A Micro Grid is a small-scale power supply network that is designed to provide power for a small community. It comprises of various small power generating sources that makes it highly flexible and efficient, and enables local power generation for local loads.
A Micro Grid is connected to both the local generating units and the utility grid thus preventing power outages. Excess power can be sold to the utility grid. The size of the Micro Grid may range from housing estates to municipal regions.
Micro grid applications have different objectives and characteristics but they share some common technical problems due to the use of Renewable Energy Sources (RES). The RES have an intermittent, aleatory, not programmable nature, which makes it difficult to provide a continuous power supply to isolated loads and to assure the energy dispatch of RES in the public distribution networks.
The introduction of powerful EnergyVAULT™ Battery Storage System in the micro grids allows to balance the variability of RES and to transform the micro grids in fully controllable power production plants.
Most RES generators (photovoltaic and wind generators) and energy storage systems are connected to micro grids through static power converters, typically inverters, which constitute regulated interfaces, capable to implement the same control logic as present in the conventional rotating generators. These “active front end” power converters, featuring advanced regulation capabilities, are defined as “Smart Inverters”.
Micro Grid Operation Modes
By definition the micro grid is a small power system composed of one or more Distributed Generation units (DG), that can be operated independently from the public electricity system. There are micro grids always separated from the bulk power system, which operate in “islanded mode” full time and micro grids that are configured to switch between “islanded” and “not islanded” modes of operation, dependent on the state of the main grid.
Grid Connection Mode
The micro grid frequency is set by the main grid and the micro-grid dynamics are also controlled by the main grid. There is a Micro Grid Controller (MGC) which governs the import/export energy flow from / to the main grid. Power unbalance between micro-grid loads and power supply are compensated by the main grid.Features:
- Utility grid is active.
- Static switch is closed
- All feeders are supplied by utility grid
- Utility grid is not supplying power
- Static switch is open
- Feeder A, B, C are being supplied by Microsources
- Feeder D (not sensitive ) is dead
The micro-grid dynamics are controlled by its own generation units. The micro-grid generation units must control the system voltage and frequency and must balance the power demand and the supply.Features:
Transitions Between Operating ModesFrom on grid to islanded operation
Mains fault: It happens as a consequence of a fault on the main grid. The fault must be detected by an Interface Protection (IP) at the point of coupling and the micro grid must be separated as soon as possible by a suitable switching device or circuit breaker.
The parameter values (normally voltage and frequency limits Vmin, Vmax, fmin, fmax,…) that define a grid fault are set by the grid code and are implemented in the IP control. If all parameters are inside the limits the micro grid has to stay connected.From off grid to grid connected operation
The main grid stability restoration must be sensed. Before closing the switching device that connects the micro grid to the main grid, several conditions must be satisfied. The micro grid frequency must be synchronised to the main grid. The voltage amplitude and phase on the node of the micro grid to be connected must be equalised to the voltage amplitude and phase of the main grid at the connection node, to avoid reactive power circulation. An intelligent static switch is necessary for fast transitions.The main objectives of the micro grid control system are:
- to guarantee the micro grid stability
- to implement voltage and frequency regulation
- to control the power sharing between generation units
- to execute the transitions between the on-grid and off-grid state of operation, without disturbance to loads
- Optimization of economical functions
- Environmental sustainability and energy efficiency
- Improvement of technical performances
- Extend components lifetime
- Master-Slave configuration
- Decentralized configuration
- Hierarchical configuration, a compromise between master-slave and decentralized solution