In solar installations, ranging from rooftop panels and larger community- and utility-scale arrays, photovoltaics (PVs) are becoming increasingly important to our energy supply. With the growing presence of such intermittent resources comes mounting concern regarding their impact on overall grid stability. The momentum toward a greener energy supply isn’t likely to slow down, so equipment manufacturers are considering new ways to help renewable generators behave a little more like traditional baseline power plants by improving system intelligence at the point where such supplies meet the grid.


The need for new ideas is becoming more obvious with every new report on the nation’s energy portfolio. In September, the U.S. Energy Information Administration (EIA) reported utility-scale solar installations were up 70 percent for the first half of 2014, versus the same period last year. Residential solar was up 79 percent in 2014’s first quarter, compared to the same period in 2013, and more than one-third of those projects received no state funding. This growth trajectory has utility planners ever more concerned regarding the voltage irregularities that passing clouds and wind lulls could cause going forward.


Because no one can control the energy going into a solar panel the way a coal or natural gas plant’s owner can, solar experts focus on the power coming out of renewable generation equipment in their efforts to improve solar/grid integration. Smart inverters that can help smooth energy output and supplies and enable new revenue sources are at the top of the list of devices under development to meet this goal. 


“As the number of distributed generation [DG] installations—and aggregated capacity—increase, the impact of DG to the existing power grid is getting more important,” said Soonwook Hong, manager of the power systems engineering group of inverter maker Solectria. “Smart inverters can provide flexible controls by adjusting both real and reactive power to increase power-system stability, reliability and power quality.”


These newer inverters’ ability to respond to utility signals in more nuanced ways than current, standard products makes them smart. Among the biggest (and most controversial) improvements is the ability to keep PV systems connected to the grid during brief low-voltage conditions (a functionality called “ride-through”). Both governing grid-interconnection standards—UL 1741 and IEEE 1547—require distributed resources to disconnect from the grid during any voltage irregularity to protect line workers from electrocution. However, dropping PV systems during brief low-voltage conditions might make a temporary problem more serious by pulling power supplies off the grid when they might be most needed.


This isn’t new technology. Smart inverters now are being retrofitted in large numbers in Germany, where the government’s Energiewende (energy transition) policy has driven grid operators to integrate a tremendous amount of solar energy into their generation portfolio. To address resulting irregularities, the country is replacing more than 315,000 existing inverters with new, smarter models. The cost has been reported in the hundreds of millions of dollars.


Now the move is on to adopt these new inverters in the United States before problems similar to Germany’s begin to plague our grid, as well. (Using smart inverters in new installations is an affordable fix because they would only add $150 or so to the total system price.) As with many of this country’s solar innovations, California is taking the lead with a Smart Inverter Working Group (SIWG) that has brought together energy and regulatory experts to outline requirements and plan a product rollout schedule. Pilot projects are underway to test the inverters as well as the software and communications capabilities making the equipment so smart. These efforts are looking at both utility-scale and rooftop systems to ensure safe operation whatever the system size. California’s SIWG anticipates commercial introduction by October 2015.


In addition to looking at voltage ride-through, the SIWG also is anticipating smart inverter use to help provide power quality support by adding reactive power when needed. This kind of value-added service could be a revenue producer, especially for larger solar arrays; however, since reactive power can’t be metered in the same way as actual power, current pricing systems would need adjusting.


“If a smart inverter and a conventional inverter are used in the same distribution feeder, and the smart inverter sacrifices the real power generation in order to provide the grid-supporting functions, it should have some type of incentives,” Hong said.


Neither the pricing nor the setting of standards are simple processes. Determining how smart functionality would work involves addendums to the Institute of Electrical and Electronics Engineers’ (IEEE’s) Standard 1547 and corresponding updates to the test procedures used by Underwriters Laboratories (UL) under its own Standard 1741 to ensure smart inverters are just as safe to operate as existing units.


“It is very important to have common industry standards for the test method and certification process,” Hong said. “Otherwise the utility companies will generate their own standards and inverter manufacturers [will] need to comply with standards from all different utilities.”