Alencon Systems teamed up with Duke Energy to look at some of the challenges faced by a utility when adding a battery storage system to an existing solar installation. DC-coupled or AC-coupled? Grounding scheme?
May 13, 2020
Solar and battery storage make such a natural pairing that it’s becoming surprising to hear about a new utility-scale solar project that isn’t incorporating battery storage. While solar and storage are now like peanut butter and jelly, this wasn’t always true, and the U.S. is home to gigawatts of stand-alone solar projects.
But what happens when the owners of these peanut butter sandwiches decide they want jelly?
Working with Duke Energy in North Carolina, researchers at Alencon Systems studied specific issues that can arise from retrofitting existing solar structures with battery storage systems. The study specifically looks at DC-coupled storage systems, which higher-ups at Duke, like Business Development Head Tom Fenimore, believe offer compelling advantages over AC coupling.
Many recent PV projects have been subject to increasing DC:AC ratios, alongside which, modern inverters have become capable of handling these DC overbuilds. With a DC-coupled battery system, any generation that exceeds the power rating of the inverter can be immediately diverted into a battery during periods of overproduction. With an AC-coupled battery, this isn’t possible and the generation is lost.
For a utility that is reliant upon being able to dispatch electricity quickly and predictably, this is a much more appealing scenario. It also helps that any storage project looking to get investment tax credit incentives must be charged directly from the PV. This is something that Fenimore identified to Alencon as being a serious consideration Duke makes in battery projects. Why make a project more expensive if you don’t have to?
Issues in installation
Deciding on DC-sided storage, however, is not the end of installation complications. Something Alencon found to be a common issue for Duke was the need to isolate the grounding of the PV system from its new battery. PV projects connected to inverters are all typically grounded. usually they are grounded negatively, but some older projects, including some included in the study, are positively grounded.
Regardless of grounding polarity, battery energy storage systems need to have floating grounds as a critical safety component. This is where galvanic isolation is utilized. Galvanic isolation is, as put forth in the study “An effective method of breaking ground loops by preventing unwanted current from flowing between two units sharing a ground conductor.” Practically, this is useful in solar projects where electric circuits must communicate, but their grounds are at different potentials.
Putting this all together: “A galvanically isolated DC-DC converter, such as those manufactured by Alencon Systems, serves the dual purpose of mapping PV voltage to battery voltage while isolating the differential grounding schemes that could be present.”
The study relied partially on Duke’s McAlpine Solar plant battery retrofit in 2019 for the real-world application of the retrofit issues and solutions presented. This installation feature positively-grounded PV panels, which meant the installation had to feature a galvanically isolated DC-DC converter. It provided about as full of a scope regarding the necessary procedures to retrofit a solar installation with batteries as there is.
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