Environmental Engineering Reference
In-Depth Information
LVRT
Low-voltage ride-through
v
Wind speed (m/sec)
Air density (kg/m
3
)
q
R
Rotor radius (m)
P
a
Aerodynamic power (W)
T
a
Aerodynamic torque (Nm)
k
Tip speed ratio (TSR)
C
p
(k)
Power coefficient
b
Pitch angle
x
mr
WT rotor speed (rad/sec)
x
mg
Generator speed (rad/sec)
T
g
Generator electromagnetic torque (Nm)
Turbine total inertia (kg m
2
)
J
t
K
t
Turbine total external damping (Nm/rad sec)
d, q
Synchronous reference frame index
s,(r)
Stator (rotor) index
V (I)
Voltage (Current)
P (Q)
Active (Reactive) power
/
Flux
T
em
Electromagnetic torque
R
Resistance
L (M)
Inductance (Mutual inductance)
r
Leakage coefficient
x
r
(x
s
)
Angular speed (Synchronous speed)
s
Slip
p
Pole pair number
2.1 Introduction
Actually, variable speed wind turbines are continuously increasing their market
share, since it is possible to track the changes in wind speed by adapting shaft
speed, and thus maintaining optimal power generation. The more variable speed
wind turbines are investigated, the more it becomes obvious that their behavior is
significantly affected by the used control strategy. Typically, they use aerodynamic
controls in combination with power electronics to regulate torque, speed, and
power. The aerodynamic control systems, usually variable-pitch blades or trailing-
edge devices, are expensive and complex, especially for larger turbines [
1
]. This
situation provides a motivation to consider alternative control approaches [
2
].
The main control objective of variable speed wind turbines is power extraction
maximization. To reach this goal the turbine tip speed ratio should be maintained at its
optimum value despite wind variations. Nevertheless, control is not always aimed at
capturing as much energy as possible. In fact, in above-rated wind speed, the captured
