Pitch-Controlled Variable-Speed
Wind Turbine Generation
February 2000 • NREL/CP-500-27143
E. Muljadi and C.P. Butterfield
Presented at the 1999 IEEE Industry Applications
Society Annual Meeting
Phoenix, Arizona
October 3-7, 1999
National Renewable Energy Laboratory
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1
PITCH-CONTROLLED VARIABLE-SPEED
WIND TURBINE GENERATION
E. Muljadi C.P. Butterfield
National Wind Technology Center
National Renewable Energy Laboratory (NREL)
1617 Cole Boulevard
Golden, CO 80401, U.S.A.
Abstract - Wind energy is a viable option to
complement other types of pollution-free generation.
In the early development of wind energy, the
majority of wind turbines were operated at constant
speed. Recently, the number of variable-speed wind
turbines installed in wind farms has increased and
more wind turbine manufacturers are making
variable-speed wind turbines.
This paper covers the operation of variable-
speed wind turbines with pitch control. The system
we considered is controlled to generate maximum
energy while minimizing loads. The maximization of
energy was only carried out on a static basis and
only drive train loads were considered as a
constraint. In medium wind speeds, the generator
and power converter control the wind turbine to
capture maximum energy from the wind. In the
high wind speed region, the wind turbine is
controlled to maintain the aerodynamic power
produced by the wind turbine. Two methods to
adjust the aerodynamic power were investigated:
pitch control and generator load control, both of
which are employed to control the operation of the
wind turbine.
Our analysis and simulation shows that the
wind turbine can be operated at its optimum energy
capture while minimizing the load on the wind
turbine for a wide range of wind speeds.
I
NDEX TERMS
Wind turbine generator, renewable energy, pitch-
controlled, variable speed.
I. INTRODUCTION
The development of wind turbine power
generation has been expanding during the past 10 years.
The global market for the electrical power produced by
the wind turbine generator (WTG) has been increasing
steadily, which directly pushes the wind technology into
a more competitive arena. Recently, there have been
positive trends shown by the utilities to offer renewable
energy to customers. Many customers who are
environmentally conscious now have the option of
subscribing to clean energy such as wind energy from
the power provider. The European market has shown
an ever-increasing demand for wind turbines.
Variable-speed wind turbine generation has been
gaining momentum, as shown by the number of
companies joining the variable-speed WTG market.
Variable-speed generation is claimed to have a better
energy capture and lower loading. The effect of
turbulence on energy capture and power fluctuations in
variable-speed wind turbines is affected by the overall
control algorithm used [1-4]. The method of
controlling the generator strongly affects the electrical
power generated by the generator [2]. Different types
of generators are used for variable-speed generation for
direct drives [2-3]; however, control algorithms for
wind turbine operation are not discussed in these
papers.
The goal of this project is to study the behavior of
the wind turbine generator operated under variable
speed with pitch-control capability under turbulent
winds. The basic comparison between a constant-speed
wind turbine and a variable-speed wind turbine will also
be explained. The constant-speed wind turbine
Power Converter
+ Generator
Pitchable Blade
Gear Box
Utility
(60 Hz)
Wind
Low speed shaft
Hi
g
h s
p
eed shaft
Figure 1. Physical diagram of the system
2
operation is a very simple case that can be used as a
baseline. The variable-speed algorithm was chosen
based on the maximum energy and steady-state limit of
the wind turbine. The steady-state limit is based on the
C
P
-TSR curve provided. Thus in the steady state
calculation, wind gust and wind shear are not
considered. It turns out that using the steady state limit
is a good approximation in the lower wind speeds, as
shown by the performance ratio and energy captured by
the wind turbine.
One concept that is fundamental to the control
dynamics is that the speed change is relatively slow
because of the large inertia involved. This makes it
difficult to use the power converter to control the speed
in highly variable wind applications. Pitch control is
relatively fast, however, and can be better used to
regulate power flow especially when near the high
speed limit.
Figure 1 shows the system under consideration.
The wind turbine is connected to a variable-speed wind
turbine. The generator output can be controlled to
follow the commanded power. The wind turbine has a
pitchable blade to control the aerodynamic power. The
dashed line indicates that the pitch angle can be
controlled. It is shown that there is a mechanical
component (such as a gearbox) between the high-speed
shaft and the low-speed shaft. The low-speed shaft is
driven by the turbine blades, which generates
aerodynamic power. The high-speed shaft is loaded by
the electric generator in the form of electrical load.
The paper is organized as follows: The next
section is devoted to the condition of the wind data.
The third section is devoted to the method of control.
In the fourth section, the discussion and analysis is
presented, and in the fifth section, the conclusion is
presented.
II. W
IND TURBINE CHARACTERISTICS
The wind turbine can be characterized by its C
P
-
TSR (curve as shown in Figure 2), where the TSR is the
tip-speed ratio; that is, the ratio between the linear
speed of the tip of the blade with respect to the wind
speed. It is shown that the power coefficient C
P
varies
with the tip-speed ratio. It is assumed that the wind
turbine is operated at high C
P
values most of the time.
In a fixed-frequency application, the rotor speed of the
induction generator varies by a few percent (based on
the slip) above the synchronous speed while the speed
of the wind may vary across a wide range.
In Figure 2, the change of the C
P
-TSR curve as the
pitch angle is adjusted is also shown. In low to medium
wind speeds, the pitch angle is controlled to allow the
wind turbine to operate at its optimum condition. In the
high wind speed region, the pitch angle is increased to
shed some of the aerodynamic power.
From Equation 1, the tip-speed ratio for a fixed
speed wind turbine varies across a wide range
depending on the wind speed. The power captured by
the wind turbine may be written as Equation 2. From
Equation 2, it is apparent that the power production
from the wind turbine can be maximized if the system is
(4)
(3)
)
(RPM
K
=
P
TSR
R
C
A0.5 =
P
3
PTARGET
3
m
TARGET
3
p
TARGET
TARGET
ω
ρ
ú
û
ù
ê
ë
é
speedrotorRPM _
data turbine windcomputed =
K
P
TSRat CpCp
)
C
p
(max power Target =
P
:where
TARGETTARGET
TARGET
=
=
(m/s) velocity wind= V
turbinewindoftcoefficien =
C
p
m
2
( areaswep =A
m
3
(kg/density air =
:where
(2)
V
3
C
p
A0.5 =
P
mech
)t
)ρ
ρ
V
R
m
= R
meter/sec wind theof speedlinear = V
meter blade theof radius = R
radian/sec mechanical speedrotor =
m
:where
(1)
TS
ω
ω
C
p
for different Pitch Angle
0
0.1
0.2
0.3
0.4
0.5
0.6
0 5 10 15
TIP-SPEED RATIO (TSR)
C
p
(Power Coefficient)
Angle = 0 deg
Angle = 6 deg
Angle = 10 deg
Figure 2. Power coefficient C
p
versus tip-speed ratio
3
operated at maximum C
P.
As the wind speed changes,
the rotor speed should be adjusted to follow the change.
This is possible with a variable-speed wind turbine.
Unfortunately, it appears that the wind speed cannot be
reliably measured. To avoid using the wind speed, the
equation to compute the target power can be rewritten
by substituting the wind speed V and the C
P
. The target
power P
TARGET
can be written as in Equation 3, or in it
can be written in its simple form shown in Equation 4. It
can be seen that the P
TARGET
is proportional to the cube
of the rotor speed.
For simulations used in this study, the wind input
data is a time series of wind data of different turbulence
conditions. To reduce the computing time, the input
data (the wind speed) of a 10-minute time series is used.
The work performed in this project was based on a
generic wind turbine. The physical dimension and the
C
P
-TSR characteristic of the wind turbine are the inputs
for this program. The inertia of the blade and the inertia
of the generator are given. The stiffness of the shaft and
the damping are given. The induction generator
modeled is a wound-rotor induction generation with the
stator connected to the utility and the rotor winding
connected to the power converter. The generator can
be controlled to respond to the torque command almost
instantaneously.
III. M
ETHOD OF CONTROL
A. Wind Turbine Power Generation
The simplest wind turbine architecture is the
constant rotor speed and constant pitch wind turbine.
Figure 3 shows a typical aerodynamic power for two
different wind speeds as the rotor revolutions per
minute (rpm) is varied. As shown in Figure 3, the
maximum C
P
operation is represented by the thick line.
The wind turbine operating in a single rpm will only be
optimized at a single wind speed. For example, if the
wind turbine is operated at 1500 rpm, at 20 m/s wind
speed, the wind turbine operates at point B’, which is
not optimum power. At 15 m/s, for 1500 rpm, the wind
turbine operates at point B, which is the optimum
operating point. Similarly, the operation of the wind
turbine at 2000 rpm is optimized only at 20 m/s, and is
not optimized at 15 m/s.
In variable speed operation, we must consider
blade inertia. Wind turbine blades have a large inertia
compared to the inertia of the generator. The inertia of
the rotor behaves like an inductor in an electrical
circuit. It helps smooth the rotor speed variation, and it
stores energy during acceleration and restores energy
during deceleration. A power flow diagram of variable-
speed operation is shown in Figure 4. In this figure, it
is assumed that the operating C
P
is constant at C
Pmax
.
P
wind
is the aerodynamic power that can be extracted
from the wind. P
captured
is the actual aerodynamic power
captured by the wind turbine. P
electric
is the power that
can be converted to electric power. When the
aerodynamic power is higher than the generator power
(P
electric
), the rotor rpm increases and the kinetic energy
in the rotor blade increases. The rate of increase in the
kinetic energy is the power difference between the
aerodynamic power and the generator power, which is
called P
accel
and P
decel
in Figure 4.
Acceleration
Deceleration
P
wind
P
electric
P
electric
K inetic
Energy
Kinetic
Energy
P
wind
P
captured
P
captured
P
wind
P
accel
P
electric
P
decel
P
electric
K inetic
Energy
P
wind
P
captured
P
captured
Figure 4. Kinetic energy in the turbine at constant C
P
during acceleration and deceleration
Figure 3. Aerodynamic power versus rpm for two
different wind speeds
1000 1500 2000
0
50
100
High speed shaft rotor speed (rpm)
Aerodynamic power (kW)
15 m/s
20 m/s
B’
B
A’
A
C
pmax
operation
