Environmental Engineering Reference
In-Depth Information
4.1 Introduction
Wind turbines are highly complex dynamical systems. They are flexible,
mechanical structures subjected to spatially and temporally distributed distur-
bances, with interconnected dynamics, poorly damped, with physical constraints,
etc. Additionally, they are operated and controlled in different modes depending on
the wind speed. The operational region of wind turbines can be divided into three
regions. On the one side, at low wind speeds one finds the partial-load region, also
called region 1, where the main control objective is energy capture maximization.
A complementary objective in this region is to reduce, or at least not to amplify,
the aerodynamic loads [
1
]. At the opposite side of the operational wind speed
range there is region 3, the full-load region. The objective there is to keep the
turbine at its rated operating point. In this region, mitigation of aerodynamic and
mechanical losses is crucial for the useful life of the wind turbine. In between,
there exists a transition region (region 2) where the objective is to achieve a
smooth transition between power tracking and regulation. Therefore, controller
performance is mainly assessed in terms of loads alleviation.
Wind turbine control has been traditionally addressed in two ways. In one way,
a multivariable controller is designed to guarantee satisfactory performance along
the whole wind speed envelope. Some problems like low controllability and poorly
damped oscillations make this task very complicated, leading to solutions that are
too conservative. In the other approach, two different controllers are designed to
fulfill the control objectives for partial- and full-load, whereas a bumpless or anti-
windup compensation avoids undesirable responses after controller switching.
This is the control structure implemented in commercial wind turbines.
For many years, the focus has been on improving controller performance in low
and high wind speeds. Less attention has been given to the transition region where
there was not a clear control objective (see for instance [
2
-
5
]). However, the
detrimental effects of loading is increasing as wind turbine size grows exponen-
tially, thus moving the attention to load mitigation. That is why operation and
controller performance in the transition zone, where the low controllability
problems appear, is now receiving special interest in the wind industry and
academy.
In this chapter, the two-controller approach with anti-windup compensation is
revisited. A robust, gain-scheduling control scheme designed in the H
1
optimal
control framework is discussed in detail, with special focus on the performance in
the transition zone. The turbine is controlled through the generator torque in the
partial-load region, and through the pitch angle in the full-load one. An optimal
anti-windup strategy is also included to achieve smooth operation in the transition
region.
