Total electricity generation must be equal to total demand second-by-second around the clock and total UK demand varies around the clock, second by second, from middle-of-night base load to peak demand rise when people reach the end of Coronation St and switch electric kettles on almost simultaneously around the nation.
This rapid rise is matched by fossil generating plant, hydro and pumped storage. This is mainly automatic by closed loop monitoring grid frequency (50Hz) which has to be maintained within +/- a small fraction of 1%.Nuclear is not used for frequency control for economic reasons (it shortens the economic life of the fuel, but it can be, and is, used as force majeure). When frequency falls below 50.00Hz, valves open automatically on all fossil plant to increase steam flow to turbine generators, and vice versa. Hydro is also used, but we don’t have much.
Wind generators can only generate what the variable wind provides (within rather tight limits as well. Off design point operation (at lower wind speeds) loses output very fast because wind energy flux varies as (wind speed) cubed. When the wind speed falls or rises, most of the compensation from other generators is fossil-sourced in UK and this standby generation has to be available at all times for grid management. It is set statistically as a strategic fraction of total generation to cover plant failures, and now also, to cover unpredictable wind output. The fossil stations on standby must be ticking over at low load with boilers and turbines primed at high temperature for fast pick-up. At this condition they are operating at very low thermal efficiency with higher atmospheric emissions. As wind generating capacity increases, more fossil stations must operate in this standby mode because the potential fall or rise in wind output to grid increases. Hence, more emissions attributable to wind, which the operators of wind do not pick up the tab for!
Not only that, but we are running out of conventional capacity we can use. The European emissions directive has given fossil plant operators a fixed budget of CO2 (plus other gases), after which they must back fit emission control plant. The operators of such plant then decide that the cost of backfitting is not worth it and shut down their plant for economic reasons. They are using their budget of emissions faster because they are compensating for more and more wind output. This is forcing many of them to review the economics of backfit earlier than they expected. The operators of Cockenzie have already said they will close down as soon as they have used up their emissions ration. This behaviour is accelerating the onset of grid frequency control difficulties. Germany has big problems already.
Grid control experts prescribe a fractional limit of wind as a % of total capacity for sensible grid management, maybe around 40% but expert opinion varies. In the days before electricity privatisation, the CEGB had a statutory duty to maintain grid stability. No single company now has that duty. The new mantra is that market forces will provide (just like it works with banks and pensions).