Droop speed control

In electrical power generation, droop speed control is a speed control mode of a prime mover driving a synchronous generator connected to an electrical grid. This mode allows synchronous generators to run in parallel, so that loads are shared among generators in proportion to their power rating.

The frequency of a synchronous generator is given by

where

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 tend 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

In this case, speed reference will be 104% and actual speed will be 100%. For every 1% change in the turbine speed reference, the power output of the turbine will change by 25% of rated for a unit with a 4% droop setting.

Droop is therefore expressed as the percentage change in (design) speed required for 100% governor action.

For example, how fuel flow is increased or decreased in a GE-design heavy duty gas turbine can be given by the formula,

where,

As frequency is fixed on the grid, and so actual turbine speed is also fixed, the increase in turbine speed reference will increase the error between reference and actual speed. As the difference increases, fuel flow is increased to increase power output, and vice versa. This type of control is referred to as "straight proportional" control. If the entire grid tends to be overloaded, the grid frequency and hence actual speed of generator will decrease. All units will see an increase in the speed error, and so increase fuel flow to their prime movers and power output. In this way droop speed control mode also helps to hold a stable grid frequency. The amount of power produced is strictly proportional to the error between the actual turbine speed and speed reference. The above formula is nothing but the equation of a straight line (y = mx + q).

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