[toast0]

When there's too much energy available for the grid, and the price goes negative, producers are paying for someone to use that energy or paying another potential producer to reduce their output.

Some industrial users have variable demand, and a lower (or negative) price could encourage them to use more. A multi-region internet service might send more traffic to a datacenter with negative electricity prices, even if in increases latency for users.

Some producers need time to modulate output, and stopping and restarting can be expensive. Solar and Wind are at least technically easy to start/stop, but subsidies may make it economic to pay the grid to deliver electricity; either because of contracts/subsidies, or because the expense to deliver unwanted electricity is less than the expense to monitor pricing and reduce production.

[bdhdbebebeb]

What happens if no one takes the power (not one solar installation but lets assume lots of surplus power). Does it screw up the grid? Increase voltage/frequency?

Would this lead to grid needing to be shut down?

[toast0]

Yes, if nobody removes the excess, grid frequency will increase. Running too high or too low frequency can damange equipment, on both the supply and demand sides.