18-09-2012, 10:29 AM
PITCH-CONTROLLED VARIABLE-SPEED WIND TURBINE GENERATION
1PITCH-CONTROLLED.pdf (Size: 321.05 KB / Downloads: 137)
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 variablespeed
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.
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.
WIND TURBINE CHARACTERISTICS
The wind turbine can be characterized by its CP -
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 CP varies
with the tip-speed ratio. It is assumed that the wind
turbine is operated at high CP 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 CP-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.
METHOD OF CONTROL
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 CP 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.
ANALYSIS AND DISCUSSIONS
We implemented the simulation of control
strategy. The rotor rpm limit rpm1 (point C) is chosen
to be 34.8 rpm. The maximum power is chosen to be
750 kW. The rpm headroom is chosen to be between
10% to 20% of the maximum rpm. The pitch rate is
chosen to be 4/0.7 degree/s. That is, the pitch angle can
be ramped up at 4 degrees per second and it can be
ramped down at 0.7 degree per second. The wind speed
chosen is 14 m/s with a rough-turbulence case. The
hysteresis rpm is chosen to be 2% of the maximum rpm.
CONCLUSIONS
We simulated a pitch-controlled, variable-speed
wind turbine operating in turbulent winds. We
described the philosophy behind the control strategy.
Slow and fast pitch rates were investigated, and their
impacts were shown on the operating characteristics of
the wind turbine.