In this section, the HAWT operational model is briefly introduced with the pitch actuator faults. Furthermore, BAPC is also considered. Finally, some technical preliminaries are given. Hereafter, to simplify the subsequent notation if there is no confusion, function arguments are omitted.
2.1. HAWT Operational Model
The wind energy is converted into the rotor kinetic energy by the blades. The effective wind speed
induces the aerodynamic torque
, thrust
, and power
modelled as [
1]:
where
and
are the air density and the rotor radius, respectively. Additionally,
,
and
are torque, power and thrust coefficients, respectively. These factors are functions of the blade pitch angle,
, and the tip speed ratio,
, defined as
[
1].
is the rotor angular speed. Furthermore, the aerodynamic power is as
, which leads to the relation
. The effect of
on the tower causes a bending oscillation [
6]. The displacement of the nacelle is represented by
, measured from its equilibrium position. The effective wind speed at the rotor plane is then obtained as
, where
is the free wind speed, i.e., the wind speed before the blades [
23].
The kinetic energy of the rotor shaft is transferred into the generator shaft, via the drivetrain, with efficiency
and speed ratio
. The rotor and generator shafts inertia are represented by
and
, respectively. Furthermore, the rotor and the generator speeds are denoted by
and
, respectively. Moreover, the drivetrain is modelled as a two-mass system, including the torsion stiffness
and the torsion damping
. Therefore, a torsion angle
is considered, where
and
are the rotation angle of the rotor and generator shafts, respectively. On the other hand, the bearings of the rotor and generator shafts impose the viscous friction, modelled by the coefficients
and
, respectively. The generator converts kinetic energy into electrical energy. Additionally, between the generator and the electrical grid, a converter is placed, regulating the power frequency [
6]. The internal electronic controller of the generator is much faster than the HAWT mechanical dynamic behaviour. So, it is reasonable to assume that the generator torque
is adjusted according to the generator reference torque fast enough to ignore the generator dynamic response. As a result, the electrical power
can be approximated by the following static function [
6]:
where
is the generator efficiency. The power regulation objective can be stated as the generation of the nominal power
under uncertain wind speed variation, while avoiding overspeeding and consequent brake engagement. Accordingly, taking Equation (2) into account, this objective is achieved by the following operation requirements: (i) setting
at its nominal value
; (ii) regulating
at its nominal value
.
The nominal power generation is then achieved as
, where
[
6]. The operation requirement (i) can be simply fulfilled by setting the generator reference torque at
. The operation requirement (ii) can be fulfilled by the pitch angle control. In this manner, the induced aerodynamic torque is controlled. Consequently, the rotor and the generator angular speeds are regulated [
5]. Therefore, the main objective of this paper is to satisfy the requirement (ii).
In order to reduce drivetrain stress, the drivetrain torsion angle variation
is to be kept as small as possible. In this regard, the ideal case can be stated as
, i.e., keeping the rotor and generator speeds at the drivetrain ratio [
24]. As the generator speed is kept at
, then the rotor speed is maintained at
. This represents the reduced drivetrain stress trajectory. Accordingly, the HAWT operational model is given by [
16]:
where,
,
,
,
,
,
,
,
and
.
Considering Equations (1) and (3), the pitch angle control leads to the adjustment of
, and consequently, the aerodynamic torque. This, in turn, regulates the rotor speed. The aerodynamic torque is not a singular function in the operational range of HAWT [
16]. By that means, in the presence of wind speed variation, there always exists a given pitch angle
, and by setting the reference pitch angle
at
, the consequent aerodynamic torque leads to the nominal power generation [
23]. Therefore, the pitch angle controller has to maintain the reference pitch angle
at
, which retains
at nominal values
. This, consequently, regulates
at
, which meets the operation requirement (ii). However, due to uncertain wind speed variation, retaining
exactly at
is impossible and there is always an error [
1]. Therefore, the main aim of this paper is to retain the tracking error as close as possible to zero within the safe-to-operate bounds, i.e., to avoid hazardous overspeeding.
As this work considers a hydraulic pitch actuator, it moves the blades to regulate
at the actuated angle
. The pitch actuator is modelled as [
5]:
with the natural frequency
and the damping ratio
. The pitch actuator operational ranges are limited as
and
. In this paper
and
indicate the maximum and minimum allowable value of the variable
, respectively. Note that HAWT operation in a harsh environment may lead to pitch actuator dynamic change, which reduces the power regulation efficiency. This causes the variation in the natural frequency and the damping ratio of the pitch actuator, which in turn leads to a slower pitch actuator response [
6]. The dynamic change is modelled by the additive signal
in the pitch actuator model, defined later [
5]. Moreover, the pitch actuator may suffer from bias, and effectiveness loss. These lead to the deviation of the actuated pitch angle
from the reference one
, defined by the pitch angle controller [
16], modelled as:
with the unknown pitch actuator bias
and the unknown effectiveness
[
9]. Note that
, where
represents full effectiveness and
is total loss [
9,
25]. More importantly,
is an unknown lower bound of the actuator effectiveness, below which the actuator is unable to keep controlling the system and it practically becomes uncontrollable [
26]. The signal
is the reference pitch angle, which is generated by the pitch angle controller. Clearly, in the case of full effectiveness and no bias,
. Associating the pitch actuator dynamic behaviour of Equation (4), with the pitch actuator dynamic change, bias and effectiveness loss, yields:
Environmental situations, such as rain, snow and dirt, lead to erosion or debris build-up on blades. This, in turn, causes BAPC. As a result, the captured aerodynamic power is reduced [
15]. Consequently, the power regulation is not efficiently achieved. BAPC can be modelled as an aerodynamic torque change
, due to a change in the power coefficient described as
[
18]. These changes are challenging to detect due to their slow-developing (incipient) characteristics. Therefore, it is difficult to determine if the decreased generated power is due to BAPC or reduced wind speed. However, as BAPC occurs slowly, this change is mostly assumed to be solved by the planned annual maintenance, when the blades are cleaned or replaced. Therefore, this paper aims to design a pitch angle controller that is insensitive to BAPC, thus guaranteeing nominal power generation up to the next planned maintenance.
Considering Equations (1) and (3), the rotor dynamic relation is represented by a non-affine function of the pitch angle [
23]. As stated earlier,
is not a singular function. Accordingly, this problem is resolved by using the mean value theorem [
15], i.e., for any given pair of
, there exists
such that
, where
and
. It is worth noting that
with constants
. It can be seen that as effective wind speed
increases, by increasing pitch angle, the aerodynamic torque decreases. Therefore, by taking the time derivative of
, the following relation is obtained:
where
is an aerodynamic torque change due to BAPC [
18]. Moreover, it is worth noting that, as the wind speed is not accurately measurable,
in Equation (7) is an unknown variable. Now, by using Equations (6) and (7) in Equation (3), one can obtain:
This describes the HAWT rotor dynamic response, which takes into account possible pitch actuator dynamic change, bias, effectiveness loss and BAPC. It is worth noting that the HAWT sensor is affected by measurement error, modelled by stochastic processes. For the sake of notation, the measured variable
is represented by the signal
, with
, where
represents a Gaussian white noise process [
1,
27]. Considering this measurement error, the computable expression of the rotor dynamic of Equation (8) has the following form:
where
,
,
and
.
Assumption 1. The bounded achievable values of pitch angle, i.e., and , are limited which leads to the boundedness of as [28]. As varies due to the variation in and , the signal is bounded as [5,16]. The debris build-up and erosion occur very slowly when compared to the scheduled maintenance of the blades. Therefore, it is reasonable to assume that is bounded as [18]. It should be noted that , and are unknown positive constants. Moreover, it is assumed that the noise processes used to represent the measurements errors have a limited bandwidth [1,24]. By considering the bounded variation in , i.e., , is bounded as , where is a positive unknown constant. It can be shown that in Equation (9) is unknown yet bounded, due to the presence of as well as [15]. More importantly, it is assumed that there is always a pitch actuator effort, i.e., , the control gain never becomes zero. Finally, considering the limited generator torque and drivetrain dynamic response of the industrial HAWTs with limited operation range, it can be shown that the induced aerodynamic torque is bounded as [16]. Considering Assumption 1, based on the information extraction technique from the system nonlinearities [
16], there is an unknown non-negative constant
and a computable non-negative function
, i.e., a core function, such that the following inequality is satisfied:
where
and
.