**2. Steam turbines control functions**

The speed regulation system is the primary control loop of the Automatic Generation Control (*AGC*) of an electrical system. The control of active power in a power system is accomplished primarily by controlling the torque of the machines primary drive system (Anderson & Fouad, 2003).

Figure 1 illustrates a system for regulating the speed of a steam turbine. By controlling the position xE of the control valve, a control can be exercised over the admission of steam (or water) at high pressure trough the turbine, and therefore the torque of the turbine. For a hydraulic amplifier, the input is the position of the pilot valve xD and the output is position xE of the main piston. A small downward movement of point E, the steam flow increases by an amount that, if measured in terms of power valve, represents an increment *Pv* . This increased flow is rendered by the increase in turbine power, *PT* .

Fig. 1. Example illustrating the mechanical speed control system of a steam turbine

The pilot valve XD, still in figure 1, can have its position changed, through the links or arms connection system, in three ways:


 Indirectly, by feedback due to changes in the position of the connection point B ( *<sup>B</sup> x* ), resulting from changes in speed.

Historically, *load-frequency primary controls* are part of the so-called *characteristics of speed governors*. However, the term "regulator" implies a mechanical speed sensor mechanically connected to control variations in input power, and since today most of the load-frequency controls are electro-hydraulic, speed control term does not describe generically this control function (Kundur, 1994).
