A partial eclipse of the sun headed for Europe next Friday has grid operators in a tizzy. On the morning of March 20 Europe's skies will darken for the first time since solar power became a meaningful piece of some countries’ power supply, and the impact could be dramatic.
“It’s a very, very big challenge for the transmission system operators in Europe,” says Enrico Maria Carlini, Head of Electric System Engineering for National Dispatching at Rome-based Italian transmission system operator Terna.
The Brussels-based European Network for Transmission System Operators for Electricity (ENTSO-E) judges in an eclipse impact analysis released last month that it poses a, “serious challenge to the regulating capability of the interconnected power system.”
While an eclipse markedly reduced solar generation in western North America last October according to energy tracking firm Opower, Europe’s far greater levels of solar power make for bigger stakes. ENTSO-E projects that the moon’s jaunt across the sun’s path next Friday could slash more than 30 gigawatts (GW) of solar generation in Continental Europe over one hour if clouds are scarce and solar generation is high. That’s the equivalent of turning off 30 big coal or nuclear power stations.
The moon’s shadow will be deepest in Northern Europe, but power grid impacts could spread across Continental Europe via the region’s 27-nation interconnected AC grid. Because the grid operates synchronously, failure by any transmission system operators (TSOs) to keep supply and demand in balance as solar power plummets and then races back will throw off the grid’s 50-hertz AC frequency Continent-wide.
In a worst-case scenario such frequency disruption could snowball. Many of Europe’s early distributed generators such as solar systems and wind turbines were programmed to shut off if grid frequency diverged from 50 hertz by just one percent, and could be triggered by the solar losses. Retrofitting with ‘smart inverters’ that can withstand much bigger AC signal glitches is a work in progress. ENTSO-E warned in January that frequency-induced blackouts remain a “significant operational risk”.
ENTSO-E’s analysis also flags a threat of transmission lines overloading during the eclipse as flow patterns over the meshed grid shift rapidly. Overload risk will be most heightened in the region around solar heavyweights Germany and Italy, which together host over three-fifths of the European grid’s solar capacity.
For Germany, ENTSO-E predicts that solar generation will drop at a rate comparable to the shutting down a 200-megawatt gas turbine every minute over one 40 minute period. In a statement released last week (in German) the country’s four TSOs predict that if Friday is sunny they will lose about 12 GW starting at 9:30am local time, then experience a rapid inrush of about 19 GW of solar power by noon as the eclipse passes. (More power comes back because the sun will be higher in the sky when it returns.)
Germany is also at greatest risk of frequency-induced shut-off by renewable energy plants. As of September 2014 nearly 35 GW of distributed generation in Germany, including wind power, biogas generators and other non-solar sources, had yet to be upgraded with smart inverters.
Italy will not be shaded as severely next Friday as Germany, but with 22 percent of Europe’s solar power capacity Carlini says the eclipse will still one of the “biggest events” the Italian TSO Terna has ever faced. He projects they will lose up to 3.5 GW of solar output in the morning, and then rapidly gain back 8 GW.
Carlini, who is responsible for positioning Italy’s power system to sustain the event, describes a series of defensive maneuvers by Terna, including:
- Restricting power trading on the transmission interconnections across Italy’s northern border from 8 GW to 1 GW next Friday. This will reduce the risk of overloading from unexpected flows and boost flexibility in case Terna and its neighbors need to exchange power for mutual support.
- Calling up extra conventional power plants to increase Terna’s ability to ramp generation up and down and thus hold the AC frequency at 50 hertz.
- Preparing Italy’s 7 GW worth of pumped hydropower storage plants for an abnormal power cycle. Such energy storage plants normally pump water uphill at night when power is cheap and then discharge during the midday peak. Next Friday they will rapidly discharge while solar power is dropping in the morning and then pump aggressively thereafter to soak up excess solar generation as the sunlight bounces back.
Grid operators in the U.K. are less concerned about a solar deficit than their colleagues on the Continent. Solar is barely 2 percent of their power supply (compared to nearly 7 percent in Germany), and the U.K. grid is not synchronized with the Continental grid.
They are more concerned with something far more capricious than sunlight: human behavior.
Jeremy Caplin, power forecasting manager for London-based National Grid, says their concern is driven by what happened during the last full solar eclipse. On August 11, 1999, power demand plummeted by 2 GW over 30 minutes as folks headed outside to directly experience the once-in-a-lifetime natural phenomenon. Power demand then shot back up by 3 GW over the next 30 minutes when they returned came back in — one of the biggest shifts National Grid’s system has ever experienced. “We could see massive swings on the network,” says Caplin.
Live coverage by BBC television attests to the festive spirit of the day. Caplin recalls sharing it on the grounds of National Grid’s research facility in South London, surrounded by 300 scientists and engineers sporting a motley array of eclipse-viewing apparatus: “They had everything from welding masks to telescopes and mirrors to project it onto walls.”
This time around Caplin is betting that power demand will drop in concert with solar supply. If he’s right and supply and demand drop together, mitigating each other’s impact on AC frequency, the 2015 event should be easier to manage than 1999. Caplin predicts that the grid’s pumped hydropower energy storage plants will do most of the heavy lifting.
As Carlini is planning for Italy’s northern transmission links, Caplin says it will important to keep extra capacity on the two pairs of power lines that connect Scotland with the rest of the country. Since much of the U.K.‘s solar capacity is installed in Scotland, the loss and rebound of generation there is likely to pull extra current over those lines. “We’ve got to make sure that we’ve got headroom on those circuits,” says Caplin.
Despite his confidence in the U.K. grid’s readiness, Caplin expects next Friday to be a long and intense day. He is the first to admit that predictions can be off, especially when humans are in the picture. Should popular response to the eclipse increase power use — if people turn on their big screen TVs and computers to watch rather than running outside — the forecasts could go out the window.
“The PV side of it we can forecast reasonably accurately. The people side is largely guesswork,” says Caplin.