3.3.1. Considered Alarms and Reaction Maneuvers

Table 1 lists all the considered alarms of the supervisory control. The first column shows the measured sensor, the second column the limit and the third column the requested maneuver. In the third column, NS stands for normal stop and ES for emergency stop. These are the only two currently implemented maneuvers of the controller and are explained below.

The alarm block considers five sensors: rotor speed Ω, electrical power *P*, yaw misalignment *γ*, pitch angles *θ* and grid status. Both Ω and *P* have two limits (n4, p4 and nA, pA respectively) that, when surpassed, trigger instantly the appropriate stop maneuver. The same simple trip-logic is implemented for the grid status signal. At the instant that the grid status is measured as offline, a normal stop maneuver is triggered.

For the other sensors, the alarm is triggered if the sensor surpasses the limit for a given time period. The alarm block foresees two limits for *γ* with two different duration times. The first limit *γmax*−<sup>1</sup> considers long-time yaw misalignments. The measured yaw misalignment *γ* has to be larger than this limit for a time interval greater than *τγ*−1, which should be fairly large. In contrast, *γmax*−<sup>2</sup> considers short-term extreme yaw misalignments. The yaw misalignment signal used to trigger the alarms is filtered with a first order low pass filter (Equation (A1)) to even out variations coming from inflow turbulence.



For the pitch angle group, there are two conditions that can trigger alarms. The first one is the difference between the individual pitch angles (Δ*θij* or Δ*θi*−*CP*). This difference is calculated differently depending on the active pitch control strategy. For the CPC strategy (Section 3.2.1), the alarm block checks the difference between all three angles. If the difference between any two pitch angles is greater than the fixed limit Δ*θmax*−*CPC* for a given time *τ*Δ*θ*−*CPC*, then the alarm is triggered and a normal stop maneuver is requested. For the IPC strategy (Section 3.2.2), the pitch angle differences are measured against the collective pitch angle signal. The limit Δ*θmax*−*IPC* is not constant but varies depending on the short-term mean difference between the IPC angle and the collective pitch angle (plus a used-defined offset angle). The short-term mean difference of the individual pitch angles is obtained by applying a first-order low pass filter to the instantaneous mean difference. The time constant for this filter is a user-defined parameter. By using this triggering strategy, the supervisory control can detect pitch angle faults even when the difference between two pitch angles lies below the maximum allowed pitch difference for the IPC strategy.

The second condition included in the pitch angle alarm group is the difference between the measured and the demanded pitch angle for each blade Δ*θset*. If the difference between the two signals in any blade is greater than Δ*θset*−*max* for a given time *τ*Δ*θ*−*set*, then the alarm is triggered and a stop maneuver is requested.

The two stop maneuvers used in this controller are defined by a sequence of actions that are taken (almost) independently of the controller signals. The only exception is the normal stop maneuver that additionally uses the pitch angle differences, as explained below. The parameters for both maneuvers are listed and explained in Table 2. Note that there are two sets of parameters: one for the normal stop and one for the emergency stop procedure.

The implemented stop maneuvers foresee that the collective pitch angle will increase at a rate ˙ *θ<sup>S</sup>* up to a maximum value *θmax*−*S*. This pitching towards feather can be delayed for a given time using the parameter *τθ*. Parallel to the pitch controller, the torque controller decreases linearly to *Qmin*−*<sup>S</sup>* at a rate given by the parameter *<sup>Q</sup>*˙ *<sup>S</sup>*. This procedure can also be delayed for a given time using the parameter *τQ*. In case of a grid loss, a chopper can keep the generator torque constant for a given time *τChopper* in order to avoid very high rotor speeds. There is also the option to enable the turbine brake during the stop maneuvers. This is controlled with an enable flag and a minimum rotor speed (ΩBrake−*S*) at which the brake is activated.

In addition to these fixed parameters, the normal stop procedure checks the pitch angle differences between all blades. If the absolute value of a given difference is above 0.5◦, then the pitch rate ˙ *θ<sup>S</sup>* is replaced by the minimum value between ˙ *<sup>θ</sup>max* and 1.5 · ˙ *θS*. This is done for all blades except the one with the highest pitch angle. This way the pitch angle differences are minimized during the stop maneuver avoiding unnecessary oscillations due to aerodynamic imbalances of the rotor. The emergency stop does not have this feature as it is designed to be hard coded in the turbine's safety mechanism.


