Advanced MicroGrids as a component for active management of distribution networks

Micro-grid can be defined as a low voltage distribution system to which small modular generation systems are to be connected. In some sense, a micro-grid corresponds to an association of electrical loads and small generation systems through a LV distribution network. This means that loads and sources are physically close so that a micro-grid can correspond for instance to the network of a small urban area, to an industry or to a large shopping center. In terms of the currently available technologies, the micro-generation systems can include several types of devices as fuel cells, renewable generation as wind turbines or PV systems, microturbines (typically in the range of 25-100 kW) powered by natural gas or bio-fuels. Apart from a LV distribution network, micro-generation devices and electrical loads, a micro-grid may also include storage equipment (as batteries, ultracapacitors and flywheels), network control and management systems and heat recovery systems aiming at recovering waste heat to power-CHP applications. From the conceptual point of view, a micro-grid (MG) can be regarded as an active cell within the power system that can be operated as a single aggregated load and an aggregated power source capable of delivering ancillary services supporting the network. To the customer it can be designed to meet his special needs and provide additional benefits, such as improved power quality and reliability, increased efficiency through co-generation and local voltage support. A MG can operate interconnected to the main distribution grid, or in an autonomous way, if disconnected from the main grid, in case of external faults. In this architecture the MG is controlled and managed by a microgrid central controller (MGCC) installed at the MV/LV substation. The MGCC possesses several key functions (such as load forecasting, demand side management, economic scheduling of micro-generators, voltage control, etc.) and heads the hierarchical control system. Controllers locate- d at loads or groups of loads (LC) and controllers located at the micro-sources (MC) exchange information with the MGCC and control local devices. Next figure describes a micro-grid with its micro-sources and controllable loads.