ABB's plan to buy Power-One reflects the growing importance of the inverter as the brain to help integrate solar energy into the U.S. electric grid.
An inverter's traditional role is to convert the direct current from the solar panels into the alternating current to feed into the grid. Its role is expanding, however, as power plant developers, utilities and grid operators increasingly turn to inverters for ways to control the flow of solar energy. Functions such as low-voltage ride through, reactive power injection, over-frequency response and ramp-up control are either under consideration or required for interconnecting solar power projects in the transmission and distribution networks.
Inverter makers knew this was coming because they have seen the need for those features in more mature solar markets such as Germany. The U.S. is a younger and fast-growing market. It's is now seeing the first wave of big solar energy projects coming online.
The amount of new solar energy generation grew 76 percent in 2012 in the U.S., according to the Solar Energy Industries Association and GTM Research. The utility segment of the business, in which developers build power plants for utilities or to sell power to utilities, is bigger than the commercial and residential sectors. In 2012, developers completed 152 utility projects, which accounted for 54 percent, or about 1.8 GW, of the solar panels installed that year. Over 4 GW of utility-scale projects are under construction, many of which are in western U.S.
With so much solar energy coming online, utilities and grid operators have been doing studies and pilot projects to figure out how best to manage the infusion of solar energy, especially given that the amount of production could fluctuate widely throughout the day because of factors such as cloud covers. A grid runs smoothly only when there is a balance of supply and demand, something that is easier to achieve with fossil fuel power plants.
Baking more intelligence into inverters is one of the ways to manage a grid with more solar power. The amount of solar energy flowing into the grid isn't significant enough to make an impact on the grid operationyet, said Chase Sun, principal engineer at Pacific Gas and Electric, told me by email earlier this year. But with big power projects coming online and the increase of smaller solar electric systems, including those on the roofs of homes and businesses, there will come a day when the grid "may not have sufficient operating margin to cover for the renewables if a large number of them tripped off unnecessarily due to a major system disturbance."
Inverters serve as a bridge between the solar panels and the transmission or distribution network. So a lot of technical work is looking at interconnection requirements for hooking a solar project to the grid. The Institute of Electric and Electronics Engineers (IEEE) is working on modifying the 1547 standard for interconnection.
Much of the new inverter functions deal with voltage and frequency regulations. Low-voltage ride through, for example, will keep the solar power flowing into the grid even when the voltage of the grid drops. In the past, the conventional wisdom was to shut off the solar power flow if an inverter detects an unusually low dip in voltage of the grid for whatever reason. But doing so may not be helpful. If a bunch of solar energy systems suddenly go offline, then the utilities and grid operators will have to scramble to make up for the shortfall. PG&E uses inverters with low-voltage ride through function in its own solar power projects that are connected to its distribution network.
Power plant developers also increasingly want to what's called "ramp-up control." This function makes it possible to turn on and dial up a power plant's production each day, and is especially critical for renewable energy generation because its rate of production isn't as steady as a fossil fuel power plant. Adding ramp-up control isn't as difficult as creating ramp-down control, though, something that would be easier accomplished with the use of energy storage.