Droop speed control basics
– Droop speed control is used for AC electrical power generators
– Power output reduces as line frequency increases
– It is commonly used as the speed control mode of the governor
– Droop speed control allows synchronous generators to run in parallel
– Power output is a monotonically decreasing function of frequency
Linear relationship between frequency and speed
– Frequency is directly proportional to the speed of a synchronous generator
– Multiple synchronous generators connected in parallel run at the same frequency
– Speed reference is set as a percentage of actual speed
– Actual speed of the prime mover tends to decrease as the generator is loaded
– Droop percentage is calculated based on the change in speed reference and actual speed
Control mechanism of droop speed control
– Speed reference is always greater than actual speed of the prime mover
– Increase in turbine speed reference leads to an increase in power output
– Fuel flow is increased to increase power output when there is a speed error
– Droop speed control helps to hold a stable grid frequency
– Power produced is strictly proportional to the error between actual turbine speed and speed reference
Sharing load in proportion to machine ratings
– Machines with the same droop speed control share load proportionately
– Fuel flow adjustments are made to change power output
– All units connected to a grid should have the same droop setting
– Stable operation of the electrical grid typically uses a four or five percent speed droop
– Changes in speed are minor due to the inertia of the rotating mass
Other related topics
– Electric power transmission and wide area synchronous grid
– Dynamic demand in electric power systems
– References to books and articles on droop speed control
– Further reading on the applicability of droops in low voltage grids
– Details about the power transmission grid in the Contiguous United States Source: https://en.wikipedia.org/wiki/Droop_speed_control
Droop speed control is a control mode used for AC electrical power generators, whereby the power output of a generator reduces as the line frequency increases. It is commonly used as the speed control mode of the governor of a prime mover driving a synchronous generator connected to an electrical grid. It works by controlling the rate of power produced by the prime mover according to the grid frequency. With droop speed control, when the grid is operating at maximum operating frequency, the prime mover's power is reduced to zero, and when the grid is at minimum operating frequency, the power is set to 100%, and intermediate values at other operating frequencies.
This mode allows synchronous generators to run in parallel, so that loads are shared among generators with the same droop curve in proportion to their power rating.
In practice, the droop curves that are used by generators on large electrical grids are not necessarily linear or the same, and may be adjusted by operators. This permits the ratio of power used to vary depending on load, so for example, base load generators will 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. With droop speed control those systems will remove energy from the grid at higher than average frequencies, and supply it at lower frequencies.
