Technical Article

An Introduction to Microgrid Systems

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A microgrid is one of the most discussed system types in commercial and industrial renewable energy right now, and for good reason. This guide covers microgrid fundamentals for PV system designers and electrical engineers: what a microgrid is, the four features that define one, and what a microgrid can actually do once it is built.

 What is a microgrid?

The answer depends on who's asking and answering. Based on our work in commercial and industrial microgrids, most share these four features, all within a defined boundary:

1. Distributed energy resources (DERs): local, on site energy storage and generation sources that can function independently from the centralized, bulk power supply infrastructure.
2. Islanding capability: the system is ready to operate independently of the grid and can transition predictably between grid-interactive and islanded modes.
3. Electrical loads: the known sets of loads that will be served in both grid-interactive and island modes.
4. Dispatchable energy source:  One or more of the DERs can be dispatched at will to serve electrical load. 

This description may feel general or incomplete, especially if you're already developing, designing, owning, or operating microgrid systems in the commercial and industrial space. So let's look at each one.

Starting with DERs: Mayfield Renewables engineers, working as third party consultants and engineers of record, predominantly work with inverter based resources such as solar photovoltaics, lithium ion battery energy storage, and generators.

Yet, with an eye for any energy future that includes a diverse abundance of renewable energy solutions, any good microgrid definition should be considerate of proven technologies such as fuel cells, modular nuclear reactors, turbine-based assets, and any number of non-lithium storage technologies like compressed air, redox flow-battery, thermal systems, pumped hydro, flywheels (to name just a few). Notice also that a simpler system consisting of loads, a generator, and proper controls for islanding capabilities could meet this four-part definition of a microgrid. This working definition is intentionally open-ended to accommodate all possible DER combinations.

Islanding

Some off grid energy systems in remote areas demonstrate features one, three, and four above, but they aren't designed for grid interaction and stay islanded permanently. We won't exclude these from the discussion below, though off grid microgrids are more the exception than the rule based on our experience.

In most cases, the transition from grid interactive to islanded and back again is a key feature of microgrid design, and it comes with real design challenges. How fast does the islanding transition need to happen, and can a blackstart of motor loads be avoided? How should a generator or uninterruptible power supply integrate with a battery energy storage system, and which source turns on first? Who defines and programs the sequence of operations when PV, battery storage, and a generator are all talking to each other?

For more information about islanding,  refer to our sequence of operations explainer article.

Backup loads: which ones, how many, and for how long?

At the onset of a microgrid feasibility study, it’s vital to begin aligning expectations among client(s), design team(s), and other stakeholders regarding load selection and the preliminary resilience targets. Optimizing a microgrid design to meet a facility owner/operator’s specific resilience targets — whether in hours, days, or weeks— usually is accomplished by 1) reducing the amount of load the system needs to serve for some or all of the resilience period; 2) increasing energy generation and storage capacity; or both.  Let’s spend a little time looking at load selection.

Schemes for managing backup loads can range in complexity.  Perhaps you’re committed to a full-facility backup, or you are tasked with backing up a whole campus.

In a partial facility backup design, the loads that the facility cannot do without during an outage can be aggregated separately from loads the facility operator might afford to leave unpowered.

Taking load management one step further in complexity, what if I want to provide full-facility backup for as long as possible, but also have the flexibility to remove some loads to extend the resilience period if needed? There may be any number of reasons to reduce load to the most important ones. When the time is right, a microgrid controller, contactors/relays, and subsystem controllers can be programmed and coordinated to shed predetermined ‘sheddable’ loads in order to keep the most important loads powered. Consider load shedding based on battery state of charge (SOC) as one example, shown below.

Here’s what programmable load shedding can look like in a single-line diagram. In this case, our microgrid includes solar PV (generation), BESS (storage), a grid isolation device (islanding), and two groups of loads (primary backup and sheddable loads).

This microgrid system has two backup behaviors: full facility and partial backup. During full-facility backup,  non-sheddable loads (see ‘Microgrid Agg.’ panel), as well as the sheddable loads to the right side of the diagram receive power.  When the load-shedding threshold of the BESS SOC is reached, the load shedding contactors, in communication with the controller integrated into the BESS, will transition from serving all loads to serving only the key ones located in the ‘Microgrid Agg.’  panel.

Lastly, as with many microgrid design considerations, note that the microgrid backup load management discussion can become even more complicated and more dynamic. We described just one example above, but there are sophisticated market solutions that use real-time data to manage a load-shedding schedule algorithmically. In all cases, which loads will be backed up? is a vitally important question to ask as early as possible in the microgrid design process.   

Dispatchability of energy and energy storage is paramount

The final important reminder is that we need energy storage to fully leverage the benefits of any microgrid. It may or may not be obvious, but a system with only PV has little control over the exact timing of energy generation: these kilowatts are intermittent in nature.  Energy generation sources that are paired with storage become dispatchable, and can fully optimize the intermittent generation sources by storing otherwise curtailed PV energy. What’s more, modern BESS products can stack multiple functions to accelerate a project’s ROI and add more value to the broader grid. 

Looking for an engineering partner to assess the techno-economic feasibility of your next microgrid project? Reach out to discuss how we can support your team today.