In the last article on this topic, we discussed the relative merits of building a DC microgrid. We, of course, won’t regurgitate that information, as you can read it for yourself by clicking here. At this point, we’ll assume you are sold on the benefits of a DC microgrid, and we’ll address some more practical technical considerations to keep in mind when setting about building one.
What is the problem you are trying to solve?
Planning a DC microgrid is no different than any other technical undertaking. You need to start with defining the problem you are trying to solve. In the realm of microgrids, several buzzwords often come to mind including resilience. But you’ll need to drill beyond just buzzwords and really understand the problem you want to solve, whether it is offsetting higher demand charges, creating greater resiliency from grid outages or something else altogether. Answering this first question will determine if a DC microgrid even makes sense versus an AC coupled topology.
Define the components
Once you have a clear understanding of your problem statement, you can go about selecting the various key components for your microgrid. While DC microgrids are still largely in their infancy, many of the ones that have been deployed typically have solar and battery energy storage connected on the same DC bus in front of a grid-tied inverter. In many cases, though certainly not all, the inverter used in a DC microgrid is grid forming, meaning it can provide stabilization to the system in the event of a grid outage. Of course, solar and storage aren’t required elements of a DC microgrid either, though they are the most common. Solar provides the least expensive, cleanest and easiest to install form of distributed generation, while battery energy storage is a great way to absorb excess energy during times of overproduction and then discharge that energy when the PV is unavailable (i.e., nighttime) or when the PV is underproducing (overcast or cloudy conditions). The beauty of the DC microgrid is that it is ideal for all manner of today’s clean generating and consuming sources and loads, including hydrogen powered fuel cells and electric vehicles.
Match the voltages
Once you’ve defined the problem(s) you are trying to solve and the key system components you’ll use to solve those challenges in a DC microgrid, know that you will need to match the voltages of these different DC sources and loads on a DC bus. This is where a device called a DC:DC converter comes into play. The role of the DC:DC converter is to “map” different voltages into a singular, common voltage level into which all of the parts of the system can operate. For example, let’s take a “typical” DC microgrid (and let’s be clear, DC microgrids are still not yet at the level of penetration where any of them can really be called typical) that consists of a PV array, a battery energy storage system and a bidirectional, grid forming, battery inverter. In such a system topology, the DC bus voltage will typically be dictated by the battery. In this case, a DC:DC converter should be placed in the PV array to harvest the solar energy at the PV panels’ maximum power point voltage (also called MPPT) and then deliver on the output a voltage that matches that of the battery at its current state of charge. That’s just one example, but the point is that a DC:DC converter is the “glue” that holds a DC microgrid together.
Isolate the grounding
One of the devilish details of DC microgrids is that their various components can have different grounding schemes. Overcoming this challenge is one of the factors that have held DC microgrids back from greater acceptance. While connecting various DC sources and loads by converting to AC is generally more expensive and less efficient than DC coupling, it does provide several inherent safety benefits by electromechanically isolating the system components from one another. However, using a galvanically isolated DC:DC converter can offer you the safety benefits of an AC coupled system while allowing you to enjoy the other benefits of a DC coupled system discussed here.
Define the control system
No discussion of any microgrid, DC coupled, AC coupled or otherwise, is complete without highlighting the importance of the control scheme. Fortunately, today there exist many excellent providers of microgrid controllers. Be sure to interview some of them and have your problem statement clearly defined before kicking off your microgrid project.
Hanan Fishman is the president of Alencon Systems, a manufacturer of DC:DC power converters.
