Rooftop solar panels can really destabilize the grid with their on-again, off-again power, if you’ve got enough of them concentrated in one place. It’s a good thing, then, that the inverters that connect solar panels to the grid can be designed to solve most of those problems -- and even add some new grid-stabilizing features of their own.
So why do the IEEE 1547 regulations governing solar-grid connections in the United States demand that those inverters disconnect any time the grid gets unstable? There’s a simple answer — it’s for safety, to make sure solar power doesn't flow through a downed power line and shock a utility worker, for example.
But that safety measure also prevents solar inverters from helping out in cases in which the instability isn’t caused by a downed line. Indeed, by turning off lots of inverters all at once, it can actually destabilize the grid even more.
What would be ideal is for the inverter to “survive that, ride through it for seconds at a time, and immediately resynchronize and continue wattage output as that fault clears,” is how Joe DeLuca, vice president of product marketing for Petra Solar, put it during a Wednesday talk at The Networked Grid 2012.
That’s just what Petra Solar’s smart energy modules -- solar panels with smart inverters and communications included -- do for the utilities like New Jersey’s PSE&G that mount them on their utility poles. They’re allowed to do so because they sit “behind the fence,” in utility parlance, not in the homes or businesses of customers.
Chalk it up to one of the many challenges facing utilities as they plan for a future of ever-increasing amounts of solar power. Most of the world’s solar PV capacity ties into the distribution grid, and as local penetration of solar grows beyond the 15 percent to 20 percent range, it becomes an increasingly destabilizing force on those local grid systems.
Germany is on the forefront of those challenges, with nearly 25 gigawatts of solar PV installed to date -- 2 gigawatts installed in the month of December alone. About 85 percent of that is tied into low-voltage distribution grids, which has forced Germany to deal with its problems earlier than the U.S., said Elie Nasr, director of utility business development for the Americas unit of German inverter giant SMA.
Germany has done so by requiring its solar inverters to perform certain functions, such as power ramping and volt/VAR control, which lead to more stability, Nasr said. It’s all part of the VDE AR-N 4105 rules that came into effect for medium-voltage connected solar in 2008 and for low-voltage solar as of January 2012.
But in the U.S., where IEEE 1547 holds sway, SMA can’t turn on those capabilities in any inverters that sit outside utility property, he said. Even so, SMA is prototyping European-style inverters in U.S. pilot projects, in hopes that IEEE 1547.8, the latest update to the standard, allows inverter manufacturers to provide those smart grid features, he said.
The Electric Power Research Institute has been working on this problem for years, said Thomas Key, a technical executive with EPRI. One way it’s helping is via its role in developing standards that set key functionalities for smart inverters and link them to the various communications standards used in the grid.
It’s also working on a string of pilots across the country, as part of the Department of Energy’s SunShot Initiative and its Solar Energy Grid Integration Systems (SEGIS) Advanced Concepts program. In September it won a $4.4 million DOE grant to demonstrate how smart inverters could assist, and is now working with utilities San Diego Gas & Electric, National Grid, DTE and Xcel on testing those out.
Of course, while inverters can help smooth out a lot of the grid-destabilizing effects of solar power’s intermittent nature, they can’t help replace the power that disappears when clouds pass over a solar array. For that, utilities will either need to secure balancing power from other sources -- fast-reacting natural gas-fired peaker plants, most likely -- or to fold batteries or other energy storage systems into the mix.
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The upcoming GTM Solar Summit includes a session called "From Grid-Tied to Grid-Integrated" which probes deeper into the inverter-grid dynamic. Go here for more event information.