355
10.2 Types of Wind Energy Conversion Systems
provided with variable rotational speed and pitch control. Variable rotational speed
turbines partly absorb gusts by the instantaneous variation of the rotational speed
as a response to the variable rotor force. Rotor inertia is thus used to buffer gusts.
Three generator concepts are used. The first system is a squirrel cage asynchronous generator with fixed rotational speed (except for slip variation). A main electrical disadvantage with asynchronous generators is their inability to generate reactive power. Capacitors are applied to compensate reactive power consumption. The
second system is an asynchronous generator with a frequency controlled wound
rotor, connected to the grid (so-called doubly fed machine). Due to the frequency
control, the rotor magnetic field rotates with adjustable speed. The machine behaves
like a synchronous generator with a rotational speed governed by the frequency difference between the stator and the rotor. A speed range of about ± 30 % compared
to the synchronous speed (i.e. the speed corresponding to the grid frequency) is
typical. At a sub-synchronous rotational speed, grid power flows to the rotor. At a
super-synchronous rotational speed, rotor power flows to the grid. A ± 30 % speed
change requires a frequency convertor power of ± 30 % of the stator power. The
third system is a synchronous generator connected to the grid through a frequency
convertor, converting the entire power. A typical ± 30 % speed change is also possible with this type. Both wound rotor generators with electronic excitation and
permanent magnet generators are applied. Synchronous generators can have a large
number of pole pairs, and thus can turn at low speed so that a gearbox may be unnecessary (direct drive). The nacelle must then be very wide. Large wind turbines
may be made with direct drive (e.g. Enercon E112: diameter 112 m, 4.5 MW, speed
range 8–13 rpm, synchronous generator with a large number of poles), applying a
one-stage planetary gearbox and a permanent magnet synchronous generator (e.g.
Multibrid M5000: diameter 116 m, speed range 5.9–14.8 rpm, transmission 1:9.92,
synchronous generator with 28 poles) or a three-stage gearbox with an asynchronous generator with rotor frequency control (e.g. REpower 5M: diameter 126 m,
speed range 6.9–12.1 rpm, transmission 1:97, six-pole asynchronous generator).
The direct drive system needs a wide nacelle (the diameter of the generator of the
E126 of Fig. 10.3 is 12 m).The system with the three-stage gearbox requires a long
nacelle (Fig. 10.4). The nacelle of the mixed concept with a one-stage gearbox and
a multi-pole generator is the most compact one.
Much simpler turbines are used in small-scale applications (200 W–10 kW).
Very small turbines have high rotational speed, enabling a direct coupling to the
generator. These are mostly generators with permanent magnets. Small turbines are
not computer-controlled. They are normally power-limited by turning out of the
wind (see next section).
10.2.4 Low-Speed Horizontal-Axis Wind Turbines
For pumping, often a type as sketched in Fig. 10.5 is used. The working principle is
identical to that of the previously discussed type, but the solidity is much larger. The
10.2 Types of Wind Energy Conversion Systems
provided with variable rotational speed and pitch control. Variable rotational speed
turbines partly absorb gusts by the instantaneous variation of the rotational speed
as a response to the variable rotor force. Rotor inertia is thus used to buffer gusts.
Three generator concepts are used. The first system is a squirrel cage asynchronous generator with fixed rotational speed (except for slip variation). A main electrical disadvantage with asynchronous generators is their inability to generate reactive power. Capacitors are applied to compensate reactive power consumption. The
second system is an asynchronous generator with a frequency controlled wound
rotor, connected to the grid (so-called doubly fed machine). Due to the frequency
control, the rotor magnetic field rotates with adjustable speed. The machine behaves
like a synchronous generator with a rotational speed governed by the frequency difference between the stator and the rotor. A speed range of about ± 30 % compared
to the synchronous speed (i.e. the speed corresponding to the grid frequency) is
typical. At a sub-synchronous rotational speed, grid power flows to the rotor. At a
super-synchronous rotational speed, rotor power flows to the grid. A ± 30 % speed
change requires a frequency convertor power of ± 30 % of the stator power. The
third system is a synchronous generator connected to the grid through a frequency
convertor, converting the entire power. A typical ± 30 % speed change is also possible with this type. Both wound rotor generators with electronic excitation and
permanent magnet generators are applied. Synchronous generators can have a large
number of pole pairs, and thus can turn at low speed so that a gearbox may be unnecessary (direct drive). The nacelle must then be very wide. Large wind turbines
may be made with direct drive (e.g. Enercon E112: diameter 112 m, 4.5 MW, speed
range 8–13 rpm, synchronous generator with a large number of poles), applying a
one-stage planetary gearbox and a permanent magnet synchronous generator (e.g.
Multibrid M5000: diameter 116 m, speed range 5.9–14.8 rpm, transmission 1:9.92,
synchronous generator with 28 poles) or a three-stage gearbox with an asynchronous generator with rotor frequency control (e.g. REpower 5M: diameter 126 m,
speed range 6.9–12.1 rpm, transmission 1:97, six-pole asynchronous generator).
The direct drive system needs a wide nacelle (the diameter of the generator of the
E126 of Fig. 10.3 is 12 m).The system with the three-stage gearbox requires a long
nacelle (Fig. 10.4). The nacelle of the mixed concept with a one-stage gearbox and
a multi-pole generator is the most compact one.
Much simpler turbines are used in small-scale applications (200 W–10 kW).
Very small turbines have high rotational speed, enabling a direct coupling to the
generator. These are mostly generators with permanent magnets. Small turbines are
not computer-controlled. They are normally power-limited by turning out of the
wind (see next section).
10.2.4 Low-Speed Horizontal-Axis Wind Turbines
For pumping, often a type as sketched in Fig. 10.5 is used. The working principle is
identical to that of the previously discussed type, but the solidity is much larger. The