## 5 Operational Descriptions of the Future Distribution Networks with the Sundom Smart Grid
### Voltage and Frequency Control via Power Balance Management Use Cases of the Microgrid Phase
The AS can be load-following, operational reserve, frequency regulation, peak shaving, black start (during island operation mode), and integrating renewables. The power balance within a microgrid can be maintained by charging or discharging the stored energy. The BESS controls the charging power by increasing it if the grid's frequency exceeds the maximum value and decreases it when the frequency reaches the minimum setpoint. The central BESS is fundamental for the voltage control in the island operation mode due to the dynamic variations such as the delayed response or some DG units' slow controllability. In this case, the BESS contains the primary and the secondary control levels only. The BESS monitors active power in the microgrid, where the system's frequency reflects the system's capacity. Each DG unit's power setpoint should be controlled through the secondary control level to maintain the BESS's zero power output [94]. In this study, the power balance management in the microgrid phase is reviewed both in the parallel and island operation modes. The OLTC, the PS-BESS, the SS-BESS, the DG units, and the controllable loads can contribute to the active voltage level management. The studied power balance management HL-UCs of the LV and the MV distribution networks in the microgrid phase are described in Table 6. \begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt}} \hline \hline
**HL-UC Name** & **Events in the LV and MV Distribution Networks** \\ \hline \multirow{4}{*}{Regulate the output voltage and control the current of the inverter-connected DER} & LV: The primary control actions of each inverter module. The inverters are associated with the dynamic loads EV-BESS, PV-BESS, class 4 PV units, class 3 b PV units, and SS-BESS. The PV and BESS inverters can be either GFL or GSI units. The control and monitoring can be executed via BAS and HEMS. \\ \cline{2-3}  & MV: The primary control actions of the PS-BESS, the CHP, and the WT interfacing units. The primary control has been in use since the self-sufficient phase in Volt/Var control, aided by the OLTC of the primary transformer, the WT converter, the CHP unit, and the PS-BESS. In this phase, the BESS, the CHP, and the WT unit can be used as the GFM units. \\ \hline \hline \multirow{4}{*}{Regulate the active and reactive power and reactive power} & LV: The secondary control actions to keep the system stable. The deviations in voltage (amplitude and frequency) between the dispatchable DER units and the grid are regulated. The controllable loads, PV inverters, the OLTC of the MV/LV transformers, and the SS-BESS can balance the power. The SS-BESS behaves as a GSI unit. \\ \cline{1-1} \cline{2-3}  & MV: The primary control actions of the PS-BESS, the CHP, and the WT interfacing units. The primary control has been in use since the self-sufficient phase in Volt/Var control, aided by the OLTC of the primary transformer, the WT converter, the CHP unit, and the PS-BESS. In this phase, the BESS, the CHP, and the WT unit can be used as the GFM units. \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Power balance management high-level use cases (HL-UCs) in the microgrid phase. Further, the seamless switching between the operation modes HL-UC in the power balance management can include the following PUCs: intentional transition to the islanded operation and unintentional transition to the islanded operation; while the control power flow at the POI HL-UC in power balance management can include the following PUCs: microgrid provides voltage control service and microgrid as a resource in the congestion management.