## 5 Simulation Analysis
### Simulation of Power Control Strategy in Grid-Connected Operation
(**a**) is the power flow of DC sub-microgrid and interlinking converter in grid-connected mode; (**b**) is DC bus voltage in grid-connected. (**c**) is the Battery #1 state of charge in grid-connected. converter to the public grid is getting less and less. At 2.5 s, the power of the interlinking converter changes from negative to positive, and the power generated by DC side photovoltaic #1 is not enough to maintain battery #1 charging and load consumption. From 3 s to 6 s, the light intensity is unchanged, and the load power is increased to 8 kW. At 3.2 s, the charging power of battery #1 on the DC side reaches the maximum value, about 21 kW, so the power of the interlinking converter is also stable at about 15 kW. From 6 s to 8 s, the DC load power continues to increase to 14 kW, while the DC side photovoltaic #1 output power decreases to 10 kW, resulting in \(P_{\text{dc}}^{\text{net}}<0\). Furthermore, the power of DC side battery #1 is still below the minimum threshold and continues to be charged at a constant voltage. At this time, the absorbed power of the DC sub-microgrid is mainly provided by the interlinking converter from the public power grid, and about 25 kW, which does not reach its maximum transmission power. Figure 12(b) shows the DC bus voltage fluctuations, from which it can be seen that the voltage is stable at the rated value of 650 V at about 0.8 s. Due to the change of system power, there is a slight fluctuation at 2 s, 3 s, and 6 s, but the rated value is quickly restored. (2) Case 2  
When the temperature is kept at 25 \({}^{\circ}\)C and the light intensity is kept at 1000 W/m\({}^{2}\), the system is connected to the grid, and the DC side photovoltaic #1 and AC side photovoltaic #2 work in MPPT mode. The initial SOC of DC side battery #1 is set to 95%. The initial conditions of AC side battery #2, interlinking converter, and AC load are the same as in case 1. At the initial moment of the system, the DC load power is 10 kW. At 3 s, the DC load power increases to 18 kW. At 6 s, the DC load continues to increase, and the power reaches 28 kW. According to the system setting, the charge state of DC side battery #1 is above the maximum threshold of 90% at this time, so the system will operate as \(SOC_{\text{dc}}\geq SOC_{\text{dc}}^{\text{max}}\) when connected to the grid. The simulation results are shown in Figure 14.  
As can be seen from Figure 13(a) that form 1 s to 3 s, the DC side photovoltaic #1 works under standard conditions, the output power reaches a maximum value of 18 kW, while the DC load power remains unchanged at 10 kW, that is, \(P_{\text{dc}}^{\text{net}}>0\). However, the internal of the DC side battery #1 has exceeded the maximum threshold, so it cannot continue to charging, that is, it is in a standby state  
Figure 14: System simulation curve of case 2 in grid-connected operation. (**a**) is the power flow of DC sub-microgrid and interlinking converter in grid-connected mode; (**b**) is DC bus voltage in grid-connected. and the output power of 0 kW. All the excess power in the DC sub-microgrid is transmitted from the interlinking converter to the AC side, and the transmission power is \(-8\) kW. From 3 s to 6 s, when the DC load power is increased to 18 kW, the power generation of DG in the DC sub-micro grid is just enough to meet the power consumption of the load, \(P_{\text{dc}}^{\text{net}}=0\). So, the transmission power of DC side battery #1 and the interlinking converter is 0 kW. From 6 s to 8 s, the DC load power continues to increase by 10 kW to 28 kW. At this time, the output power of DC side photovoltaic #1 cannot meet the DC load, then \(P_{\text{dc}}^{\text{net}}<0\). The DC side battery #1 switches from standby mode to constant voltage mode to maintain the DC bus voltage with the interlinking converter and the output power is 2 kW. The transmitted power of the interlinking converter is 8 kW. According to Figure 14, the DC load power will increase at 3 s and 6 s, and the DC bus will decrease slightly.