Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 199

For most power systems, about two thirds of the stored energy will be provided
by generation, in the form of synchronous machines driven by multi-stage turbines,
with the remainder coming from the load. As wind penetration levels increase,
these synchronous machines will be displaced gradually with a mixture of wind
turbine generators: fixed-speed (induction generators); partial variable-speed
(doubly fed induction generators); and full variable-speed (direct-drive synchro-
nous generators). For a given turbine rating, the inertia, and hence the stored
energy, will be of similar magnitude for each design. However, the ease with which
this stored energy can be extracted by the power system varies greatly depending
on the technology.
Until the late 1990s, wind turbines were predominantly based on fixed-speed
squirrel-cage induction generators. Such machines operate above synchronous
speed, with a small speed variation of around 2 per cent corresponding to an
increase from no-load to full-load output (Figure 3.14). Should the frequency fall
by say 1 per cent (0.5 Hz) following a loss of generation event, the wind turbine
generator could possibly double its electrical output, as illustrated by the fre-
quency-shifted torque-speed characteristic of Figure 5.32. If the wind speed, and
hence the mechanical input power, remain more or less unchanged over a short
time period, then the induction machine will decelerate, releasing stored energy to
the system. The magnitude of this response will depend on the combined inertia of
the induction generator and the wind turbine rotor. Theoretical calculations
and practical studies have suggested typical inertial constant, H gen , values of
3-5 seconds, comparable with conventional generation (Holdsworth et al. , 2004;
Littler et al. , 2005). It could, therefore, be expected that large-scale growth in fixed-
speed designs will not unduly affect the system inertia, and, hence, the ROCOF
during a system transient. Some wind turbine manufacturers have introduced
modifications to the basic induction machine design to increase the operating speed

Synchronous speed
(∝ system frequency)

Speed (rpm)

1

2

Figure 5.32 Induction machine torque-speed characteristic