The main turbine controller aims at controlling speed
using pitch control [Fig. 13(c)]. Up to the time instant
t=350 s, the pitch control is not very active because maximum
power is not reached. Hence, the main controller
seeks to maximize output power according to the maximum
efficiency curve shown in Fig. 4. Beyond 350 s, one
can see wind speed going up to approximately 15 m/s. Fig.
13(d) shows that the wind turbine controller now limits
the torque command at 100%. The actual output power
delivered to the grid is shown in Fig. 13(e) and matches the
command value perfectly.
Notice that in this constant, maximum power mode,
the pitch controller sets the blades to keep speed within
bounds. At the time instance t=450 s, the elasticity of the
variable-speed DFIG wind turbine system is demonstrated.
For a short while, wind speed rapidly reaches 17
m/s. The pitch controller is not capable of following this
fast gust of wind. Hence, the speed of the turbine blades is
allowed to increase storing energy into the turbine’s inertia.
During this transient, output power remains practically
constant, avoiding power surges into the power grid.