Wind power technology - Wind Power Integration: Connection and System Operational Aspects - page 71

Induction
generator

Gearbox

Horizontal
axis rotor

Pendant
cable in
tower

Power factor
correction
capacitors

Turbine
transformer

Anti-parallel
thyristor soft
starter

Figure 3.16 Schematic of fixed speed wind turbine

and wind shear effects. By an unfortunate coincidence these aerodynamic torque
variations are often close to the natural frequency of oscillation of the connection of
a synchronous generator to the network. Thus, it is not possible to use synchronous
generators directly connected to the network and a simple mechanical drive train in
fixed-speed wind turbines. Some early wind turbines, using synchronous gen-
erators, included mechanical dampers in the drive train but modern fixed-speed
wind turbines all use induction machines.
Although supplying significant damping, induction generators suffer from a
number of important disadvantages. The damping is proportional to the slip (the
difference between rotor speed and that of the stator field) and is developed by power
losses in the rotor. Thus a high slip (say 1%) that is desirable to damp drive train
oscillations results in 1% of the generator output being generated as heat in the rotor.
An induction machine does not have a separate field circuit and so there is no
direct control over-reactive power. There is a fixed relationship between real and
reactive power (Figure 3.17). The operating locus in the generating region is shown as
the line A-B. Even at zero real power output, reactive power (MVAr import) is
required to energise the magnetic circuits of the machine. As the real power export is
increased, additional reactive power is drawn from the network. The effect of the
power factor correction capacitors is to translate the operating curve vertically
downwards. When the generator and capacitors remain connected to the power
system then the network determines their voltage. However, if a network fault occurs
and the generator and capacitors are isolated, then there is the possibility of a resonant
condition, known as self-excitation, which can result in significant over-voltages.