## 4 Higher-Level Power Coordination Control Strategy  
### Multi-System Mode Power Coordination Control Strategy in Grid-Connected Operation  
In grid-connected operation, the AC bus is directly connected to the public power grid, and its voltage and frequency will remain stable. Therefore, this section focuses on adjusting the DC bus voltage. For the DC sub-microgrid, it is necessary to maintain the power balance together with the photovoltaic power generation unit photovoltaic #1, the energy storage unit battery #1, and the interlinking converter on the DC side. In order to maximize the utilization of solar energy resources,  
Figure 10: Interlinking converter control strategy diagram. Figure 9: Interlinking converter topology. photovoltaic power generation units will operate in MPPT mode. Besides, to maintain the DC bus voltage stability, the interlinking converter adopts DC bus constant voltage control. To maintain the DC bus voltage, the DC side battery #1 and the interlinking converter need to work together, and the state of charge is an important factor to determine whether the battery can be charged or discharged. Therefore, the state of charge \(SOC_{\text{dc}}\) of battery #1 is taken as an important basis for the division of system mode; the maximum value is \(SOC_{\text{dc}}^{\text{max}}\) and the minimum value is \(SOC_{\text{dc}}^{\text{min}}\). At the same time, since the photovoltaic power generation unit operates in MPPT mode, the power \(P_{\text{dc}}^{\text{pv}}\) emitted by the DC side photovoltaic #1 is constant under steady-state, and the power \(P_{\text{dc}}^{\text{load}}\) absorbed by the DC load when the voltage is stable and also constant. Therefore, the balance between the two is also one of the bases for the division of system mode. The power \(P_{\text{dc}}^{\text{net}}\) of the DC sub-microgrid is defined as:  
\[P_{\text{dc}}^{\text{net}}=P_{\text{dc}}^{\text{pv}}-P_{\text{dc}}^{\text{load }}, \tag{10}\]  
In addition, for the interlinking converters, the transmission direction from the AC side to the DC side is set to be positive, the maximum conversion power of a single interlinking converter is \(P_{\text{ILC}}^{\text{max}}\), and the maximum conversion power of two parallel converters is \(P_{\text{ILCs}}^{\text{max}}\)\(=2\times P_{\text{ILC}}^{\text{max}}\). Therefore, the actual conversion power \(P_{\text{ILCs}}\) of the two interlinking converters will also be referred to in the system mode division. In summary, the operation mode is analyzed according to the situation:  
(1) \(SOC_{\text{dc}}\)\(\geq SOC_{\text{dc}}^{\text{max}}\)  
At this time, the DC side battery #1 has reached the maximum charged state, can only discharge. When \(P_{\text{dc}}^{\text{net}}\)\(\geq\) 0, the DC load is not enough to consume the power generated by the DC side photovoltaic #1. In order to protect the battery, battery #1 should be in a standby state, and the excess power generated by photovoltaic #1 should be transmitted to the public grid by an interlinking converter. The power balance relationship can be expressed as:  
\[P_{\text{dc}}^{\text{pv}}+P_{\text{ILCs}}=P_{\text{dc}}^{\text{load}}, \tag{11}\]  
where \(P_{\text{ILCs}}\)\(\leq\) 0, the interlinking converter is in the inverter state. When \(P_{\text{dc}}^{\text{net}}\)\(<\) 0, the generation power of photovoltaic #1 on the DC side is greater than the consumption power of the load, resulting in the DC bus voltage drop. For faster adjustment to stabilize the DC bus voltage, battery #1 should be in the DC bus constant voltage mode and adjust the DC bus voltage to the rated value together with the interlinking converter. The power relationship at balancing is as follow:  
\[P_{\text{dc}}^{\text{pv}}+P_{\text{ILCs}}+P_{\text{dc}}^{\text{bat}}=P_{ \text{dc}}^{\text{load}}, \tag{12}\]  
where \(P_{\text{dc}}^{\text{bat}}\)\(>\) 0 indicates the output power of battery #1 and \(P_{\text{ILCs}}\)\(>\) 0 indicates that the interlinking converter is in a rectified state.