Authors: Dr Maria Brucoli, and Kevin O’Halloran, Arup
A microgrid is a cluster of loads and localised distributed generation and storage sources operating as a single controllable unit. Usually, a microgrid is a small part of the medium- or low-voltage distribution network, where the power and, sometimes, the heat demand are supplied by local energy sources (e.g. photovoltaic units, micro turbines and fuel cells) and storage devices (e.g. flywheels and batteries).
A microgrid can operate in grid-connected mode by using the energy produced on site and importing/exporting any excess from/to the grid. However, in response to a grid power quality event or for economic reasons, it can be disconnected from the main grid and operate in islanded mode.
Control is the key element that enables the microgrid to appear as a single, controlled unit to the distribution network operator, independent of how individual sources or loads behave within the microgrid network.
[caption id="attachment_15531" align="alignright" width="4882"] Figure 1: Typical microgrid network layout (click to enlarge)[/caption]
A typical example of a microgrid with its key components is shown in Figure 1 below. Distributed generation (DG) and distributed storage (DS) units are directly connected to the distribution network, together with the local loads. Often, DG and DS units are referred as distributed energy resources (DERs).
The term DG generally refers to a heterogeneous group of power sources. Sometimes, the term DG is associated with renewable energy sources (RES); however, not all DG technologies are environmentally friendly. DG technologies available on the market range from traditional power sources like diesel generators to new technologies like microturbines, fuel cells, photovoltaic systems and wind turbines. Typical DS units include batteries and flywheels.
The microgrid is connected to the main grid at the point of common coupling (PCC) through a separation device, which in Figure 1 is referred to as a microgrid disconnecting device (MDD).
Control is a key component in the microgrid. A microgrid central controller (MCC) is responsible for regulating power production and consumption within the microgrid and performing actions like grid synchronisation. Control and other functions like protection require monitoring and communication capabilities, which add to the costs and therefore need to be considered at an early stage of the design.
The requirements of the interconnected loads shape the microgrid generation mix and operation. High-specification microgrids are designed to provide customised energy services to improve resilience and power quality. On the other hand, rural microgrids have a profound impact on society as they supply loads that improve education, health, safety and enable manufacturing and trade.
Because of the close match and interaction between generation and consumption in a microgrid (i.e. a stiff grid capable of providing constant voltage and frequency is not always available as power is locally generated and consumed), loads cannot be considered as a passive element; on the contrary they can be defined as 'prosumers'.
A ‘prosumer’ is a load that can consume power, but also participate (at different levels) in demand response (DR) by reducing its load consumption or shedding.
DR enables the microgrid to manage peak loads, generation shortage/unavailability and, depending on the controls installed at load points, participate in the system stability. End users can also get a financial return by committing to curtail/shed power loads through the MCC.
HOW DOES IT WORK?
[caption id="attachment_15533" align="alignright" width="486"] Figure 2: Microgrid operating and transition modes[/caption]
As a single controllable unit, a microgrid can be controlled in four different modes as shown in Figure 2.