The short answer is that they don't over produce. If there is more energy being generated than used, turbines (for example) will start speeding up. In the very

The short answer is that they don't over produce. If there is more energy being generated than used, turbines (for example) will start speeding up. In the very short term, the extra energy goes to accelerating the turbines. This can be observed by the grid frequency increasing. Immediately, generators will be throttled back to reduce production. If this can't happen fast enough, generators will be cut out of the system entirely, and their excess energy will almost certainly be released as heat somewhere. Imagine you are driving a car downhill and the car starts speeding up. You let off the accelerator in order to not go too fast. Worst case, you apply the brakes and dissipate the energy as heat.