## 5 Simulation Analysis
### Simulation of Power Control Strategy in Isolate Island Operation  
During the island operation, the overall power balance should be considered. At this time, the interlinking converter works in V/F control mode to provide voltage and frequency support for the AC bus. (1) Case 1  
The system works in island mode with a constant temperature of 25 \({}^{\circ}\)C, the DC side photovoltaic #1 works in MPPT mode, and the AC side photovoltaic #2 works in P/Q mode. The AC side photovoltaic #2 active power reference value is set to 16 kW, and the reactive output is set to 0 kV. The \(SOC\) of DC side battery #1 and AC side battery #2 are both 85%, they operate in \(SOC_{\text{dc}}^{\text{min}}\leq SOC_{\text{dc}}\leq SOC_{\text{dc}}^{\text{ max}}\) mode in Figure 12. The DC load power is 10 kW, and the AC load active power is 20 kW. At the initial moment, the light intensity was 1000 W/m\({}^{2}\). At 5 s, the light intensity decreased to 800 W/m\({}^{2}\). The simulation results of active power flow in the system are shown in Figure 15.  
Figure 15: System simulation curve of case 1 in isolated island operation. (**a**) is the overall power flow of AC/DC sub-microgrid in island mode; (**b**) is the transmission power of interlinking converters in island mode. As can be seen from Figure 15(a) that at the beginning of the system operation, the output power of DC side photovoltaic #1 reached 18 kW under standard conditions, and the output power of AC side photovoltaic #2 is 16 kW, which is 34 kW in total. At this time, the sum of the active power of AC load and DC load is 30 kW, then \(\mathrm{p_{island}^{net}}>0\). According to Figure 12, the DC side battery #1 works in constant voltage mode and can absorb the excess power of the system. Therefore, the active power reference value of AC side battery #2 is 0 kW, and the absorbed power of DC side battery #1 is about 3 kW. Form 5 s to 8 s, the, and the output of DC side photovoltaic #1 is reduced to 14 light intensity is reducedkW, then \(\mathrm{p_{island}^{net}}=0\). Therefore, the active power reference value of AC side battery #2 is still 0 kW. However, due to system losses, DC side battery #1 still needs to maintain a power output of approximately 1 kW to stabilize the system. The AC bus voltage and frequency curves are shown in Figure 16,b, and the DC bus voltage curve is shown in Figure 16.  
According to Figure 16,b, the voltage and frequency of the AC bus do no obviously fluctuate during 5 s, and the rated value is always maintained during system operation. As can be seen from Figure 16, the DC bus voltage is maintained at the rated value of 650 V from 1 s to 5 s. At 5 s, due to the sudden decrease in light intensity, the output of DC side photovoltaic #1 will immediately decrease, while the DC side battery #1 will not immediately reduce its absorbed power. Therefore, the DC bus voltage will have a downward trend. When it falling to 622 V, the DC side battery #1 no longer absorbs power, so the DC bus voltage immediately returns to the rated value. Figure 16: System simulation curve of bus voltage and frequency for case 1 in the island operation. (**a**) is the three-phase voltage interception part of AC bus in island mode; (**b**) is the frequency interception part of AC bus voltage in island mode; (**c**) is the DC bus voltage in island mode. (2) Case 2  
The system works in island mode, the ambient temperature is 25 \({}^{\circ}\)C, the light intensity is 1000 W/m\({}^{2}\), and the DC side photovoltaic #1 runs in MPPT mode. Moreover, the AC side photovoltaic #2 runs in P/Q mode, with the active power constant at 16 kW and reactive power output at 0 kVar. The settings of the DC side battery #1 and the AC side battery #2 are the same as that of isolated island in case 1. At the initial moment, the DC load power is 20 kW, the AC load active power is 50 kW, and the reactive power is 0 kVar. At 3 s, the active power of the AC load increases to 55 kW. At 6 s, the active power of the AC load continues to increase until it reaches 60 kW.