*4.4. Hydraulic Variables*

The flow rate and HT mechanical power plots for the KS and no-KS cases in solid and dotted line, respectively, are presented until *t* = 45 s in Figure 11. As commented on, the frequency transient after the connection of +40 kW at *t* = 1 s is managed by the DL PID, but the small frequency variations also causes the HT PID to slightly react and this explains the small perturbation in the flow rate after *t* = 1, as seen in Figure 11. At *t* = 5.1 s both KS and no-KS flow rate curves left the zero value due to the response of the HT-PID to the frequency falling after the −1 m/s wind step at *t* = 5 s. The flow rate in the KS case increases faster due to the added signal from the KS transfer function triggered by the active power deficit detector activation. The time when the flow rate reaches the *qnl* value (0.1 p.u.) is for the KS and no-KS cases 6.2 and 6.43 s, respectively. From that time on, the mechanical power produced by the HT, *Ph*−*mec*, rises from the initial value of zero as can be also seen in Figure 11, so the interval from *t* = 5 s until times 6.2/6.43 s is a dead time where there is a system active power deficit, as the WTG power does not reach the demanded active power and therefore the system frequency falls due to the imbalance. Now, it is seen by means of graphics the importance of a small *qnl* in the HT efficiency curve commented in Section 2.1 in order to minimize the negative frequency peak in the WO to WH mode transition. In the HT mechanical power plots, it is also seen that the KS case follows the flow rate curve with less delay and the reason is a better transient of the head at the turbine admission.

*Energies* **2020**, *13*, 5937

**Figure 11.** Hydraulic variables.
