Solar Eclipse Will Test European Power Grids

Solar Eclipse Will Test European Power Grids

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.

Its 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, Europes far greater levels of solar power make for bigger stakes. ENTSO-E projects that the moons jaunt across the suns 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. Thats the equivalent of turning off 30 big coal or nuclear power stations.

The moons shadow will be deepest in Northern Europe, but power grid impacts could spread across Continental Europe via the regions 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 grids 50-hertz AC frequency Continent-wide.

In a worst-case scenario such frequency disruption could snowball. Many of Europes 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-Es 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 grids 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 countrys 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 Europes 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 Italys power system to sustain the event, describes a series of defensive maneuvers by Terna, including:

  • Restricting power trading on the transmission interconnections across Italys 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 Ternas ability to ramp generation up and down and thus hold the AC frequency at 50 hertz.
  • Preparing Italys 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 Grids system has ever experienced. We could see massive swings on the network, says Caplin.

Source: IEEE Spectrum

SMART GRID Bulletin March 2017

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