Real power output is controlled through the force or torque exerted by the prime mover, for example, the steam turbine driving the generator rotor. Intuitively, this is straightforward: if more electrical power is to be provided, then something must push harder.

The rotor’s rate of rotation has to be understood as an equilibrium between two opposing forces: the torque exerted by the turbine, which tends to speed up the rotor, and the torque exerted in the opposite direction by the magnetic field inside the generator, which tends to slow it down. The slowing down is directly related to the electric power being supplied by the generator to the grid.

This is because the magnetic field that provides the retarding effect (the armature reaction) is directly proportional to the current in the armature windings, while the same current also determines the amount of power transmitted.

For example, if the load on the generator suddenly increases (someone is turning on another appliance), this means a reduction in the load’s impedance, resulting in an increased current in the armature windings, and the magnetic field associated with this increased current would slow down the generator. In order to maintain a constant rotational frequency of the generator, the turbine must now supply an additional torque to match.

Conversely, if the load is suddenly reduced, the armature current and thus its magnetic field decreases, and the generator would speed up. To maintain equilibrium, the turbine must now push less hard so that the torques are equal and the rotational frequency stabilizes.

The torque supplied by the prime mover is adjusted by a governor valve (Figure 4.10). In the case of a steam turbine, this increases or decreases the steam flow; for a hydro turbine, it adjusts the water flow. This main valve can be operated manually (i.e., by deliberate operator action) or, as is general practice, by an automated control system.

In any situation where a generator must respond to load fluctuations, either because it is the only one in a small system or because it is designated as a load-following generator in a large power system, automatic governor control will be used; in this case, the generator is said to operate “on the governor.”

The automatic governor system includes some device that continually monitors the generator frequency. Any departure from the set point (e.g., 3600 rpm) is translated into a signal to the main valve to open or close by an appropriate amount.

Alternatively, a generator may be operated at a fixed level of power output (i.e., a fixed amount of steam flow), which would typically correspond to its maximum load (as for a baseload plant); in this case, the generator is said to operate “on the load limit.”

Various designs for governor systems are in use. Older ones may rely on a simple mechanical feedback mechanism such as a flywheel that expands with increasing rotational speed due to centrifugal force, which is then mechanically connected to the valve operating components.

Newer designs are based on solid-state technology and digitally programmed, providing the ability to govern based on not just the frequency measured in real time but its time rate of change (i.e., the slope). This allows anticipation of changes and more rapid adjustment, so that the actual generator frequency ultimately undergoes much smaller excursions.

In any case, such a governor system allows the generator to follow loads within the range of the prime mover’s capability, and without direct need for operator intervention.

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