*2.3. Control Strategies Used in Hybrid Micro-Grids*

Two particular and opposite approaches are used with respect to the power system's control strategies: Centralized and decentralized. A compromise between fully centralized and fully decentralized control schemes can be achieved by means of a hierarchical control scheme consisting of three control levels: Primary, secondary, and tertiary. Micro-grids three-level hierarchical control consists of controlling the voltage and frequency, as primary and secondary controls (droop control), while the tertiary control is responsible for economic and optimization operations, including Battery Management System (BMS), control of the Battery Storage System (BSS), as well as managing the import and export energy between the micro-grid and power systems [28,35]. The innovative and integrated energy management and control systems will follow a hierarchical control structure, based on three levels of hierarchy:


The integrated energy management system, for optimizing future aggregators' assets, is coordinated through an advanced ICT data hub platform that allows DSOs (Distribution System Operators) and aggregators to deal with innovative aspects of their distribution network operation and planning with smart energy storage solutions, including V2G integration, power electronics based interface, with bi-directional power control of DER and ESS and integrating IoT into the micro-grids.

Hybrid micro-grids and the integration of DER units introduce several operational challenges that need to be incorporated into the control systems, in order to make certain that the present levels of reliability are not considerably affected, and the potential benefits of DG are fully exploited. The main role of micro-grid control is to assist the active and reactive power generated by the DER components, as well as load demand.

The most relevant provocations in the control of a micro-grid, include bi-directional power flow, stability issues, power balance, uncertainties among DERs coordination, and transition between stand-alone and grid connected operation modes.

Providing technical support to the grid during the transmission of power from generating units to the consumers can be referred to as an ancillary service and involves the adjustments of both the active and reactive power of the flexible resources. For power management in modern micro-grids and the utilization of ESS in different applications, the applied control strategy is the most critical part, in order to have full benefit of all the elements of the power system.
