## 5 Simulation Analysis
### Simulation of Power Control Strategy in Grid-Connected Operation  
Since the AC bus is directly connected to the public power grid during the grid-connected operation, the changes of the power, voltage, frequency, and other values of the AC side microgrid are not analyzed. (1) Case 1  
When the system works in grid-connected mode, the DC side photovoltaic #1 and the AC side photovoltaic #2 work in MPPT mode. The initial SOC of the DC side battery #1 is set to 10%, the AC side battery #2 works in P/Q control mode, and the initial SOC is 90%. The active power of the AC load is 50 kW and remains unchanged. The interlinking converter works in the DC bus constant voltage control mode. During the operation of the system, the ambient temperature remains unchanged at 25 \({}^{\circ}\)C, the initial light intensity is 1000 W/m\({}^{2}\), and the DC load power is 2 kW. At 2 s, the light intensity was reduced to 800 W/m\({}^{2}\). At 3 s, the DC load power increased to 8 kW, and at 6 s, the light intensity continued to decrease to 600 W/m\({}^{2}\), while the DC load power continued to increase to 14 kW. The simulation results are shown in Figure 13.  
As can be seen from Figure 13c, at the beginning of the system operation, if the DC side battery #1 is discharged, and the state of charge will instantly fall below the minimum threshold of 10%. Therefore, the system will operate according to the situation of \(SOC_{\text{dc}}\leq SOC_{\text{dc}}^{\text{min}}\) in Figure 11, where battery #1 will be charged at a constant voltage. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Symbol** & **Value** & **Unit** \\ \hline AC bus rated line voltage & \(u_{\text{ac}}^{\text{ref}}\) & 380 & \(V\) \\ AC bus rated line frequency & \(f^{\text{ref}}\) & 50 & Hz \\ DC bus rated line voltage & \(u_{\text{dc}}^{\text{ref}}\) & 650 & \(V\) \\ Battery \#1 rated power & \(p_{\text{dc}}^{\text{bat\_N}}\) & 20 & \(kW\) \\ Battery \#1 maximum threshold of state of charge & \(SOC_{\text{dc}}^{\text{max}}\) & 90\% & - \\ Battery \#1 minimum threshold of state of charge & \(SOC_{\text{dc}}^{\text{min}}\) & 10\% & - \\ Battery \#1 capacity & \(S_{\text{dc}}^{\text{bat}}\) & 50 & \(Ah\) \\ Battery \#1 rated voltage & \(U_{\text{dc}}^{\text{bat\_N}}\) & 200 & \(V\) \\ Battery \#1 droop control coefficient & \(k_{2}\) & 1.5 & \(V/kW\) \\ Reference power when battery \#1 droops and discharges & \(p_{\text{dc}}^{\text{bat\_ref}}\) & 10 & \(kW\) \\ Maximum power under photovoltaic \#1 standard condition & \(p_{\text{dc}}^{\text{bat\_max}}\) & 18 & \(kW\) \\ Photovoltaic \#1 droop control coefficient & \(k_{1}\) & 1.67 & \(V/kW\) \\ Reference power for photovoltaic \#1 droop control & \(p_{\text{dc}}^{\text{pv\_ref}}\) & 9 & \(kW\) \\ Battery \#2 rated power & \(p_{\text{ac}}^{\text{bat\_N}}\) & 10 & \(kW\) \\ Maximum power under photovoltaic \#2 standard condition & \(p_{\text{ac}}^{\text{pv\_max}}\) & 29 & \(kW\) \\ Filter capacitor & \(C\) & 10 & \(\mu\)F \\ The capacity of a single interlinking converter & \(S\) & 15 & \(kVA\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Basic parameters of AC/DC hybrid microgrid simulation. According to Figure 13a, form 0 s to2 s, the DC side photovoltaic #1 will work in the standard environment with an output power reaching 18 kW and the DC load power reaching 2 kW, resulting in \(P_{\mathrm{dc}}^{\mathrm{net}}>0\). The excess power of the DC sub-microgrid will be absorbed by the DC side battery #1, but its charging power will not reach the maximum value soon, and the battery charging current will increase slowly. Therefore, the power flowing to the large power grid through the interlinking converter gradually decreases. From 2 s to 3 s, the light intensity decreases, and the output power of DC side photovoltaic #1 is about 14 kW. At this time, the load power remains unchanged, and the charging power of DC side battery #1 continues to increase, so the power transmitted from the interlinking  
Figure 13: System simulation curve of case 1 in grid-connected operation.