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
from pitch controlled wind turbines [2]. The increased frequency excursion, on the
other hand, is due to the fast action of the pitch regulator, which reduces the transient
load-sharing capability of the wind turbine. More specifically, an increase of the loadpower is initially shared among the DG and WT electrical generators, resulting in
a transient increase of the WT output power. If a pitch controlled machine is used,
this increase is detected by the pitch controller and is partly compensated by an
increase of the pitch angle. Thus, the diesel generator initially undertakes a larger part
of the load increase and therefore its frequency undergoes a more severe transient.