Environmental Engineering Reference
In-Depth Information
3.3 Power Governing
3.3.1 Power Conversion Chain
To understand the impact of RE sources on power systems it is essential to understand the
way in which thermally driven generators are controlled. This is also of direct application to
biomass fuelled systems, where in many cases the plant is similar to a standard fossil fuelled
plant.
Figure 3.3 shows the power fl ow diagram for a simple fuel fi red generator [1], supplying
electricity to consumers as depicted in Figure 1.12 in Chapter 1. Energy is fi rst converted
from chemical into thermal form in the boiler, from thermal into mechanical form in the
turbine, from mechanical into electrical form in the generator and fi nally back to thermal,
mechanical, light or chemical form by the action of the consumers. For the frequency to
remain fi xed the fuel energy input to the boiler must be controlled to balance the variable
consumer demand.
Importantly, the chain of power conversion units in Figure 3.3 contains two stages with
intrinsic energy storage. The boiler consists of kilometres of piping that carry superheated
high pressure steam. Because of its large physical size it constitutes a substantial thermal
store, in fact it contains enough energy to supply the turbine at full output for a few minutes.
The turbogenerator itself, because of its substantial rotating mass and its high rotational speed,
also contains inertial stored energy, in this case only of the order of a few full-output seconds.
These stored energies can be used to follow variations in demand over short periods. Rota-
tional energy in the turbogenerator is instantly available in response to decreases in system
frequency. A sudden increment in demand will result in a frequency drop that will extract a
part of the stored energy from the spinning mass. The more signifi cant stored energy in the
boiler is also available, with some delay, to supply any increments of electrical generation
through the action of the governor.
Trans-
mission
system
Fuel
Boiler
Turbine
Generator
Load
Energy from
chemical
Thermal
mechanical
chemical
light
Thermal
Mechanical
Electrical
Main feedback
control loops
Boiler
Governo r
Figure 3.3 Power fl ow diagram for a thermal power station [1]. (Reproduced with permission of
McGraw - Hill)
 
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