## 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  
Power balance management aims to maintain the power system frequency that results from the balance between electricity production and consumption. The demand can be predicted, but deviations occur due to unexpected load variations, fluctuation of the renewable generation, or disturbances. Power balance management and a hierarchical control strategy are required for the microgrids [94] because of the autonomous operation requirements. In microgrids, the system's stability is achieved by optimizing active and reactive power flows through the DG units, the BESS units, and the controllable loads. The BESS system should support the microgrid reliability and efficiency. The loads have to be classified and prioritized for demand flexibility. In the grid-connected mode, the power output of DG and BESS units is to be optimized. The controllable loads are committed according to the requirements of the energy markets in which they participate. Considerable is the limit of the active and reactive power flow at the POI without losing the possibility of safe islanding operation. The DSOs are in charge of the safe power supply in the electricity distribution networks, and they have the control and management systems for that. For the utility-connected microgrids, the dedicated functions and functionalities have to be integrated with the existing control and management systems, at least to some extent. For example,  
Figure 4: High-level use case diagram of energy management of the MV distribution network in the intelligent microgrid network phase. [PERSON] and [PERSON] [95] present the functions and the functional requirements for a controller that manages the local generation, the storages, and the controllable loads, and the smart switches control the normal, emergency, and island operation modes. They present two functions: the local grid reconfiguration and the optimization functions related to economic, environmental, and customers' comfort. These can be considered as energy management functions and power balance functions operated by the MEMS in the microgrids [43]. If there is an OLTC in the secondary substation, it can be used for active management [96; 97; 98]. The DSOs (or the microgrid operators) must have permission to configure the alarm settings, rank the load priorities with the customers, enter the profiles, and configure the regional generation [95]. The power balance management in microgrids is presented in [94; 99], consisting of four control levels, according to the standard IEC 62264. Hierarchical control of microgrids is also presented in [100], according to the ANSI/ISA-95 standard. All four control levels manage the power balance, but the highest level, the tertiary control, is not used in the island operation mode. The basic operations in the hierarchical control of microgrids are described and presented by the UCs in the following. The internal control or level 0 manages the operating point between the DG and the BESS units and their interfacing devices or the power electronic interface [94]. These interfaces can be classified into the current-source inverters (CSIs) and the voltage-source inverters (VSIs) [94]. The CSIs are used for injecting current to the grid, and the VSIs for maintaining stable voltage in the grid [99]. Further, the VSIs are connected to the BESS, typically using droop control. The CSIs are connected to the PV or WT generation units, which require the maximum power point tracking (MPPT) algorithms [99]. The primary control or level 1 is responsible for regulating the voltage (frequency and amplitude) delivered to the zero-control level [94; 99]. There are different methods to regulate the voltage within the microgrid.