Renewable Electric Power and Grid Inertia
The Telegraph published an interesting opinion piece on unanticipated problems with increasing dependence on renewable energy sources in the UK. The author was Dr Capell Aris PhD, a former Fellow of the Institute of Engineering and Technology of the UK. The thrust of the article has to do with grid inertia. It turns out that rotating generators have energy stored in heavy rotating turbines that can compensate for short interruptions in the power grid. Wind and solar power generating stations do not have inertia like a steam or gas turbine does at a conventional power plant.
A critical parameter that power generating stations must watch is the line frequency. The entire US power grid must be producing 60 Hz power all of the time for stability. The load on a system must be in line with the supply. If the load exceeds what the grid inertia can supply, then overloaded generators will begin to slow with reducing line frequency and voltage. This forces the plant to disconnect neighborhoods on the power grid to bring the frequency and voltage back into line. This is called load shedding. If the plant generator frequency drops outside of specifications, the plant must disconnect. Allowing the generator frequency to stray too far from 60 Hz +/- 1 % can lead to physical damage of power transmission equipment.
Power sources that lack this inertia remove stability from the grid. Solar and wind cannot participate in this stabilizing feature. It’s just physics.
The power outage in Texas on Feb 21, 2021, relates to this in some ways. There is a very informative article by Houston Public Media here which I won’t repeat in detail. The upshot is, due to a winter storm a number of power plants in Texas went off-line, leaving too few plants remaining to service the load on the grid. Oh, yes, somehow Texas has constructed a power grid that is isolated from the rest of the US. It is managed by ERCOT, Electric Reliability Corporation of Texas. So, as power plants dropped out, there were fewer and fewer remaining stations providing inertia to the Texas grid. Plants remaining in operation faced higher loads than they could service and saw reducing line frequency, causing them to gradually blackout certain areas. Operators made a last call for blackouts when the line frequency dipped to 59.3 Hz. More blackouts did the trick. Only after widespread blackouts were they able to achieve a steady state and avoid a total collapse.