In this April 3, 2017 article in Green Tech Media, author Bentham Paulos, thoroughly explores and explains the issues and challenges involved with integrating solar and wind energy into California’s electric grid.  One key excerpt:

“It’s an interesting growing pain of our increasingly green grid,” said Shannon Eddy of the Large-Scale Solar Association. “We’re curtailing the cleanest and newest resource on the grid, and leaving alone the 2,000+ megawatts of mostly fossil imports and in-state gas.”

And the growing pains will likely continue. The latest U.S. Solar Market Insight report from GTM and SEIA counts solar projects in the works that will double California’s capacity from 17 gigawatts in 2016 to 34.5 gigawatts by 2022. With springtime power demand peaking at well under 30 gigawatts, we may see a very solar future.”

Curtailment and negative prices come from a surfeit of generation. Lots of generation relative to supply means low prices, as generators compete to be called on by the ISO market optimization software. Too much generation, or “oversupply” as CAISO calls it, can create reliability problems, and must be curtailed.