Solar Power and Grid Stability: Scenarios and solutions

hawaii.gridx299Renewable energy is often intermittent, and that variability presents a variety of challenges to power grids. The nature and magnitude of the challenges depends on the time frame — from fractions of a second to seasonal or even multiyear variations — as well as the nature of the grid itself. The latter is evident in two of my articles from last week looking at how seconds-to-minutes fluctuations in solar power complicate grid controllers’ efforts to maintain alternating current at the 60 hertz frequency and the roughly 110 volt power levels required by North American devices.

Fluctuating AC frequency stars in my Technology Review dispatch from the paradise of Kauai, where the island utility is riding an electrical roller coaster as it pushes solar towards 80 percent of peak power flows. Clouds form around the Hawaiian islands’ mile-high peaks and wreak havoc as they float over solar farms, eliminating three-quarters of their generation in less than a minute. On Kauai’s tiny island grid the resulting power supply dips are deep enough to slow down Kauai’s handful of diesel-fuelled generators, causing the AC frequency to crash below 60 hertz.

My story tracks the Kauai Island Electric Cooperative’s hitherto troubled experience with using batteries to smooth out the solar power flows. Discharging a first set of batteries to fill the solar power gaps burned out the battery cells.

Larger continental grids experience solar power quite differently, as I described last week in IEEE Spectrum. Fluctuating output from even the biggest solar farms has little impact on a big grid’s AC frequency, which is maintained by the collective momentum of literally hundreds of conventional power plants spinning in sync. However, fluctuating output from lots of small rooftop solar systems can cause gyrating voltage on a big grid’s local distribution lines.

My piece for Spectrum shows how California is empowering solar systems to solve the voltage problem. The inverters that turn each system’s DC output into AC power for the grid are being deputized to serve as miniature grid regulators that monitor and dynamically adjust the voltage levels on the local lines.

California’s ‘smart inverters’ mark an about-face in how grid operators think about distributed power generation, and are definitely a good-news story. As reader @etlipman commented: “You know exciting changes are afoot when regulators and utilities that have in the past perceived something as a nuisance are starting to see it as asset!”