Negative power prices are only possible because there are other resources to provide the production baseline. As there exists no system to store power at scale, production must match exactly consumption at any time; so when the wind blows or the sun shines and there is no demand, the generated power is basically worthless.

But this is only possible because renewables represent only a small fraction of all power in most places. They can at peak generate a significant amount of power, but overall most produced TWh come from other sources.