Michelle Gerdes / Creative Commons

A worker installs an inverter on a solar project in Lakewood, California. Many inverters already have "smart" functionality installed but switched off by default.

How an obscure piece of technology will help put more solar on the grid

An esoteric smart-grid technology is gaining prominence as expanded solar capacity poses new challenges to utilities and grid operators.

Advanced inverters, or smart inverters, are sophisticated versions of the devices long used to convert the direct current output of solar panels into the alternating current used by consumers across the electrical grid. Whereas traditional inverters are programmed to shut off during disturbances on the electrical grid, advanced inverters can continue to operate and even assist in smoothing out an increasingly variable grid.

Only a handful of U.S. states make significant use of this emerging technology, but that’s changing as electricity standards and procedures are updated to reflect a modernizing industry. In Illinois, ComEd plans to test smart inverter functionalities as part of a microgrid pilot project on Chicago’s South Side. In January, the utility received a $4 million grant from the Department of Energy (DOE) toward utilizing smart inverters in the so-called “Microgrid-Integrated Solar-Storage Technology” project.

The project will address “one of the key barriers for the high penetration of solar PV systems: the seamless integration of these systems in utility grids,” reads a release on DOE’s website.

Additionally, as part of the company’s Next Generation Energy Plan, ComEd proposed a smart-inverter rebate of $1,000 per kilowatt of generating capacity for residential customers and $500 per kilowatt for commercial and industrial users. The plan, part of broader legislation which faced opposition for its continued financial support for struggling nuclear plants, stalled in the Illinois Senate.

Across the US, widespread use of advanced inverters could more than double solar capacity from about 170 gigawatts (GW) to 350 GW, according to a report released in May by the Department of Energy.

It’s why experts view advanced inverters as a key lynchpin in the suite of technologies needed to accelerate deployment of renewable energy and build a smarter, more flexible grid for the 21st century.

“[Advanced inverters help] make these resources – solar, storage – full-fledged providers of energy services to the grid, instead of … just participating, but not necessarily providing, all these things that are needed,” says Bryan Palmintier, one of the authors of the Department of Energy report and a senior engineer at the National Renewable Energy Laboratory in Golden, Colorado. “While maintaining a reliable energy system, we can have much higher penetrations of these types of technologies.”

A workhorse for the grid

Between 2010 and the first half of 2015, U.S. solar connected to the distribution system (lower-voltage transmission lines that deliver electricity to the end-user) grew from a capacity of 1.8 GW to 11 GW, according to the Department of Energy. That rapid sixfold increase in distributed generation is a tall order for an electric grid that has operated as a one-way street for roughly a century.

Traditional inverters have helped manage this change by converting solar energy into the kind of electricity used on the grid, and by shutting off when there’s a potentially unsafe change in system-wide voltage or frequency. That kind of basic functionality is sufficient for low levels of solar but higher penetrations could pose technical problems.

That’s where the additional functionality of smart inverters comes into play, says Chris McClurg, a senior associate at Rocky Mountain Institute, a nonprofit research foundation based in Boulder, Colorado. Advanced inverters have multiple ports that allow them to manage the flow not just between solar panels and the grid, but also between solar panels and an on-site battery, or between solar panels and an electric vehicle charging station, or between an on-site battery and the grid itself.

On a more macroscopic level, McClurg and others note, smart inverters can help the grid balance the fluctuating quantity and quality of power that comes with increased intermittent resources like solar and wind. Merely shutting solar panels off when the grid destabilizes (for example, when a centralized power plant goes offline) can cause even more headaches by depriving the system of supply. Advanced inverters offer functions that allow solar panels to continue operating and make adjustments to help balance the grid.

“The grid is now much more dynamic,” McClurg says. “It’s having to respond to all these distributed resources that are changing based on wind and sun. So now these smart inverters not only have to be able to deal with sending power to multiple places, but they also have to be able to be smart enough to figure out how to help the grid manage all these fluctuations in power.”

‘It’s really just a software change’

Perhaps surprisingly, many of these advanced features are already installed in inverters around the country, but they are disabled by default. The reason, experts say, is that established technical standards and codes have long required that distributed resources trip offline during grid disturbances. Those standards take time to catch up with evolving technology, but there are signs that these codes are becoming more flexible.

In 2014, for example, the Institute of Electrical and Electronics Engineers 1547 Standard for Interconnecting Distributed Resources with Electric Power Systems (IEEE 1547) was amended to allow utilities to require that distributed energy resources regulate voltage or ride through grid disturbances. A full revision of that standard is underway, but the timeline of that revision is unclear.

In most jurisdictions, another important safety standard, Underwriters Laboratories Standard 1741 (UL 1741), is required for inverters to connect to the grid. UL 1741 is based on IEEE 1547, so it too is undergoing a full revision. In the meantime, renewable-friendly states like California and Hawaii have introduced their own rules encouraging utilities to implement and experiment with advanced inverter technologies.

“It’s really just a software change in the controls and the technology is out there today, literally asleep on the shelf, and, in many cases, even installed in people’s’ houses,” Palmintier says. “It’s just turned off because of the grid interconnection rules in the US that have recently allowed these capabilities to be used and previously didn’t.”

This reporting was made possible by a grant from the Illinois Science & Energy Innovation Foundation.

David Unger is a Chicago-based journalist whose work has appeared in the Christian Science Monitor, InsideClimate News, and other outlets. 

12 thoughts on “How an obscure piece of technology will help put more solar on the grid

  1. Daniel H. Clark, P.E. Engineering Supervisor Columbia Water & Light Dept on said:

    The purpose of UL1741 is to ensure that the deployed solar systems in a neighborhood cannot backfeed a downed utility line during a storm. When the primary voltage fused cutout on the pole feeding the neighborhood with the solar panels is blown, the electrical linemen need to know that the line has been disconnected from all of its energy sources so that they can safely work on it. UL1741 provides this important safety function by requiring that solar inverters are unable to self sustain 60Hz frequency. When the utility 60 Hz frequency goes away, the solar inverters become intentionally unstable as regards frequency, pull away from 60 Hz and trip very quickly. I would like to know how it is possible to have it both ways: to have batteries be able to backfeed the utility lines while making sure that linemen don’t get electrocuted when working on a failed power line.

    • I would guess that the inverters can distinguish between varying levels of instability/disturbance on the grid. Surely a “blown primary voltage fused cutout” looks a lot different than sagging voltage or wandering frequency or phase if a major generator fails, right? I’m handwaving a little bit here, I’ll admit.

  2. Easy answer, at the sign of a disturbance the inverter stops feeding the line and feeds a local battery bank instead. The power is stored in the battery until the repairs are made and when the inverter senses the line is operating normally again it feed the stored power into the grid.

    • In case of breakdown of distribution line connecting distributed roof mounted residents power station , the lineman as per electricity rules shall take permits to work from all those residents power station ,test the power line with a live lineteter, apply earths on both sides of the line, shall carry out the power line repair work. After completion of work ,the lineman being authorised nominated person shall cancel all the permits to work and shall in writing allow energising of the power line. The above cited procedure shall have to followed to carry out repair work on damaged power line as per electricity rules whatever high tech system may be used for connectivity for distributed generators .

  3. This is TOO Funny! The only picture they could find was a picture of the Xantrex GT inverter… Almost 100% failure rate and several million dollars of recalls.

  4. There are two large-scale questions left unanswered. First, the cost of all those inverters, both dollar and resources and CO2 footprint manufacturing and recycling. That adds to the same question about solar panels as the number in use increases. Right now the percentage of grid power from Solar or wind is very small. To ramp up will be extremely expensive compared to MSR/LFTR (Molten Salt Reactors/Liquid Fluoride Thorium Reactors)

    Second is the inevitable problem of solar and wind variability. In order to consider a grid largely powered by “renewables” (meaning mostly solar and wind) you must somehow plan for solar/wind farms widely dispersed over the entire area of the US, requiring massive redundancy, storage, and huge mileage of grid extensions capable of transferring the power from where it happens to be available to the other side of the continent when necessary. The cost of grid extensions is much smaller than the cost of the wind/solar installations, but is still significant, running about a Million dollars per mile, just in dollar cost, and then we need to factor in the CO2 footprint and the maintenance costs. Another very large expense of wind/solar, often ignored, is the relatively short lifespan compared to MSR/LFTR, and the resulting cost of replacement and maintenance.

    Another advantage of a widespread fleet of MSR/LFTR (totally fail-safe, 80-100 year or more lifespan, uses existing stockpiles of nuclear waste as fuel) is that grid extensions are not needed. As the fleet of reactors grows, much of the existing grid would become unnecessary. The cost difference in dollars and CO2 footprint is immense. MSR/LFTR are about half as expensive to build as PWR (Pressurized water reactors, the present industry standard).

    For detailed and fascinating information on the MSR/LFTR and related topics, see timothymaloney dot net.

    Read “Super Fuel: Thorium, the Green Energy Source for the Future” by Richard Martin – for an excellent history of the Nuclear Industry, the invention and testing of the first MSR at Oak Ridge National Laboratory, and the absurd mistakes made leading to the decision to concentrate all future nuclear development to the solid fuel PWR.

  5. Net metering systems do not make long term strategic sense to countries and their grid energy suppliers. These companies have to lay the high tension and backbone network, power management systems and distribution, maintain transformers, service and maintenance crews and we all go solar and alternate energies and give them nothing back? That’s crazy. Net metering works only to stimulate the interest into renewable energy in the short term and make the long term switch to actually going off – grid. Or, pay a high monthly premium to the power supply company so that would actually maintain the grid supply to you which reverts from the traditional role of standard grid supply to one of “back – up” supply when your alternate energy system runs into problems.

    • These are largely sunk costs. Where is new transmission and distribution lines being laid? Net metering so far has only served to offset the deep subsidies given to coal and natural gas – from extraction to transport and refining/packaging, these two energy sources have subsidies that are at least 1x the cost.