*3.3. Hybrid Plant Set Point Tracking*

Although the PV plant can perform set-point tracking, it can only do so within certain limits of available PV power. Thus, the reserve provided by the PV system was used for primary frequency response and TPSH response enhancement only. When the set-point for the hybrid plant changes, the PV plant momentarily responds to correct the error between the hydro plant and the reference model as dictated by the plant control system in Figure 3.

In Figure 9b,d,f, at *t* = 10 s, when the reference increased, the PV system was first to respond and then attempted to absorb the reverse response of the hydro turbine governor system. This capability decreases the response time of the plant and prevents the hydraulic nonlinearities from interfering with the grid, as can be seen in Figure 9a,c,e. However, the PV plant did not compensate for the tracking error completely due to the delays in the inverter control. Moreover, due to its nonlinear nature, the TPSH model exhibited greater oscillations on set-point reduction, which resulted in an increase in transient set-point tracking error. Nonetheless, this capability also provided the plant with pump mode flexibility, as can be seen in Figure 9e.

**Figure 8.** Plant control capability when the (**a**) reference was equally divided, and (**b**) reference was divided proportional to the reserve; (**c**) inverter control performance.

#### *3.4. PV Firming Results*

TPSH can perform firming only in generation mode and pump mode with HSC active. While mechanical governors cannot be used to compensate for rapid changes in PV, they can be used for slow changes in PV. A low-pass filter is used to filter out the fast transients in the PV output as shown in Figure 3, while the self-compensation feature through ΔPPV\_comp of the PV inverter controls is used for compensating fast changes in PV and for the damping transients due to mode change from pump to generation mode of the TPSH. Figures 10–12 show a case study for one day. To ease the simulation burden, certain sections of the day were picked and simulated separately.

**Figure 9.** Set-point tracking capability and individual power output of the PV and TPSH plant in (**a**,**b**) generation mode, (**c**,**d**) pump mode with hydraulic short-circuit, (**e**,**f**) pure pump mode.

**Figure 10.** PV firming from *t* = 420 min to *t* = 450 min showing (**a**) irradiance profile, (**b**) required TPSH power to meet baseload, (**c**) firmed PV power (**d**) power output from PV and TPSH during dynamic simulation to meet set-point (**e**) speed and (**f**) terminal voltage during simulation.

**Figure 11.** PV firming from *t* = 610 min to *t* = 640 min showing (**a**) power output from PV and TPSH during dynamic simulation to meet set-point (**b**) speed and (**c**) terminal voltage during simulation.

**Figure 12.** Transition firming showing (**a**) net response of the hybrid plant and (**b**) power output from PV and TPSH during dynamic simulation to meet set-point.

The steady-state simulation of the day is shown in Figure 10a–c. In Figure 10a, we see the PV profile with and without curtailment and the base load. The aim of the controls was to modify Figure 10a into Figure 10c using actions in Figure 10b. The base load in the morning hours was ignored, and the plant was allowed to ramp up to the base load, which would support load increase. Figure 10d–f display the dynamic simulation results for a period of 30 min (from *t* = 420 min to *t* = 450 min) of the profile in Figure 10a. During this period, the TPSH operated in pump mode with HSC and was seen to adjust its output to minimize the error in set-point tracking. By operating in pump mode with HSC, it prevented curtailment of the PV system. However, since the TPSH was prevented from transitioning into the generation mode and due to reserve limitations, it could not reduce the plant's tracking error to zero.

Figure 11 shows the firming effect of the controls using the generation mode of the TPSH. Due to the availability of upward reserve, the set-point was tracked with small transient errors. As shown in Figure 10a, the TPSH changed its mode from HSC to generation, as scheduled to meet the increase in base load from 50% to 70%. The dynamic simulation of this phase is shown in Figure 12. The TPSH had a minimum generation of 40%, and the available PV was 50%. The controls curtailed the PV system to compensate for the transition transients of the TPSH and meet the base load of 70%. The PV system saturated and was unable to completely compensate for the transition transients.
