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6 Electrical power systems ± an overview

1.3.1 Generation

The large demand for electrical energy coupled with its continuous varying nature
and our inability to store electrical energy in significant quantities calls for a diversity
of generating sources in the power network. The traditional view is that the use of
different primary energy resources helps with continuity of supply and a more stable
pricing mechanism.
Most of the electricity consumed worldwide is produced by three-phase synchron-
ous generators (Kundur, 1994). However, three-phase induction generators will
increase their production share when wind generation (Heier, 1998) becomes more
widely available. Similarly, three-phase and single-phase static generators in the form
of fuel cells and photovoltaic arrays should contribute significantly to global elec-
tricity production in the future.
For system analysis purposes the synchronous machine can be seen as consisting of
a stationary part, i.e. armature or stator, and a moving part, the rotor, which under
steady state conditions rotates at synchronous speed.
Synchronous machines are grouped into two main types, according to their rotor
structure (Fitzgerald et al., 1983):
1. salient pole machines
2. round rotor machines.
Steam turbine driven generators (turbo-generators) work at high speed and have
round rotors. The rotor carries a DC excited field winding. Hydro units work at low
speed and have salient pole rotors. They normally have damper windings in addition
to the field winding. Damper windings consist of bars placed in slots on the pole faces
and connected together at both ends. In general, steam turbines contain no damper
windings but the solid steel of the rotor offers a path for eddy currents, which have
similar damping effects. For simulation purposes, the currents circulating in the solid
steel or in the damping windings can be treated as currents circulating in two closed
circuits (Kundur, 1994). Accordingly, a three-phase synchronous machine may be
assumed to have three stator windings and three rotor windings. All six windings will
be magnetically coupled.
Figure 1.2 shows the schematic diagram of the machine while Figure 1.3 shows the
coupled circuits. The relative position of the rotor with respect to the stator is given
by the angle between the rotor's direct axis and the axis of the phase A winding in
the stator. In the rotor, the direct axis (d-axis) is magnetically centred in the north
pole. A second axis located 90 electrical degrees behind the direct axis is called
the quadrature axis (q-axis).
In general, three main control systems directly affect the turbine-generator set:

1. the boiler's firing control
2. the governor control
3. the excitation system control.

Figure 1.4 shows the interaction of these controls and the turbine-generator set.
The excitation system control consists of an exciter and the AVR. The latter
regulates the generator terminal voltage by controlling the amount of current sup-
plied to the field winding by the exciter. The measured terminal voltage and the