## 5 Result and Discussion
### Simulation Results
#### 5.2.1 IEEE Test System  
To demonstrate the validity of the proposed approach, an IEEE test system in which general European MGs [34] connected the modified IEEE 33-bus DN [35] was used for the simulation and analysis is shown in Figure 7. Details about the IEEE 33-bus DN can be found in [36]. The factors should be taken into account when modeling an MG integrated with the DN. Here, the MG was always connected to the DN by means of the tie-line. The maximum limit of the exchange power of the grid tie-line was 400 kW. The scheduling period was assumed to be a single day. Figure 5: Market price information. Figure 6: Residual energy of BESS. Figure 8 illustrates the resulting scheduled power scheme for the distributed units of the integrated MG, when the energy band was not considered, where the tie-line power accounted for the majority of the load. Moreover, the BESS charged and discharged less frequently. The batteries were charged from 02:00 to 04:00, while they were discharged at 07:00 and 19:00. Furthermore, the MG purchased energy from the DN to optimize the operational cost, because it was significantly lower than the MT cost. The manner in which the MGO shares the responsibility for ensuring balance of the supply and demand with the DSO under the fluctuating renewable energy depends on the operating condition of the tie-line flow. The tie-line flow results of Case 1 are based on the predetermined (contractual) tie-line flow scenario. These results demonstrate that the MG rescheduled receiving the control signal from the DN for operating cost minimization with no tie-line constraints. Interestingly, the tie-line flow changed significantly to approximately 140 kW from 08:00 to 09:00, with a corresponding change in the load of 360 kW. Such substantial changes involve an increase in the potential risks or operational costs of the DN, even though this results in significant reductions in the operating costs of the MG. The important issue is the reduction of the reliability for the entire DN, which is far more significant than the minimization in the operating costs of the MG. This means that the scalability, namely the ability to  
Figure 8: Scheduled power scheme for distributed units for Case 1. Figure 7: IEEE (Institute of Electrical and Electronics Engineers) test system. incorporate MGs into the DN, may be restricted. In this case, fluctuations as large as 4334 kW could occur in the power transmission via the tie-line during the day. We also evaluated the energy band operation scheme where the MG optimized the control signal of the tie-line flow within the energy band (Case 2). The sharing of reliability should be carefully considered when establishing the energy band size as an operational condition, owing to the trade-off between reliability and cost in the grid-connected system. We considered energy bands ranging from 30% to 80% and compared the results with those of Case 1, as indicated in Table 2. According to the results, the ratio of the 80% energy band was almost equal to the contract power. In the proposed bi-level optimization model, ensuring the proposed ramping capability in the MG is essential for mitigating operational risks. The scheduled power scheme for Case 2 with an energy band of 80% is plotted in Figure 9. It can be observed from Figure 9 that the scheduled power scheme was similar to the contract power. Limited control over the tie-line flow could cause the power system reliability of the MG to decrease owing to an increased supply-demand unbalance. As shown in Figure 9, the BESS was charged for longer than that during Case 1 (from 03:00 to 05:00). The MT also generated power, which was used to ensure reliable operation between the DSO and MGO, because of the ramping capability of the proposed energy band scheme. Consequently, these results demonstrate that the increased responsibility for balancing the supply and demand requires preparation and planning to make sure that the MGO is capable of ramping.