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Modern utility-scale wind turbines have at least five independent control actuators for power production control. These are the electrical generator, the three individual blade pitch drives and the nacelle yaw drive which are used to operate the turbine preferably in optimal con-ditions (cf. Fig. 3.9). Therewith, the WT is eligible for variable-speed variable-pitch (VSVP) operation, cf. [72, 34], which means that the rotor speed can be adjusted for MPPT and that the pitch actuators are actively controlled to regulate the power in the full load regime.

The VSVP control strategy is preferable since it not only increases the energy capture below rated but also limits the rotor power effectively for above rated wind speeds. In addition,

16 The actuator dynamics are not considered in the simplified models. Thus, the blade pitch angles as manipulated variables can theoretically change arbitrarily fast, knowingly that this is physically impossible for an angle or position (mechanical states).

Extended Baseline Controller

n_g

¨ xt

γ p_ref f_ref γ_ref

M_g β₁ β₂ β3

γ_w

Figure 3.9: Block diagram of the extended baseline controller with measurement and reference inputs (on the left) as well as computed control actuations (on the right): p_ref ∈ [0.5, 1] is the power reference, f_ref ∈ [0, 1] the fatigue reference¹⁷ and γ_ref ∈ [0, 2π] the yaw reference (a separate generator speed reference is not needed necessarily)

0 5 10 15 20 25 30 35

0 1.0 2.0 3.0 4.0 5.0

R1 R2 R3 R4

Wind Velocity v_w in m/s ElectricalPowerPginMW

Sharp Cut-Out Soft Cut-Out

Figure 3.10: Illustrative wind turbine power curve with a nominal power of 5 MW including the operating regimes R1 to R4 where vc,i = 4 m/s, v0 = 11 m/s and vc,o = 25 m/s are the cut-in, nominal and cut-out wind velocities (note that the transition between R3 and R4, namely the storm management, is usually realized with a soft cut-out strategy rather than a sharp cut-out)

the mechanical fatigue loads during transition from partial load to full load operation (et vice versa) are considerably lower compared to fixed-pitch machines [17].

The typical operating regimes are displayed in Fig. 3.10. In the region R1 the contribution to the AEP is low so that the turbine cannot be operated economically here. The region R2 is also known as the partial load regime (PLR) where the available power in the wind is not enough to produce the nominal power. Therefore, the controller seeks to maximize the

power output. In contrast, region R3 is denoted as full load regime (FLR) where there is more power in the wind available than can be accommodated by the generator. In region R4 the turbine is shut-down to avoid damages due to stormy weather conditions. The regions R1 and R4 are therefore less relevant for power production control since these do not contribute to the AEP.

A wind turbine must be operated fully autonomously in all operating regimes. For this purpose, there are three main functions in every wind turbine control system, cf. Burton et al. [34] (p. 197, p. 476 ff):

1. The supervisory control manages and supervises the overall power production (ref-erence values), the orderly start-up and the shut-down routines, standby, alarm man-agement and external communication.

2. The safety system jumps in whenever critical operating parameters (generator speed, generator power or component vibration levels, etc.) exceed their nominal range.

Thereby, the wind turbine is brought into a safe operational mode. Depending on the critical parameters and the emergency, it may or may not start independently again. The safety system intervenes only if an event or problem is serious or poten-tially serious. It is thus vital in any turbine.

3. The closed-loop control system takes responsibility for the orderly operation in changing wind conditions and wind regimes. This system is denoted as WTC in the following. As a matter of fact, it controls the above mentioned five actuators in order to achieve all expected control objectives. There are different control loops which have different real-time requirements. For instance, the blade pitch angles must be adjusted rapidly.

The WTC has direct economic implications on the LCoE because it has a lasting effect on the AEP and OPEX (cf. Sect. 1.1 and [45]). As a consequence, the most important control objectives are deduced as follows:

• Maximize the energy harvest:

The controller must maintain the optimum power coefficient by operating the turbine at the optimal tip-speed-ratio λ^∗ in the PLR. This is especially critical for maximizing AEP.

• Limit the rotor torque:

The available aerodynamic power in the wind flow exceeds the allowable maximum generator power in the FLR. Thus, the WTC must make sure that the nominal elec-trical power is not exceeded on average which requires a rapid pitch control actuation (also to limit rotor over speed).

17 The fatigue reference f_ref adjusts the tower damping gain (which puts emphasis on the feedback of the nacelle acceleration ¨xt). fref= 0 means no damping and fref= 1 the maximum allowed damping.

• Reduce nacelle oscillations:

Mechanical vibrations in fore-aft direction correlate with alternating tower bending moments and thus fatigue loads on the tower structure, cf. [34] (p. 492). In order to achieve the desired lifetime of the tower and possibly beyond that, the oscillations must be kept in allowable ranges by active pitch control (mainly in the FLR).

• Minimize yaw misalignments:

The turbine must face the wind in order to produce energy properly which requires an active yaw controller (less relevant in the FLR). Though, due to the cosine effect – meaning cos γ ≈ 1 for small angles – this is not as time-critical as pitch control and needs to be adjusted only from time to time. Additionally, too much yaw control action would result in unacceptable wear on yaw drive system (increasing the OPEX).

In order to achieve these control objectives, a suitable plant controller must be designed.

The WTC used in the present thesis is inspired by the controller architecture proposed by Jonkman et al. [95] which is widely considered as the typical industrial turbine controller, cf. [19] and [185] (p. 47). It consists of two separate controllers for the generator speed ng

which use the generator torque and the collective pitch angle to maximize/limit the generator power. Since this is not sufficient to achieve the third and the fourth objective (see above), the existing baseline controller structure is enhanced by a fatigue controller with variable gain to reduce nacelle oscillations and also by a simplified yaw controller to correct misalignments.

Moreover, a power reference input is introduced in order to be able to vary the desired power production.

In summary, there are in total three measurement inputs to the WTC (cf. Fig. 3.9): The generator speed ng, the nacelle acceleration ¨xt and the yaw angle γw. In addition to that, there are three reference inputs: The power reference pref, the fatigure reference fref and the yaw reference γ_ref. These are processed by the control algorithm and produce the desired control actuations. Further details on the controller are omitted due to sake of brevity and due to the availability of extensive literature [34, 22, 140, 17].