*3.1. Power Curtailment*

To address the power curtailment, the PV generator cannot be working at the maximum power point. Instead the control works close to the reference of active power (*Pref*) given by the PPC. Thus, a Reference Power Point Tracker (RPPT) is used. Considering the P-G curve, it can be seen that the MPPT control is applied until a point of solar irradiance depending on the power reference. Although, the solar irradiance increases, the PV generator can only supply the reference power by using the RPPT control. In this case, the RPPT control can be applied in Region II and III of the P-G curve (Figure 8a).

The control of active power will be managed according to two variables: dc voltage variation and the active power reference (*Pref*). For that purpose, any algorithm used for MPPT can also be used in RPPT but the target point is what changes. The most common algorithm is Perturb and Observe, that is used in the present study. In this case, the dc voltage is changed by small steps (Δ*v*) until the active power generated by the PV array is the same as the power reference. Each time the solar irradiance changes, the algorithm should only decide if the dc voltage reference should increase or decrease its value. On this control, the dc voltage limits are also considered according to the PV array and the inverter limitations (*vmin*, *vmax*). However, as the solar irradiance changes, the reference of active power could be higher than the maximum possible power that the PV generator can supply. In this case, the algorithm starts to work as a normal MPPT (Figure 8b).

**Figure 8.** Active power curtailment (**a**) P-G curve when a reference is given and (**b**) logic between maximum power point tracker (MPPT) and Reference Power Point Tracker (RPPT).

An example of this control is illustrated in Figure 9, where the first point is for a given solar irradiance (blue line) and with a voltage equal to the open circuit voltage (*voc*). At this point, the active power that the PV generator can supply is equal to 0. Then, the dc voltage will reduce its value in small steps Δ*v* in order to be close to the reference (2). In the case, the solar irradiance reduces, the new point of operation will be in (3). Then, again the control will change its dc voltage to ge<sup>t</sup> close to the reference. As the maximum power at the new solar irradiance is less than the reference, the control will change to MPPT instead of RPPT until it gets the maximum power (4). If the solar irradiance suddenly increases (red line), the PV generator operates in a new point (5). As the dc voltage is equal to *vmpp*, this has to increase in small steps until the active power generated is the same as the reference given by the PPC (6). This algorithm is presented in the block diagram illustrated in Figure 10.

**Figure 9.** RPPT operation in a PV generator. (1) Initial point. (2) Point of operation by using the algorithm. (3) Change of point of operation because of solar irradiance. (4) New maximum power point. (5) Reduce of power. (6) Reference of active power.

**Figure 10.** RPPT control algorithm.
