All generators are driven by a prime mover, which is the generator’s source of mechanical power. All prime movers tend to behave in a similar fashion – as the power drawn from them increases, the speed at which they turn decreases. The decrease in speed is in general non linear, but some form of governor mechanism is usually included to make the decrease in speed linear with an increase in power demand.
Whatever governor mechanism is present on a prime mover, it will always be adjusted to provide a slight drooping characteristic with increasing load. The speed droop (SD) of a prime mover is defined as:
SD = %100 x (n nl – n fl)/n fl
Where nnl is the no-load prime mover speed and nfl
is the full-load prime mover speed.
Typical values of SD are 2% – 4%. Most governors have some type of set point adjustment to allow the no-load speed of the turbine to be varied. A typical speed vs. power plot is as shown below.
Since mechanical speed is related to the electrical frequency and electrical frequency is related with the output power, hence we will obtain the following equation:
Where P = output power
= no-load frequency of the generator
= operating frequency of system
= slope of curve in kW/Hz or MW/Hz
If we look in terms of reactive power output and its relation to the terminal voltage we shall see a similar shape of curve as shown in the frequency power curve.
In conclusion, for a single generator:
a) For any given real power, the governor set points control the generator operating frequency
b) For any given reactive power, the field current controls the generator’s terminal voltage.
c) Real and reactive power supplied will be the amount demanded by the load attached to the generator – the P and Q supplied cannot be controlled by the generator’s controls.