Power Generation

The high demand of electricity together with the continuously variable nature, and our inability to store electricity in a significant number of calls for a diversity of production in the grid. The traditional view is that the use of different primary energy resources contributes to the continuity of supply and stable price mechanism.

Most of the electricity consumed worldwide is produced by three-phase synchronous 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 electricity 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).

Fig. 1.2 Schematic diagram of a synchronous machine.

In general, three main control systems directly affect the turbine-generator set:

• the boiler's firing control
• the governor control
• the excitation system control.

Fig. 1.3 Coupled windings of a synchronous machine.

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 supplied to the field winding by the exciter. The measured terminal voltage and the desired reference voltage are compared to produce a voltage error which is used to alter the exciter output. Generally speaking, exciters can be of two types: (1) rotating; or (2) static. Nowadays, static exciters are the preferred choice owing to their higher speed of response and smaller size. They use thyristor rectifiers to adjust the field current (Arco, 2000).

Fig. 1.4 Main controls of a generating unit.

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