# Cooperative Multi-Objective Optimization of DC Multi-Microgrid Systems in Distribution Networks
## 5 Case Study and Discussion
### Simulation Results  
#### 5.3.1 Inner-Loop Optimization Results  
1. Case 1  
In this case, when EVs are connected to the MG, they are charged immediately. In case 1, the 24 h curve of RESs, EVs, load, and DC/AC converter active power output in OBMG/RMG is shown in Figure 8. EVs are charged as soon as they reach RMG/OBMG.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**MG Type** & **RES Type** & **Installed Capacity/kW** & **Power Generation Cost/\(\forall\)\(\forall\)\(\forall\)\(\forall\)kWh\({}^{-1}\)** \\ \hline \multirow{2}{*}{OBMG} & PV1 & 800 & 0.24 \\  & WT1 & 800 & 0.38 \\ \hline \multirow{2}{*}{RMG} & PV2 & 400 & 0.24 \\  & WT2 & 400 & 0.38 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Installed RESs and the power generation cost.  
Figure 7: (**a**) Prices of energy exchanging in OBMG; (**b**) prices of energy exchanging in RMG.  
The active power curve of the DC/AC converter represents the active power output curve of the MG to the distribution network. When it is below the \(X\)-axis, it means that the MG sells electric energy to the distribution network. When it is above the \(X\)-axis, it means that the MG purchases electric energy from the distribution network. The power curve of EVs has a similar definition. In Figure 8, RMG will allow EVs to be charged at maximum power from 19:00-20:00, and when REs are insufficient, MEMS will purchase electricity from the distribution network. OBMG is also charging EVs at 9:00-10:00. The total daily operating cost of RMG is CNY 2776.3, and the total daily operating cost of OBMG is CNY 5732.6. Therefore, the total daily operating cost of MMGS is CNY 8508.9.  
Case 2  
In this case, since EVs can participate in the energy dispatching of independent MGs, their across-time energy transmission is used. When the total generated power of the RESs in the MGs is greater than the load, the MEMS will sell the remaining energy to the distribution network or charge the EVs according to the energy prices. When the total power generation of REs is less than the load, the MEMS will purchase electricity from the distribution network or EVs according to the energy prices. In Figure 9, the active power output of RES, EV and DC/AC converters in OBMG and MG are optimized. In OBMG, due to the high energy prices of the distribution network and EVs from 9:00 to 12:00, MEMS choose to let EVs release electric energy. OBMG lowers costs by selling energy to the distribution network. When energy prices are low between 12:00 and 15:00, MEMS fully charges EVs. In RMG, MEMS chooses to charge EVs at 23:00 when energy prices are low. This is to avoid additional battery charge-discharge cycle costs due to discharge, so EVs are only charged. The across-time energy transmission of the EV in the independent MG is fully utilized. Through optimization model calculation, the total daily operating cost of RMG is CNY 2644.1, and the total daily operating cost of OBMG is CNY 5642.1. Therefore, the total daily operating cost of MMGS is CNY 8286.2. Compared with Case 1, the across-time energy transmission of EVs can reduce the overall operating cost of MMGS.  
Figure 8: (**a**) Power output of RESs, EVs, and the DC/AC converter in OBMG; (**b**) power output of RESs, EVs, and the DC/AC converter in RMG.