The Danes are finally going nuclear. They have to

Electrical equipment is highly sensitive to this frequency and can break if it deviates too much from 50 Hz. For this reason, power stations, substations, switching equipment and other devices in the grid have fail-safes which will cause them to trip out should frequency fall outside acceptable bounds.

This frequency property is also connected to the balance between supply and demand: if there is more generation than consumption, frequency increases and the turbines speed up, and if there is more demand than generation, the opposite happens.

Fortunately, conventional generators are large heavy lumps of metal whose speed of rotation is hard to change. They oppose changes to their speed of rotation, providing resistance to changes in grid frequency, a property known as “inertia”.

However, wind and solar do not produce synchronous alternating current. Although wind turbines rotate, they do not do so at a constant speed, and solar has no moving parts at all. They produce direct current which is converted to alternating current using electronic devices known as inverters. Wind and solar also have no inertia.

The situation in Iberia highlighted the difficulties with running low inertia power systems: an as yet unidentified grid fault caused a disturbance and frequency changed. There was not enough inertia to contain the frequency shift and inverters dropped out, widening the gap between supply and demand and worsening the frequency deviation. This caused other equipment to disconnect, quickly leading to a cascading blackout. The process was too fast for grid operators to take action to contain it, due to the lack of inertia. If there had been more time, generators could have been shut down or started up in order to match supply to demand and get the grid frequency back to 50 Hz.