That doesn't sound right. According to [0] that is 2/3 of total electricity generated, from all sources, in the whole of US.
[0] http://www.wolframalpha.com/input/?i=average+electricity+gen...
The key word is "curtailment" for renewables: https://electrek.co/2017/03/27/california-solar-wind-renewab...
For wind farms, this just involves turning the blades on their axes like "feathering" a propellor. This reduces efficiency and hence power output, potentially to zero. Turbines may also need to feather if the wind is too high to avoid exceeding max speed.
All larger systems - coal, CCGT etc - have some kind of system for throttling back input of steam or gas to turbines. Nuclear power is slightly different in that it takes a long time to cool down the reactor even once you slow the reaction, and heat is still generated from intermediate decay products. So they have "bypass" systems instead for dumping steam.
All power plants have to be able to survive the loss of their upstream output load without self-destructing. In case the wires go down.
Spot prices may also go negative for short periods, but that's a different thing. https://www.cleanenergywire.org/factsheets/why-power-prices-...