## 4 Higher-Level Power Coordination Control Strategy
### Multi-System Mode Power Coordination Control Strategy in Isolated Island Operation  
In isolated island operation, the whole AC/DC hybrid microgrid system does not have the support of the public power grid, and the power balance of the entire system needs to be considered during control, so the control process is more complex than when it is connected to the grid. This paper adopts the mode of DC sub-microgrid as the main micro-grid, and the interlinking converter needs to work in the V/F control mode to maintain the voltage and frequency stability of the AC bus. In order to improve the utilization rate of solar energy, the DC side photovoltaic #1 will be in MPPT mode at the initial moment. The AC side photovoltaic #2 can work in MPPT or P/Q control mode. Moreover, the AC side energy storage unit battery #2 works in P/Q control mode. At the same time, battery #2 is  
Figure 11: Power coordination control strategy of AC/DC hybrid microgrid during grid-connected operation. regarded as an energy storage device with rated charging and discharging power, regardless of its _SOC_. During operation, the control mode of battery #1 should be switched according to the power balance state. Meanwhile, the reference power of photovoltaic #2 and battery #2 should be adjusted. When the system is overloaded, the secondary load should be cut off. Since the DC sub-microgrid is the main microgrid, the charging state of DC side battery #1 is still an important basis for operation mode analysis. The power \(P_{\text{island}}^{\text{net}}\) of the hybrid AC/DC microgrid in isolated island is defined as:  
\[P_{\text{island}}^{\text{net}}=P_{\text{dc}}^{\text{pv}}+P_{\text{ac}}^{\text {pv}}-P_{\text{dc}}^{\text{load}}-P_{\text{ac}}^{\text{load}}, \tag{14}\]  
where \(P_{\text{ac}}^{\text{load}}\) is the absorbed power of the AC load and \(P_{\text{ac}}^{\text{pv}}\) is the output power of the AC side photovoltaic #2. According to the _SOC_ operating mode in isolated island, the analysis is as follow:  
(1) \(SOC_{\text{dc}}\geq SOC_{\text{dc}}^{\text{max}}\)  
The DC side battery #1 reaches the maximum power and cannot continue to charge. If \(P_{\text{island}}^{\text{net}}\geq 0\), the power generated by DG in the whole microgrid system is greater than the total power consumed by the AC and DC side loads, so measures need to be taken to reduce the power generated. First, we should consider whether AC side battery #2 can be used for charging to maintain power balance. If the rated output power of battery #2 is \(P_{\text{ac}}^{\text{bat}}\)\({}_{N}>P_{\text{island}}^{\text{net}}\), then battery #2 can be used to absorb excess power. At this time, set the reference power \(P_{\text{ac}}^{\text{bat}}\)\({}_{N}\) of battery #2 as:  
\[P_{\text{ac}}^{\text{bat}}\)\({}_{N}\)\({}_{N}\)\({}_{N}\)\({  
absorbed into the battery within the rated power of battery #1. At this time, the power balance state can be expressed as:  
\[P_{\text{dc}}^{\text{bat}}+P_{\text{island}}^{\text{net}}=0. \tag{19}\]  
In the situation of \(P_{\text{island}}^{\text{net}}-P_{\text{dc}}^{\text{bat}\_N}>0\), the AC side battery #2 is also required to absorb power. When \(P_{\text{island}}^{\text{net}}-P_{\text{ac}}^{\text{bat}\_N}-P_{\text{dc}}^{ \text{bat}\_N}\leq 0\), it means that when the batteries on the AC and DC sides absorb the excess power together, the system can reach a new balance. At this time, \(P_{\text{ac}}^{\text{bat}\_ref}\) should be set to:  
\[P_{\text{ac}}^{\text{bat}\_ref}=P_{\text{island}}^{\text{net}}-P_{\text{dc}}^ {\text{bat}\_N}, \tag{20}\]  
where the minimum value of \(P_{\text{ac}}^{\text{bat}\_ref}\) is 0. When \(P_{\text{island}}^{\text{net}}-P_{\text{ac}}^{\text{bat}\_N}-P_{\text{dc}}^{ \text{bat}\_N}>0\), it means that the power generated by the micro-source is too much. At this time, the processing method is the same as when \(SOC_{\text{dc}}\geq SOC_{\text{dc}}^{\text{max}}\). The AC side battery #2 absorbs power at rated power.