## 4 Higher-Level Power Coordination Control Strategy
### Multi-System Mode Power Coordination Control Strategy in Grid-Connected Operation
In addition, even in the situation of grid-connection, the load power on the DC bus cannot be allowed to increase indefinitely because the interlinking converter capacity is constant. Therefore, when \(P_{\text{dc}}^{\text{net}}\)\(<\) 0 and \(P_{\text{ILCs}}\)\(>P_{\text{ILCs}}^{\text{max}}\), the secondary load on the DC side should be cut off to protect the normal operation of the converter. (2) \(SOC_{\text{dc}}\)\(\leq SOC_{\text{dc}}^{\text{min}}\)  
This situation indicates that the DC side battery #1 has reached the minimum charged state and cannot continue discharging, but it can be charged. The battery should be switched to a constant voltage charging state, limiting its maximum current to be no greater than 0 and charging at rated voltage, and the power balance relationship can be expressed as:  
\[P_{\text{dc}}^{\text{pv}}+P_{\text{ILCs}}+P_{\text{dc}}^{\text{bat}}=P_{ \text{dc}}^{\text{load}}, \tag{13}\]  
where \(P_{\text{dc}}^{\text{bat}}\)\(<\) 0 indicates that the DC side battery #1 absorbs power. Although the DC side battery #1 is in constant voltage charging mode at this time, the power source is different. When \(P_{\text{dc}}^{\text{net}}\)\(>\) 0, it means that although the battery power is low, the output power of the photovoltaic unit is sufficient to meet the demand of the DC load. Therefore, on the premise that the interlinking converter maintains the DC bus voltage, battery #1 can absorb the excess power generated by the photovoltaic unit at a constant voltage. When \(P_{\text{dc}}^{\text{net}}<0\), the internal power supply of the DC sub-microgrid is scarce, but the capacity of the interlinking converter does not reach the rated value and battery #1 can still absorb power from the public grid in a constant voltage charging state. If \(P_{\text{ILCs}}>P_{\text{ILCs}}^{\text{max}}\), first put battery #1 in the standby state, and then continue to determine whether the interlinking converter is overloaded. If so, remove the DC load, otherwise, it will operate normally. (3) \(SOC_{\text{dc}}^{\text{min}}\leq SOC_{\text{dc}}\leq SOC_{\text{dc}}^{\text{ max}}\)  
In this mode, the DC side battery #1 is in the normal range of charging and discharging. When \(P_{\text{dc}}^{\text{net}}>0\), the running state at this time is the same as \(SOC_{\text{dc}}\leq SOC_{\text{dc}}^{\text{min}}\). Because battery #1 is charged at a constant voltage. Then \(SOC_{\text{dc}}\) will be greater than the minimum threshold, the power balance relationship can also be expressed as (13). When \(P_{\text{dc}}^{\text{net}}<0\), the system running state under this condition is the same as \(SOC_{\text{dc}}\geq SOC_{\text{dc}}^{\text{max}}\). Because battery #1 constant DC bus voltage discharge, after a period of time, \(SOC_{\text{dc}}\) will fall below the maximum threshold, and the power balance relationship can also be expressed as (12). Based on the above analysis, this paper divides the control strategy of the whole AC/DC hybrid microgrid system into multiple modes during grid-connected operation. The central controller needs to send commands to the lower converter in accordance with the power coordination control strategy to ensure the normal flow of power, as shown in Figure 11.