In alternating current (AC) microgrids, there’s always the need to convert direct current (DC) storage, solar and other elements to AC. But with DC microgrids, all that could change — and possibly reap simplicity, quickness, and cost savings, said Abraham Ellis, program manager for Sandia National Laboratories’ renewable and distributed systems integration program.
Discovering whether those possibilities can become reality is the aim of a joint research project between Sandia National Labs and Emera Technologies, an energy technology startup and subsidiary of Emera, which aims to boost the use of clean energy. The startup is focused on researching and developing DC microgrid technology.
What’s difficult about AC voltage? “The AC microgrid requires that all the components of the system be carefully synchronized with the AC waveform. It goes up and down,” Ellis said. That waveform is a sinusoidal curve that cycles up and down 60 times a second, or at 60 hertz.
Simpler way to connect DC sources
“In a DC system, there is no waveform. You can attach elements to this DC power system, and it’s potentially an easier way because you don’t have to deal with synchronization,” he explained. “Assuming we overcome some challenges, DC microgrids are a simpler way to get DC sources connected together.”
In an AC system, synchronization requires matching the electrical frequency, which is generally 60 hertz, and controlling the timing with respect to the system voltage.
“The timing has to do with when the AC waveform crosses zero. Power sources that involve an inverter, such as a PV or energy storage system, use a phase-locked-loop (PLL) control to stay synchronized,” he said.
Power sources such as hydro or diesel generators must adjust their mechanical speed to match the system frequency and AC waveform timing. “The power sources ‘close the switch’ and begin to inject power only after synchronization is achieved,” he said.
On the other hand, in a DC system, it’s not necessary to match frequency because the system voltage does not oscillate in the same way an AC waveform does.
“In this case, the sources only need to match the magnitude of the DC voltage,” he said. And that’s why the DC microgrids are so much simpler.
Emera initially approached Sandia seeking help in is efforts to develop DC microgrids.
The lab and startup then signed a Coooperative Research and Development Agreement (CRDA) providing for the research, which will be funded by the US Department of Energy and Emera.
“The focus of this work we are doing with Sandia is to be able to make clean, community-scale direct current (DC) microgrids mainstream,” said Sasha Irving, vice president, market and business planning for Emera Technologies. Her company and Sandia seek to develop DC microgrids, which she described as “small-scale versions of interconnected electric grids that locally manage energy storage and resources, such as solar, wind and thermal systems, that may connect to a larger host grid.”
Part of the work focuses on creating microgrids that are safe, Ellis said.
“DC microgrids are powered by electronic converters, which can monitor grid conditions and respond much faster than conventional generators can. These fast controls, in conjunction with other safety elements, could be used to engineer a system that is safe for use in homes and businesses,” he said.
The cooperative agreement also focuses on developing microgrids that are secure.
“Some of the CRADA has to do with making sure the systems are secure against cyber and physical vulnerabilities by design. We will apply state-of-the-art methods into the design. We think these types of microgrids are appropriate for applications that require high security,” he said.
DC microgrids for communities
The DC microgrids will be a good fit for community microgrids, he said.
“Say there is a new subdivision created. It might have 50 homes, a grocery store and a gas station,” Ellis said. “If that community wanted to be more robust, you would want to add energy storage and some local solar PV. In order to have those resources, you have to add resources that are inherently DC.”
With an AC microgrid, it’s necessary to convert the power to AC, and connect to the utility. “We would need to contend with AC components that need to work together and be synchronized and controlled. Our thesis is that it would be easier if it were DC in the first place,” he said.
Under the subdivision scenario, a DC microgrid could “live happily connected to the grid,” Ellis said. “But it can also be an autonomous unit, or islanded microgrid. It the large grid is not available, it can function on its own.”
Under the project, the hope is that a DC microgrid will live happily in communities, military installations and other applications — and ultimately be less expensive, simpler, and safer than an AC microgrid.
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