Microgrid

A microgrid is a discrete energy system consisting of distributed energy sources (e.g. renewables, conventional, storage) and loads capable of operating in parallel with, or independently from, the main grid. The primary purpose is to ensure reliable, affordable energy security for commercial, industrial and federal government consumers. Benefits that extend to utilities and the community at large include lower greenhouse gas (GHG) emissions and lower stress on the transmission and distribution system

: Configuration of microgrid

The idea of linking the power sources with consumer demands without interruptions leads to a complex configuration. This kind of operation needs to connect the control system using communication systems with information technologies. Distributed energy resources like microturbines, fuel cells, photovoltaic (PV) arrays, wind farms together with storage devices and controllable loads present control capabilities for the network operation

Microgrids are formed as islands that have at least one distributed energy resource and controllable loads. Sources and loads can be connected and disconnected to islands which can do so to the medium voltage (MV) distribution system. Disconnecting microgrids from the utility is called islanding mode. This mode result when the utility experience faults or instability. Once the disturbance in the utility has cleared, microgrids could be connected again

Microgrids interconnected to the utility grid will do so at the point of common coupling (PCC), which is where the microgrid will interface with the macro-grid at either medium or high voltages. It is from this point that the microgrid will transfer between two states of operation, grid-connected and grid isolated, or islanded mode. Transformers are located at this point to either step-up microgrid exports to grid voltage or step-down imports to the microgrid distribution voltage. The fast switch, which is essentially an advanced circuit breaker, is able to sense conditions on the macro-grid and rapidly connect and disconnect the microgrid from the macro-grid. Each of the three feeder lines in Figure 1-1 (identified as serving sensitive, adjustable and shedable loads) can also be connected or disconnected via separate circuit breakers. This design highlights the important role of demand response as an internal resource for a microgrid as interconnected loads are designated for varying – or heterogeneous – levels of service reliability

The operation and management of the microgrid is controlled and coordinated via both local micro-source controllers, which are sited with generation or storage devices interconnected to the microgrid, and a central controller, which executes the overall control of the microgrid and coordinates the operation and protection requirements of the micro-source controllers. These devices collect and share information to ensure that voltage and frequency conditions on the microgrid are optimized. Sectionalizing circuit breakers or automated switches (i.e., small green boxes in Figure 1-1) located on each of the feeders, combined with multiple power feeds, allows faults to be isolated on the distribution system preventing outages from affecting the entire microgrid area. An advanced metering and communications infrastructure allows real time monitoring of energy use on the microgrid and automated controls through the central control system. An energy management system is programmed to ensure that conditions on the microgrid stay within pre-determined levels (e.g., active and reactive power or voltage and frequency) and dispatch generation, storage devices or control shedable loads to ensure that supply and demand is balanced