Microgrids

What Technologies make up Microgrids?

Technologies

There are few ‘typical structure’ definitions of a microgrid, as they can be applicable with a range of constituent technologies, energy vectors and design topologies.

Some of the possible components are listed in Figure 1 and Figure 2 on the following page. Common characteristics of a microgrid include that they are typically small in scale with peak power demand, e.g. ranging from tens of KWs up to tens of MWs, that they are usually able to operate independently from the main power grid or in synchronization with it, and that they generally have a rather small/localized geographical/spatial footprint.

They can incorporate multiple Distributed Energy Resources (DERs) that usually feed electricity as the main energy supply component, but can also incorporate links to gas networks, thermal energy and water supply elements. There can also be tangential connections to the transport sector, via electric or even hydrogen vehicles.

Distribution

Where available, a Point of Common Coupling (PCC) is used to provide the microgrid with connectivity to a larger scale main grid. When the PCC is temporarily or permanently unavailable, they become isolated microgrids, which is the default setup in the case of geographically remote sites. PCC connectivity is managed through either conventional AC switchgear in conjunction with power electronic converter controls.

Based on the characteristics of the electricity distribution system, microgrids can be classified into AC, DC or hybrid microgrids (those that combine elements of AC and DC technology that are controlled separately).

AC microgrids can present different distribution types: single phase, three phase without neutral and three phase with neutral.

In DC microgrids, there are three main types of distribution: monopolar, bipolar and homopolar. Since there is no reactive power element of a DC system, DC distribution can present some advantages, such as reduction of the reactive power losses and an increased utilization of the capacity of the electrical lines . However, this is perhaps at the cost of technical control and protection design complexity when compared to a traditional AC solution.

Full DC distribution systems have only been used for limited applications so far, such as telecommunication systems, vehicles, ships and traction systems [4]. Hybrid AC/DC microgrids can be seen as a beneficial compromise to facilitate the connection of various AC and DC power sources and load demands with the power system in order to minimize installation and operational costs.

Monitoring and Control

Certain microgrid management strategies gain commercial benefit from maximizing the use of their indigenous renewables or other local generation capacity and selling energy back to the grid at times of suitable price differential. However, they can also independently decouple from the main grid if unexpected outages are caused by severe weather or natural disasters.

For those that are forced to be independent and isolated by virtue of their circumstances and operational needs, for example geographically remote sites or off-limits military camps [1], then the objective is solely to manage the supply and demand resources locally available to best effect.

In any case, a key element required for such a system is a fast, responsive and highly functional techno-economic control management system, to coordinate the dispatch decisions and monitor the technical integrity of the system across timeframes ranging from sub-seconds to a daily or weekly basis.

Given the smaller size of microgrids, their lower inertia, the power electronic characteristics of many renewable supply sources and the limited smoothing of demand spikes across a wider customer base, there can be significant stability control, security and power quality challenges inherent in most applications. It is important that these technical challenges are quantified and understood, that suitable control systems are put in place to manage them, and that their implications for constraint on economic operability are properly captured in any modeling and control framework.

There are many parameters in a microgrid that must be continuously supervised by means of a monitoring system, such as voltage, frequency and power quality. Both AC and DC microgrids can have a mixture of hierarchical and decentralized control systems that carry out such tasks, generally comprising three levels: primary control, secondary control and tertiary control.

1. Primary control is often applied in a decentralized manner to require individual elements of the system to be automatically robust to both short-term frequency and voltage disturbances. One can implement microgrid voltage control with the voltage regulated (𝑃𝑉) and unregulated (𝑃𝑄).  
   • 𝑃𝑄 control is an unregulated bus, which holds the active power 𝑃 and reactive power 𝑄 constant at a bus while the voltage is allowed to vary (unregulated) over a limited range.
   • 𝑃𝑉 control is a regulated bus, which holds the active power 𝑃 and the bus voltage 𝑉 constant. Since the voltage is held constant, it is considered regulated while allowing the reactive power to vary within limits.
2. The function of the secondary control is to help the microgrid achieve global controllability by restoring the frequency and amplitude deviations produced by the primary control. Often a Microgrid Central Controller (MGCC) is used.
3. The MGCC may also include tertiary control. Tertiary control ensures economic optimization, based on energy supply and demand trends over a longer timeframe.

“Mars will be the ultimate microgrid. With no centralized power sources, communities will one day rely on decentralized energy systems.”

- SIEMENS, MICROGRID ON MARS