Droop speed control

Lore & Background

Droop speed control operates by setting a speed reference as a percentage of actual speed. As the generator is loaded from no load to full load, the actual speed of the prime mover tends to decrease. To increase power output, the prime mover speed reference is increased. Because the actual prime mover speed is fixed by the grid, the difference between the speed reference and the actual speed is used to increase the flow of working fluid (fuel, steam, etc.) to the prime mover, and hence the power output increases. The reverse occurs for decreasing power output. The prime mover speed reference is always greater than the actual speed, which is allowed to 'droop' or decrease with respect to the reference, giving the mode its name.

Reader's Guide

Droop speed control is significant because it enables multiple synchronous generators to operate in parallel on an electrical grid, sharing load changes in proportion to their power ratings without requiring external communication. This mode provides primary instantaneous control of an individual power plant's power output (kW), alongside the inertia of parallel synchronous generators. For stable operation of large electrical grids, such as that of North America, power plants typically operate with a four or five percent speed droop. The droop curves used by generators are not necessarily linear or identical and may be adjusted by operators, allowing base load generators to generate a larger proportion at low demand. Stability requires that over the operating frequency range, the power output is a monotonically decreasing function of frequency. Droop speed control can also be used by grid storage systems to remove energy from the grid at higher than average frequencies and supply it at lower frequencies.

Did You Know?

• With droop speed control, when the grid is at maximum operating frequency, the prime mover's power is reduced to zero.
• For a unit with a 4% droop setting, every 1% change in turbine speed reference changes power output by 25% of rated power.
• Droop speed control can be used by grid storage systems to remove energy from the grid at higher than average frequencies and supply it at lower frequencies.