## 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
In addition to the active load sharing (usually in the islanded mode), the most common control is the droop-based control (grid-connected mode typically) for regulating the active and reactive power consisting of the active power/frequency (\(P\)/\(f\)) and the reactive power/voltage (\(Q\)/\(V\)) droops. The primary control's tasks are to stabilize the voltage and frequency after islanding, guarantee interconnection capability for DERs, share the active and reactive power between DERs (even without communications), and reduce circulating currents [101]. The primary control manages the power balance between the central BESS and the inverter-connected DG and BESS units. In addition, frequency-controlled load types such as the dynamic loads, for instance, V2H or V2G, can be worthwhile to connect to the primary control system. The secondary control or level 2 is responsible for correcting the grid frequency and amplitude deviations. A (central) controller is needed for regulating voltage and frequency deviations toward zero after any change in demand and generation within the microgrid [99]. The secondary control's tasks are to compensate for the voltage and frequency deviations after primary control actions, for the reason the BESS cannot provide long-term power control [101]. Without this control, both the frequency and amplitude of the voltage would depend on the loading [94]. The frequency and voltage at the DERs' terminal are compared with the reference values sent by the primary controllers, and the deviation or the error values are sent to the primary controllers for compensation [101]. The tertiary control or level 3 is responsible for optimizing the power flow in the microgrid and the power flow between the microgrid and the utility or main grid. The tertiary control also enables electricity suppliers' and aggregators' participation in the electricity markets. This control level is responsible for managing the active and reactive power flows through POI by regulating the voltage and frequency in the parallel operation mode [99]. The active and the reactive power are compared with the reference values at the POI. The active power flow at the POI can be controlled by adjusting the reference frequency in the grid-connected mode [94]. An islanding detection algorithm is needed to disable the tertiary control for detecting islanding [99]. The microgrid frequency and voltage are managed mainly by the power electronic interfaces and BESS (although direct load control affects power balance, it is not desirable in the first place). The power converters of DER units can be divided into grid following (controls current and phase angle), grid forming (controls voltage magnitude and frequency), and grid supporting based on their functioning types. Grid following inverters (CFLs) are connected with non-dispatchable DG units, and they operate as a CSI with a unitary power factor (\(\cos\varphi=1\)). This inverter requires a voltage reference, and it tracks the grid by injecting current in phase with the grid voltage. The maximum MPPT algorithm modifies the operating point (voltage or current) of a micro-source by the DC/DC converter that sends the DC power to the inverter. The CFLs operate similarly in the grid-connected and islanded microgrid. The grid-forming inverters (GFM) are connected with the BESS units. In the grid-connected mode of the microgrid, the GFMs tasks control the active and reactive power injected into the AC bus to maintain the SOC of the BESS and improve the power quality. They generate voltage on the bus in the islanded mode, thus acting as a VSI. The GFM usually uses the \(P/f\) and the \(Q/V\) droop control methods. The grid supporting inverters (GSIs) aim to maintain the power quality in the islanded microgrid by supplying active and reactive power with the droop controllers. The GSIs can be connected with the dispatchable DG units and the BESS units [102]. The BESS control is essential in the microgrid's power balance management and AS.