The U.S. Department of Energy Microgrid Exchange Group defines the following principles for a microgrid:
“A microgrid is a group of interconnected loads and distributed energy resources within clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid. A microgrid can connect and disconnect from the grid to enable it to operate in both grid-connected or island-mode.”
A modern microgrid might include multiple renewable generation units (e.g., photovoltaics, wind turbines), various types of energy storage systems, and a diesel backup to ensure continuous reliability and quality of the power generation.
Challenges and opportunities:
Modern microgrids are changing the structure of the grid from traditional one-way vertical configuration—from power plants to consumers—to more formless, intensely meshed distribution systems, which include a host of renewable sources with unpredictable characteristics. This new structure necessitates the implementation of power inverters to change direct current (DC) to alternating current (AC) in place of traditional power generators.
Considering these new levels of complexity, the design principles of modern microgrids can be characterized as follows:
Computer Simulation to Maximize Capabilities:
To ensure the reliable operation and cost-efficient microgrids, proper design is critical. This design relies on a computer optimization simulation that is formulated to determine the optimal size (and location) of renewable generators, energy storage systems, and loads.
Adaptive Protection for Safe Operations:
The protection system plays a substantial role in ensuring the safe operation of a microgrid. Various types of distributed energy resources (DERs) and several modes of the operation result in different protection schemes. The protection structure in conventional systems mostly depends on protective devices (e.g. fuses, sectionalizers, and reclosers). However, the operation of microgrids necessitates the use of an adaptive protection scheme which can be adjusted in response to changing conditions. For instance, an electric power line might be designed with relays set to trip if the current flowing from the upstream exceeds a preset level. If Microgrid was added to the downstream, it might require that the existing relay settings be changed to provide optimal protection.
Effective Damping to Enhance Stability:
Unlike conventional electric grids, modern microgrids are weak grids. The weak grid characteristics may result in additional stability issues that can be resolved using either active or passive approaches. Passive damping can be realized by implementing physical components; whereas in active damping, the physical components are mimicked by a control system with flexible parameters.
Software Tools for Assessment:
Different software tools have been developed in order to evaluate the microgrids with respect to the operations and planning, which can apply different control strategies and adapt to different time intervals.
In the near future, the energy sector will completely its change configuration. Challenges of this evolution are: achieving cost-effective, stable and optimal design as well as meeting the users’ performance requirements. Nevertheless, different types of software tools are available to help the proliferation of modern microgrid implementation with acceptable resilience expectation and reliability. With these advances in microgrid design, there are more opportunities to develop better solutions for the current grid.