Grid operator unplugs huge volume of wind plant -- Weekend storms in Spain trigger controversial shutdown of generation

Wind plant operators in Spain suffered the biggest curtailment of wind power production yet last month, when national grid operator Red Eléctrica de España (REE) ordered the shut down of 2800 MW on a weekend morning of high winds and low demand. As hurricane-force gusts hit Spain's north-eastern seaboard, national wind power production soared from 1868 MW to 7517 MW over an eight hour period, starting shortly before midnight on November 1. With demand for electricity at rock bottom in the early hours of a Sunday, REE ordered dispatch control centres to unplug enough turbines to bring wind power generating capacity down to 4780 MW.

The massive curtailment has left the Spanish wind industry smarting, not certain if the full force of the smack was entirely necessary or whether REE's hand also carried a little extra, as a warning. The system operator has spent years saying Spanish wind power penetration is growing faster than its system can cope with in practice. Wind was thought to be serving 38% of the national load when REE pulled the plug on it, although that may have been higher, given that more wind power was apparently online at the time than the operator had knowledge of.

The fast ramp up in wind production that night represented a 5469 MW variation over eight hours, the biggest so far experienced in Spain. With generation soaring at 1400 MW an hour between 03:00 and 07:00, alarm bells went off at REE. "If it had been after nine o'clock, demand would have absorbed the extra wind power," says Miguel Duvisón, REE's operations chief.

Before curtailing wind power, REE had removed fossil-fuel generation from the system. At 06:15, with demand from consumers hovering around 20 GW, REE unplugged four combined cycle gas stations, totalling 1500 MW. Generators at thermal power plant had dropped to "absolute technical minimum" an hour earlier, says Duvisón. The system had already stretched its offloading options to the limit -- both into two-way hydro pumping and into exports across the feeble interconnection with France. But that was still not enough. "There was nothing left but to cut wind generation," says Duvisón.

A step too far

"Fair enough, but more was curtailed than necessary," says Alberto Ceña of national wind association Asociación Empresarial Eólica (AEE). All REE needed to cut was 1400 MW, but 2800 MW was curtailed, he argues. True, admits Duvisón, but that was not REE's fault, he insists.

Under predetermined merit order arrangements for wind power, REE required a rolling shutdown of plant, he explains. Data from wind operators indicated that national wind capacity was running at 6200 MW, requiring that just 1400 MW go off line. If double that was cut to meet the order, that was because there was really 7517 MW online, not 6200 MW, argues Duvisón.

The polemic raises the question of the adequacy of Spain's control and monitoring systems for wind, which both the wind industry and REE have previously lauded as the best anywhere in the world. As of January 1, 2008, all wind generation in Spain has been under obligation to operate through dispatch control centres, giving the system operator remote and real-time emergency override control over plant production when required. As the first such obligation for wind on a national scale anywhere in the world "there are clearly still teething problems, not with the orders emitted but with the measuring by dispatch offices," says Duvisón. AEE and REE have agreed to work on the matter together.

The problem is not likely to go away immediately. Three weeks after the incident and almost during the same time interval, even higher winds pushed penetration to a record 43% from 9253 MW of wind plant. But with the wind power peak coming at 05.00 on a Monday morning, rather than a Sunday, demand, at 21.3 GW, was just that bit higher than it had been three weeks previously, providing more flexibility for regulating supply. No wind was curtailed.

Duvisón says there is no fixed wind penetration limit during periods of low demand. It depends on numerous variables for data processing, including: whether less sophisticated, older wind turbines are in operation, or newer machines; the output forecasts wind plant operators are providing for the hours ahead; generation forecasts from other technologies; hydro pumping availability and weather forecasts, including temperature, which affects transmission line capacity -- the hotter the grid wires the more they sag and the less electricity they can carry. Cooling winds can help.

More cuts coming

With Spain's official objective to double installed wind capacity from over 15 GW today to 29 GW in 2016, "more and more curtailment is going to happen with less and less dramatic weather," according to Ceña. So far this year there have been eight significant curtailment incidents, the other seven involving hundreds of megawatts, not thousands. The total cost to wind power operators has been around EUR 6 million in lost revenues, according to rough AEE calculations.

Better transmission links would alleviate much of the problem. AEE and REE are pushing hard to speed up the long promised upgrade of the interconnection with France. Construction of more hydro pumping stations would help, too. Both developments would provide REE with more flexibility to export or offload wind generation. "A lot of clean energy was lost. Hydro pumping could have stored a lot of it," says Duvisón. Water pumped into reservoirs can be released for hydro electricity generation in periods of lower wind production.

Hydro capacity

Spain currently runs 5500 MW of hydro pumping capacity. With a further 2000 MW, hydro pumping could cater for all excess wind production contingencies to 2016, estimates Duvisón. Opposition to building new hydro dams, however, makes construction of new pumped storage a long and arduous process. Spain is already one of Europe's biggest large-hydro producers.

Another option could be to offload peak night time generation into electric car batteries. "There will be a lot coming on this in the near future," promises Ceña.