In this post by HOMER CEO Dr. Peter Lilienthal, we consider the problem of how to classify microgrids. This is a special entry in our microgrid learning series.
Although microgrids are becoming more and more prevalent in both the press and in energy world, there is no common definition of exactly what constitutes a microgrid or how to differentiate among types of microgrids. The term microgrid is not well defined or distinguished from other terms, such as mini-grids. As microgrids become a more well-established part of our global electrical system, the need for a common set of definitions will become increasingly important.
In this post I suggest there are four main categories of microgrids and present a set of criteria that distinguish them.
The two most important criteria are: (1) whether the microgrid is ever connected to a larger grid and (2) the type of dispatchable generation. The type and extent of the distribution within the microgrid is another important distinction, but distribution has multiple alternatives, which may make it hard to agree on defining thresholds.
Size could also be an important criteria, but it is a continuum and there are multiple ways to define size. The other criteria may be a more clear way to distinguish between different size systems.
Finally, controls are a potential differentiator, and there are many different kinds. Switchgear to handle connecting and disconnecting to a larger grid is covered by the first criteria. Controls to handle paralleling of generators is handled by another criteria. The remaining controls are mostly about load management. This is a very important issue, but it is challenging to categorize as there are many options.
The following table summarizes these various criteria and the issues with using them to categorize microgrids:
|Is it ever connected to a larger grid?||Yes/No||The USDOE definition precludes systems that are never connected to a larger grid|
|Dispatchable generation||Multiple genests that can operate in parallel,Single genset (although there may be redundant spares, only one can run at a time),Batteries only|
|Voltage level of distribution system||Low voltage DC: 12 – 48 voltsHouse current: 100 – 250 voltsLow voltage distribution: ??? – ???Medium voltage: < ???||Some island grids have medium voltage distribution.|
|Phase||Single- phase or 3 phase||Could be part of voltage criteria|
|Size||KW of peak load||Continuum with no clear thresholds|
|Size||KW of generation capacity||Continuum with no clear thresholds|
|Size||KWHs||Continuum with no clear thresholds|
|Size||# of customers||Continuum with no clear thresholds|
|Load management||Many||Really important topic, but not obvious how to categorize|
|Metering||Lots of innovation||Really important topic, but not obvious how to categorize|
The intersection of size (large or small) and grid connectivity (connected or remote) results in four main microgrid types, although the last type probably deserves further categorization:
Large grid-connected microgrids, such as military bases and large campus applications, are connected to a traditional utility, but capable of operating in island mode. They have multiple generators and may have substantial distribution and sophisticated controls within the microgrid.
Small grid-connected microgrids have a single genset, but the genset is supplemented with storage and renewable, as appropriate. Grid-connected microgrids are typically in developing countries with unreliable grids where the backup generator is used frequently. These would not have significant distribution within the grid. They could have additional load management and other controls, but those would more commonly be manual. Many people may not consider these to be microgrids, but they share many of the characteristics of other types of microgrids. I argue that as long as they can operate independently from the grid, they are a valid (and potentially very important) form of microgrid.
Large remote microgrids, such as island utilities, have multiple generators and substantial distribution.
Small remote microgrids could be categorized more finely. They will usually not have more than one genset, and the smallest may have none. They should not have step-up transformers for distribution. Some of the very small ones may have DC distribution. Innovations in billing and payment methods could greatly enhance the potential for these types of microgrids.