Environmental Engineering Reference
In-Depth Information
6.3 System Control
6.3.1 General
6.3.1.1 Optimum Control by Model Characteristic
Control concepts operating without measuring the wind speed can be used when
wind turbine parameters are known. Considering the variable wind direction and
velocity the determination of correct test values is not a simple task. Methods out-
lined in the following may therefore be of advantage for small up to medium speed-
variable systems.
The torque produced by a wind turbine can be described in dependence of the
rotor speed by rearranging (2.5) using (2.5) in the equation:
=
2
A
D
3
c
p
(
λ
)
P
Ω
2
T
=
Ω
(6.53)
2
3
λ
According to Fig. 2.6 a turbine operates at
c
p
,
max
when the tip speed ratio is equal
to the design value
A
in (6.53) the torque is proportional to
the square of the rotational speed, the turbine operating at maximum power. In (6.54)
K
is a constant for given turbine parameters:
T
=
λ
A
. When setting
λ
=
λ
2
A
D
3
c
p
(
λ
A
)
λ
A
3
2
=
K
2
Ω
·
Ω
(6.54)
2
Equation (6.54) is then the control rule for the torque set-value in dependence of
rotational speed. The algorithm is valid between cut-in speed and rated operation
for turbines with fixed blade position, i.e. keeping the pitch angle constant.
6.3.1.2 Optimum Control by MPP Tracking
Another method of optimum control of a variable speed wind turbine is based on
the maximum power point (MPP) tracking. The method may be illustrated using an
analogy with photovoltaic system control. Figure 6.11 (left) shows in principle the
characteristics of a PV-generator: currents
I
versus voltage
U
with the parameter
solar radiation
E
; the line of maximum power is indicated. In Fig. 6.11 we see the
characteristics of a wind turbine: torque
T
versus speed
with the parameter wind
velocity
v
; the relevant
P
max
line also indicated. A similarity is obvious; of course
the rotating system will have a larger time constant than the PV system has.
Similar to MPP tracking in photovoltaic systems [Qua07], a searching algorithm
can be established [Schi02] which checks variations of power
P
and rotational speed
Ω
Ω
at equally spaced time intervals, and adjusts the set-point speed by increments
according to a suitable algorithm:
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