This.

For those not in the industry, the drivers for this are green tags and production tax credits (more common in wind).

Green tags are basically attaboys for funding the generation of renewable electricity, and are tradable.

Production tax credits are a $/MWH tax incentive for generating renewable power, and are, again, tradable.

In both cases, then, there are incentives for renewable projects to keep producing power even when the wholesale power price at the point of interconnection is negative, as there are generation incentives that still make it better than idling.

From an environmentalist perspective, this is fantastic, as virtually all of this renewable generation represents offset coal and gas peaker plant generation.