The response of the system to a step increase of the load and the wind speed,
without any change in the speed governor and pitch controller parameters
(R=8%, Ki=2.5, AP=10, BP=4), is shown with a solid line in Figs. 14 and 15. Comparing
Fig. 14 with Fig. 10, where an equivalent stall controlled AWT is used, it is
noted that the behaviour of the system is significantly more oscillatory and the
maximum frequency excursion is increased. The origin of the slow oscillations in
Fig. 14 is the underdamped mode resulting from the interaction between the WT
pitch controller and the DG speed governor. Such a ‘governor–pitch control’ mode
has been observed in existing autonomous power systems with significant penetration
The large frequency excursions can be reduced by selecting a lower droop value.
Hence, the speed governor parameters can be set to the values for operation without
the wind turbine (R=5%, Ki=4). In order to increase the damping of the pitch–governor mode, the gains of the pitch controller are reduced to AP=5 and BP=2. The
response of the system using the new settings is shown with dashed line in Figs. 14
and 15. The dynamic behaviour is clearly improved, exhibiting much better damping,
more effective frequency control and reduced pitch movement, particularly in case
of load disturbances. The price for this is an increase in the transient overshoot of
the wind turbine output power in case of wind gusts, as it can be seen in the corresponding
diagram of Fig. 15. Yet, the observed over-power is of the same order as for
operation against an infinite bus (Fig. 13) and therefore it is regarded as acceptable.