Comment by jerven

The plants that are willing to give supply for the most negative price are the ones that will not be curtailed. So market forces. Basically at such points power plants are paying for the privilege to be allowed to supply power. This is dominated by restart costs and as such is often paid by classic "baseload" plants such as nuclear ones. i.e. they will accept losing money during one part of the day/week so that they can make money during a different part of the day/week.