*5.2. PV-PRC-VSG*

In the previous section, the PV-PRC implementation and the voltage operating point were verified. The PV-PRC-VSG technology aims to enable the PV system to provide a certain power support for the power system. This requires DC/DC side and DC/AC side coordinated control. When the system power shortage causes the frequency to change, the DC/DC side provides power support for the power system by adjusting the standby rate R to release or expand the reserve power. The simulation parameters are shown in Table 3.


**Table 3.** Simulation parameters.

• System frequency rise

The simulation time is 1.5 s, and the system frequency does not change from 0 to 0.7 s. At 0.7 s, the power system has the problem of excess power, which leads to the sudden change of the system frequency from 50 Hz to 50.3 Hz, and it returns to normal at 1.3 s. The frequency variation of the system is shown in Figure 7a. Because of the inertia, the rise curve of the system frequency is smoother. Equation (7) shows that when the system frequency increases, the reserve power rises to 5 kW, and the reserve rate increases to 25% as calculated by Equation (8), and as shown in Figure 7b. The active power of PV output is further reduced to 15 kW, as shown in Figure 7c, and the output voltage and current of the inverter are shown in Figure 7d. The results are in agreemen<sup>t</sup> with those of the formulas of Equations (7) and (8). The whole response process essentially maintains power system stability by abandoning a part of PV energy.

**Figure 7.** System output waveform when frequency increases by 0.3 Hz.

• System Frequency Drop in a Small Range

The simulation parameter setting and the initial reserved power amount are the same as above. When the system loses part of the external power due to generator failure at 0.7 s, the system frequency is reduced from 50 to 49.9 Hz, and it returns to normal at 1.3 s, as shown in Figure 8a. When the system frequency drop is detected, the PV system needs to release a part of the reserve active power. According to Equations (7) and (8), the standby rate reference value needs to be changed from 10 to 5%, as shown in Figure 8b. During the frequency drop, a part of the active power is released by the reduction of the reserve ratio to participate in the primary frequency response of the power grid, and the output power is as shown in Figure 8c, and Figure 8d shows the output voltage and current of the inverter.

**Figure 8.** System output waveform when frequency decreases by 0.1 Hz.

• Serious Decrease of System Frequency

The initial parameters of the simulation are the same as above. When the large load is suddenly cut in 0.7–1.3 s, the system frequency is reduced from 50 to 49.7 Hz. According to the setting, the PV system reduces the reserve ratio and increases the active output. Due to the limited active reserve of the PV system, the frequency response will be limited. Equation (7) shows that when the frequency drops by more than 0.2 Hz, the PV system will release all reserve power. Therefore, under this condition, the PV system reserve ratio is changed from 10 to 0%, all the active reserve is released, and the grid is integrated into the grid in MPPT mode. The simulation diagram is shown in Figure 9a–d. It can be seen that the frequency modulation effect of the PV-ESSs-VSG system in reference [13] can be achieved without external ESSs, and the active power support time provided is longer.

**Figure 9.** System output waveform when frequency decreases by 0.3 Hz.
