Environmental Engineering Reference
In-Depth Information
Fig. 3.2
the block diagram of the MPPT control system
x
wopt
¼
k
opt
r
ð
3
:
9
Þ
v
w
Although this method is simple, it needs an additional anemometer for mea-
suring the wind speed in the tip of the wind turbine, which is much difficult.
Another more popular method for MPPT is based on the active power of the
generator. Suppose that the generator in the wind turbine is an induction generator.
The block diagram of the MPPT control system for the wind turbine is shown in
Fig.
3.2
.
For the induction generator, the active and reactive power can be expressed as
follows, respectively:
P
¼
i
qs
u
qs
þ
i
ds
u
ds
ð
3
:
10
Þ
Q
¼
i
qs
u
qs
i
ds
u
ds
ð
3
:
11
Þ
where i
ds
and i
qs
are the stator currents in d-q axes, u
ds
and u
qs
the stator voltages in
d-q axes.
In Fig.
3.2
, the search algorithm is basically based on the calculation of the
power gradient to the speed of the wind turbine. Since the curve of the power versus
the turbine speed is convex, there is no local maximum among the whole turbine
speed. Hence based on the power gradient, the search algorithm can find the
maximum point corresponding to the maximum power point of the wind turbine
and finally produce the reference speed of the generator. To obtain maximum power
from the wind, the speed of the wind turbine should be always controlled to rotate at
the optimal speed x
xopt
by controlling the speed of the generator.
3.4 Model of Wind Energy Conversion System
The block diagram of the WECS is shown in Fig.
3.3
. The back-to-back PWM
converters are utilized in the WECS, and consist of the generator-side PWM
converter, the intermediate DC circuit and the grid-side PWM converter. As shown
in Fig.
3.3
, the left is the distribution power networks, the right the generator,
