The charts below illustrate what’s become known as the ‘duck curve’. It’s a phenomenon first decsribed in California, but the principles of which apply in other power systems with substantial solar capacity.
The top chart shows how, on a sunny day, generation from solar rises rapidly in the morning, is substantial during the middle of the day and then falls rapidly again in the late afternoon/early evening.
The middle chart shows the impact on the market for electricity from conventional thermal power plants (the purple line). As the sun rises, demand is eaten into by solar, shrinking the market available for those thermal plants (as well as requiring rapid ramp rates up and down on the eveing and morning respectively).
The bottom chart shows how this shrinking daytime market for thermal power plants impacts prices: on this particular day they dropped right down to zero for a substantial time.