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.

Contents

  • 1 Linear
  • 2 See also
  • 3 References
  • 4 Further reading

Linear

The frequency of a synchronous generator is given by

F = P N 120 {\displaystyle F={\frac {PN}{120}}}

where

  • F, frequency (in Hz),
  • P, number of poles,
  • N, speed of generator (in RPM)

The frequency (F) of a synchronous generator is directly proportional to its speed (N). When multiple synchronous generators are connected in parallel to electrical grid, the frequency is fixed by the grid, since individual power output of each generator will be small compared to the load on a large grid. Synchronous generators connected to the grid run at various speeds but they all run at the same frequency because they differ in the number of poles (P).

A speed reference as percentage of actual speed is set in this mode. As the generator is loaded from no load to full load, the actual speed of the prime mover tends to decrease. In order to increase the power output in this mode, the prime mover speed reference is increased. Because the actual prime mover speed is fixed by the grid, this difference in speed reference and actual speed of the prime mover is used to increase the flow of working fluid (fuel, steam, etc.) to the prime mover, and hence power output is increased. The reverse will be true for decreasing power output. The prime mover speed reference is always greater than actual speed of the prime mover. The actual speed of the prime mover is allowed to "droop" or decrease with respect to the reference, and so the name.

For example, if the turbine is rated at 3000 rpm, and the machine speed reduces from 3000 rpm to 2880 rpm when it is loaded from no load to base load, then the droop:% is given by

D r o o p % = N o   l o a d   s p e e d − F u l l   l o a d  ... ...read more

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