An obvious barrier to adopting wind and solar power for electricity supply is their intermittency—when the wind isn’t blowing, and the sun isn’t shining, substitute sources are required. This issue is given much attention by conservative media, as it should.
Yet one of the less well-known roadblocks for these renewable technologies is frequency control, even though it becomes a critical concern much sooner.
Since the 1890s, electricity networks and devices all around the globe have used alternating current (AC) systems, which means that the flow of electricity in the system is repeatedly changing direction.
In Australia, it alternates 50 times a second, that is, at a frequency of 50 Hertz (in the USA, it is 60 Hertz).
Supplying electricity at a consistent frequency is very important because appliances and electronics on the network are designed for a specific frequency/voltage input. Therefore, they can be damaged by the wrong electricity supply.
As a rule, networks would rather supply no electricity than bad electricity. Automated controls through the electricity system will disconnect the supply if the frequency or voltage is “off-spec.”
South Australians will not soon forget when this happened to the entire state network in 2016. The state-wide blackout started late in the afternoon during some poor weather conditions, and thousands of people had to drive out of the city without any streetlights or traffic signals.
There were a range of contributing causes, including gusty winds taking down some transmission lines and a lightning strike on a power station.
After those physical causes, automated protection systems took over. Wind turbines disconnected themselves from the network. The system naturally started drawing more load from all remaining supplies, which maxed out the capacity of the interconnector to the rest of the East Coast network, which consequently disconnected.
From that point, the shutdowns cascaded throughout the whole network. This all happened in less than a second.
The potential for a cascading shutdown can never be entirely eliminated; automated protection systems must make decisions at a speed that prevents any human involvement.
Nevertheless, the vulnerability of the whole system can vary, and increasing intermittent renewables contribute to decreasing the system’s stability.
Traditional vs Renewable Generators
Traditional generators use turbines—steam turbines, open-cycle turbines, and water turbines (hydroelectricity). This equipment is called “synchronous” because the frequency of the electricity they produce is directly linked to the speed that the shafts of the turbines rotate.
Because these machines are large and heavy, it takes time and energy to speed them up or slow them down, which means that the frequency of the electricity cannot change too quickly. This is called “inertia.”
As you may imagine, solar panels, having no moving parts, do not provide inertia. They match whatever frequency is already in the system; they do not help stabilise it.
Wind turbines, though they do have large spinning components, change speed all the time merely due to wind conditions. Hence they are not designed to synchronise with the AC network. So they do not provide inertia either.
If a system does not have inertia, then instead of gently responding to a change in load, the frequency can flail about like a cyclist getting speed wobbles (any engine can have the same problem if it doesn’t have a sufficiently heavy flywheel)….