Building a microgrid from off-the-shelf components, putting them all together, and successfully running them through their paces, on- and off-grid -- without blowing anything up?
Usually, that kind of job could take months, if not years. But using the power of common technology, Duke Energy and some two dozen partners were able to put together a working microgrid and run it through its paces, in less than a month -- and set up a live demo at the DistribuTech conference in Orlando in early February to show how it was done.
“We’ve been able to do this in about two to three weeks, with many different pieces of off-the-shelf equipment that don’t require a whole lot of customization,” Jason Handley, director of operations and projects for Duke’s Emerging Technology Office, said in an interview before DistribuTech.
This result from Duke’s Mount Holly, N.C. microgrid test site (PDF) was made possible by several years of work beforehand, however. We’ve been following the saga of Duke’s efforts on this front, which has led to the creation of a new standard, the Open Field Message Bus (OpenFMB), and a growing roster of grid vendors willing to make it part of their future plans.
Duke has been planning its microgrid for a year now -- we covered the start of the Mount Holly project in February 2015. But all the gear to allow the utility lab to island itself from the grid was installed only three months prior to the beginning of the testing period, and the 250-kilowatt battery system was only installed three weeks beforehand, he said.
The use cases being tested are microgrid optimization, unscheduled islanding transition and island-to-grid connected transition -- three of the capabilities most microgrids might be expected to handle. Unscheduled islanding is perhaps the most obvious scenario, allowing the microgrid to keep running when grid power goes down.
“We’re able to island the microgrid, using the battery and the solar, to support the load of the lab without any interruptions,” he said. This requires some very fine-tuned synchronization of the inverters and other power electronics systems that step in with alternating current to replace grid power, and Duke struggled in its first test runs with equipment that didn’t activate in proper sequence, Handley said.
“The sensing was a particular problem for us,” he said, and required Duke to measure phase angle and frequency at multiple locations throughout the microgrid. The same challenge arises in the second use case of island-to-grid transition -- reconnecting to the grid -- which requires synchronization with the power flowing from the grid.
“The problem with doing it wrong is that you could potentially damage equipment or have an outage,” he said. “We’re not 100 percent yet, but we’re doing islanding and reconnecting where it’s invisible.”
The optimization use case involves making the most of the on-site solar power and energy storage, Handley said. “It doesn’t matter if you’re grid-tied or islanded, you’ve got to use your distributed energy resources in the most optimal fashion,” he said. “We’re coordinating how much battery input, how much solar input, is going to the grid.”
Duke does face a complication in this task, however, he said -- “Our interconnection agreement with Duke states we can’t push any power back onto the grid. If we’re over-generating, we have to consume more somehow.” To deal with this, Duke installed a resistive load bank, “essentially a big fan with a heater on it,” able to absorb up to 500 kilowatts of power at a time.
Eventually, Duke wants to be able to connect its back-office SCADA systems to its microgrid, to allow the utility to interact with this two-way power providing capability. In all three cases, multiple devices are required to share data with each other -- “Without the sharing of the data, none of this would be possible without a whole lot of back-office integration work.”