What is droop setting of Turbine and how does it effect electrical power system in the condition of overload on a power system?

In turbine control droop control applies to generator turbines only. Normally the setting id 4% droop. It may be changed.

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.

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

DROOP % = (NO LOAD SPEED – FULL LOAD SPEED) * 100 / NO LOAD SPEED

= (3000 – 2880) *100/ 3000

= 4%