adjust output up or down rapidly. Wärtsilä power plants can ramp from 10% to 100% load (or down) in just 42 seconds, with an effective operational ramp rate of over 100% per minute.
Figure 2: Screen shot from a dispatch center shows the drop off in wind generation (green line) and rapid ramp up of Wärtsilä's Plains End power plant to compensate. Compared to the fast ramping of the Wärtsilä plant, gas turbine output (purple line) increases more slowly.