Page 390 - Wind Energy Handbook

364 CONCEPTUAL DESIGN OF HORIZONTAL-AXIS TURBINES

6.10 Type of Generator

Fixed-speed wind turbines differ from almost all conventional generating plant by
using induction rather than synchronous generators. This choice is driven by the
requirement for significant damping in the drive train due to the cyclic variations in
the torque developed by the aerodynamic rotor.
Both synchronous and induction generators have similar winding arrangements
on the stator which, when connected to the three-phase network voltage, produce a
fixed-speed, rotating magnetic field. However, the rotors of the two machines are
quite different (Hindmarsh, 1984, McPherson, 1990). A synchronous machine has
magnets mounted on its rotor and the rotor magnetic field then locks into that
produced by the stator leading to operation at synchronous speed. For power
generation applications, electromagnets are used on the rotor excited by an
externally applied direct current. Although the rotor operates at the same speed as
the stator magnetic field it leads the stator field by an angle depending on the
applied torque. In contrast, the rotor of a conventional induction machine has a
‘squirrel cage’ winding into which currents are induced as the rotor bars cut the
magnetic field produced by the stator. Hence, an induction generator can only
develop torque at a rotational speed slightly greater than that of the stator field.
This ‘slip speed’ is proportional to the applied torque.
Therefore, to a first approximation, the behaviour of a synchronous machine may
be considered to be analogous to a torsional spring. The torque is proportional to
the angle between the rotor and the stator field. This angle is known as the load or
power angle. In contrast, an induction generator can be thought of as a torsional
damper where the torque is proportional to the difference in speed between the
rotor and the stator field (the slip speed). This is illustrated in simple schematic
form in Figure 6.14. It may be seen that if the simple model of a fixed-speed wind
turbine, equipped with a synchronous generator, is excited by the cyclic torque
from the wind-turbine rotor then there is no damping in the drive train to control
the torsional oscillations. It is a simple two-spring, two-mass system. In contrast,
with an induction generator, the connection of the generator to the network is
represented by a torsional damper. The main cyclic torque of the wind turbine rotor

Blades Network
Generator
Synchronous
generator
Transmission Connection

Blades Network
Generator
Induction
generator
Transmission Connection
Key
Spring Damper Mass
Figure 6.14 Mechanical Analogue of Fixed-speed Wind Turbines