Largely the negative prices are a side-effect of feed-in tariffs, where (some) renewable projects get paid a fixed price/kWh regardless of what the actual spot

Largely the negative prices are a side-effect of feed-in tariffs, where (some) renewable projects get paid a fixed price/kWh regardless of what the actual spot market price is. So when there's an oversupply of these subsidized renewables it pushes the spot price to negative. I think newer renewable support schemes have been designed to avoid this problem, either by cutting off the feed-in tariff when the spot price goes to zero, or then by restructuring the subsidies as some kind of investment support (state guaranteed very low interest loans or such) rather than paying for the generated electricity. That of course doesn't change the fundamental issue, in that in a grid with increasing amounts of variable production renewables and the price determined by marginal cost of the most expensive producer, the price swings will be larger than what we previously were used to.