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The grid needs to maintain supply within certain levels.

If you produce more than your share, someone else needs to shutdown.

Shutting down certain power sources (e.g. coal) comes at a non zero cost. Someone still has to pay the power plant that needs to shutdown or send people home unexpectedly. These plants might be have pre secured contracts to sell at a given price over a given time period further adding to the cost of shutting off their supply.

Some more info here:

https://www.epexspot.com/en/company-info/basics_of_the_power...