The Generator

30.1.1.3 The Generator

into the rotor so that it has the same frequency as the elec- The main objective of the generator is to transform the tromotive force induced in it and an arbitrary magnitude and

mechanical energy captured by the rotor of the wind turbine phase. The techniques used to vary the supplying frequency into electrical energy that will be injected into the utility grid. permit a wide range of variation of the speed, from 0 to 100%

Asynchronous generators are commonly used in wind tur- or even greater than the synchronous speed. Another variable- bine applications with fixed speed or variable speed control speed technique is achieved by changing the number of poles strategies. Also, in large power wind turbine applications syn- which permit a regulation of the speed in discrete steps. If we chronous machines are used. In the asynchronous generator, proceed to vary the slip, then the range of variation of the the electric energy is produced in the stator when the rotating speed is within a narrow margin of regulation. speed of the rotor is higher than the speed of the rotary field

Among all these techniques, only the variation of the volt- of the stator. The asynchronous generator needs to take energy age can be actually implemented using a squirrel cage machine

from the grid to create the rotary field of the stator. Because of with a short-circuited rotor. The rest are implemented by this, the power factor is decreased and so a capacitor bank is means of a wound-rotor machine. needed. The synchronous generator with an excitation system

The stator voltage can be varied by means of a power con- includes electromagnets in the rotor that generate the rotat- verter [4, 8, 9]. This converter should be connected in series

ing field. The rotor electromagnets are fed back with a DC to the generator and to the grid. Since it is only necessary current by rectifying part of the electricity generated. Another to vary the voltage of the generator and not its frequency, an kind of generator recently used is the permanent magnet [5]. AC–AC converter can be used. Furthermore, the power con- This type of machine does not need an excitation system, and verter bears all the power of the generator so it deals with all the it is used mainly for low power wind turbine applications. disadvantages of the other wide-range of control techniques. The advantages of using an asynchronous generator are low

For the speed to be varied by changing the slip, it is necessary cost, robustness, simplicity, and easier coupling to the grid, to work with wound rotor induction machines.

yet its main disadvantage is the necessity of a power factor compensator and a lower efficiency.

30.1.1.3.2 Synchronous Machine with Excitation System As it is well-known, the general principle to change the speed

30.1.1.3.1 Induction Machine The induction generator, as of a synchronous machine is summarized in the following can be deducted from its torque/speed characteristic, has a equation [3, 6, 7]: nearly constant speed in a wide working torque range, as they are positive (working as a motor) or negative (work-

60 ·f 1 ing as a generator). This characteristic curve is very useful

N r =N 1 =

(30.2) p

for machines with constant speed, as the machine is auto- regulated to keep the synchronous frequency. But the situation

The only way to control the speed is by changing the number is very different when we proceed to change the speed of of pole pairs or by supplying frequency into the machine, f 1 . the generator. It is then necessary to use power converters in Therefore, wide range or discrete steps are permitted.

794 J. M. Carrasco et al. The synchronous machine will always be controlled in a wide

C p (λ,β)

range of the rotor speed, ω r . In this kind of system, the

excitation current permits an easier torque and power control.

30.1.1.3.3 Permanent Magnet Synchronous Machine As 0 ° with the synchronous generator with excitation system, the

1 ° permanent magnet synchronous machine can be controlled

in a wide range of rotor speeds ω r . In this case, a mag-

netic field control has to be made from the power converter.

0.1 2 The advantage of this machine is better performance and less ° complexity [3, 6, 7, 10].