PV to stabilize grid around the clock?
A new inverter technology allows inverters to act as phase-shift oscillators, thereby providing the grid with reactive power from solar plants even overnight – and lowering the cost of grid expansion considerably in the process.
Large solar power plants are crucial for the switch to renewable power. "All technologies that produce power irregularly are a burden on the grid," says Bernhard Beck, head of Germany's Belectric, one of the world's largest providers of turnkey solar power plants. "With new central inverters, power plants with a capacity of 4 to 8 megawatts can help keep the regional grid stable. For the past year, we have been using inverters that even provide reactive power at night, which provides us with completely new ways of stabilizing the grid."
The German grid is broken up into four levels. Ultra high-voltage lines have at least 380 kilovolts and high-voltage lines at least 110, whereas medium-voltage lines that regionally distribute power have 20 kilovolts – and local distribution grids (to which retail consumers are connected) have 400 volts and three phases. If too much energy is exported to the grid, the voltage increases along with frequency. Lines then began to overheat, and grid operators provide additional reactive power from central plants. Unfortunately, Germany has less reactive power in the south now that so many nuclear plants have been shut off there.
"But utility-scale solar can do the job," Beck says. "A solar farm with hundreds of megawatts can produce enough reactive power to offset the grid-stabilizing effect of a nuclear plant." The new inverter function with software from SMA can also be used in inverters for wind turbines. But small inverters for rooftop arrays cannot perform this function because the equipment would be too expensive.
Germany has a growing share of solar power, wind power, and power from biomass. But increasingly, reactive power – not real power – is turning out to be the bottleneck when it comes to stabilizing the grid. "If a large solar plant in Bavaria produces reactive power, ultra high-voltage lines to the North Sea might even be able to be utilized better," Beck surmises. "In our estimation, it would then only cost half as much to expand the grid" because local reactive power and high-temperature cables would mean that the amount of power on the grid could be tripled without having to build a single new pylon.
Phases of power shift when the three phases go out of sync. By pushing the three phases back into rhythm, the grid can take up much more power. As Beck explains, "During the day, the solar array affects the phase shift via the energy it produces. At night or when the sky is cloudy, it then takes some of the real power off the grid and puts back a much larger phase shift with very low losses."
In the process, grid control is partly passed down to distributed solar plants and wind farms connected to the low-voltage or medium-voltage grid. Transformers can work in both directions. "Germany is currently talking about the cost of solar," Beck argues, "but instead of focusing on reducing feed-in tariffs for solar, PV arrays should be required to help stabilize the grid. Ground-mounted arrays larger than 30 megawatts should be required to provide reactive power regardless of how much they are producing at the time. And only then should they receive the full tariff for ground-mounted PV." (Heiko Schwarzburger / Craig Morris)