Effect of brutal cutoff of wind capacity – Spanish Wind ramping, intermittency back up

Spain’s variable wind and stable electricity networks
Sunday 15 November 2009 in uncategorized by Chris Goodall

One of the frequent criticisms of wind energy is that national distribution systems (‘the grid’) cannot cope with large number of turbines because of the variability and unpredictability of their output. Grids need to match supply and demand precisely, the critics say, and because wind varies so much it causes huge problems. Recent data from two meteorologically unusual days in Spain – the world leader in the management of renewable energy supplies – shows this assertion is almost certainly false.

• During part of 8 November, Spain saw over 50% of its electricity come from turbines as an Atlantic depression swept over the country’s wind parks. (They are so big that no one seems to call them ‘farms’.) Unlike similar times in November 2008, when Spanish turbines were disconnected because the grid had an excess of electricity, the system accepted and used all the wind power that was offered to it.
• A very different event in January of this year saw unexpectedly high winds shut down most of the country’s turbines with little warning. The grid coped with this untoward incident as well. These two events show that a well run transmission system can cope with extreme and unexpected events even with a large fraction of power provided by wind.

 The 8/9 November peak was predicted by REE. 23 January 2009 was very different. The Spanish grid was expecting very high winds from Atlantic Storm Klaus and projections were for the country’s turbines to produce about 11 gigawatts (11,000 megawatts) of electricity for most of the 23rd and the night of 23/24.

It didn’t quite turn out that way. Wind speeds were unexpectedly low on the morning of the 23rd and electricity output hovered around 9.5 gigawatts. As wind speeds rose output began to rise from about midday and peaked at about 16.00. By that time the winds were sufficiently strong to trigger automatic shut-down of many turbines in the north of the country. (The technical term for this is ‘over-speed protection’ and means the turbines cease to provide any electricity at all.) The grid hadn’t predicted this and the amount of wind power began to fall far short of what it was expecting. By 21.00 wind generation had fallen to about 7 gigawatts (7,000 megawatts), almost 4 gigawatts below the prediction for that time. Output continued falling throughout the night and only began to recover early next morning.

At the moment of minimum output (about 06.30 on the 24th), the gap between the forecast and actual wind turbine production exceeded 7 gigawatts, or the equivalent of four or five large coal-fired power stations.

This event was unusual in two separate ways. First, it was extreme. I haven’t obtained detailed records for Spain but the grid says that some wind parks experienced winds of up to 220 km/h or about 150 miles per hour. French evidence suggests that Atlantic Storm Klaus produced the most destructive winds experienced in the south-west of the country – the part just north of northern Spain – for ten years. Some French areas reported a loss of one third of all trees in some forests.

Second, the severity was not predicted. The grid was expecting very high levels of output but did not think that the speeds would reach the level that forced turbines into automatic shut-down. The combination of these two factors produced almost the most difficult imaginable set of circumstances for an electricity transmission system: wind was expected to provide a large fraction of all electric power but this electricity suddenly became unavailable. If this Spanish electricity system could manage this, it could survive almost all problems that renewable generating plants could throw at it.

And survive it did. The control centre ramped up the supply of hydro-electric power, including pumped storage, and temporarily imported power through international connections. Within a few hours of the storm’s unexpectedly powerful arrival, the system was back in equilibrium and began to export its power again by 22.00 on the 23rd, well before the period of minimum wind output. Coal- and gas-fired power stations ran hard until about 23.00 and then fell away as total demand declined during the night.

The crucial thing about wind is that it usually fades gracefully, even in extremely severe circumstances such as Klaus. Below is a chart that shows total output from Spain’s 15 or so gigawatts of wind turbines, spread across the country but concentrated along the Atlantic coasts from Vigo northwards. You will see that the decline during the passage of Klaus is almost a straight line, with only minor unexpected bumps. By 03.00, the position is beginning to stabilise. The chart logs estimated wind output every ten minutes and in only two of the 65 ten-minute periods did output fall by more than 5%. The mean percentage variation from one reading to the next was 0.72%.[*]

 The maximum loss of power in any ten minutes was about 450 MW and this occurred quite late in the course of the storm, when the control room was already directing surplus electricity back into pumped storage reservoirs. The control could instantaneously reduce the amount of power being sent to pump water into high reservoirs in response to the few cases of sharp declines in wind turbine availability.

The lesson from this event is profoundly cheering. The Spanish electricity system was expecting up to about 45% of its power on the night of 23/24 January to come from wind. The outturn was about 16% for most of the night. The system handled the problem with no outages.


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