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What Happened in ERCOT

Voltage sag shows value of accurate wind forecasting.

Fortnightly Magazine - May 2008

Variability is a well-known characteristic of windpower, and system operators know they must plan for changes in wind generation over the course of a day. But when those plans fall short, voltage levels can drop quickly, forcing grid operators to dispatch resources to make up the difference—either by shedding load or bringing reserve generation online.

That’s exactly what happened in ERCOT on an evening in February, when a combination of events left the system operator short on power and long on demand  (See Figure 1) .

“On a macro level we forecasted load pretty close to what it turned out to be,” says Kent Saathoff, vice president of operations at ERCOT. “But there was a substantial difference between what we were getting from wind generators and what was reported to us in our look-ahead studies.”

Ultimately what happened in ERCOT was a non-event, in the sense that only interruptible load was affected and no involuntary outages occurred. Nevertheless, the episode illustrates the value of modern forecasting technology to provide accurate data for utilities and system operators who must keep the lights on.

Perfect Storm

While a rare combination of events created the February 26 emergency, none of the individual factors was unusual by itself.

ERCOT suffered unexpected shortages in both wind and non-wind generation sources—totaling nearly 1,600 MW—starting at around 6:00 p.m., just as its load was rising, roughly in accord with forecasts.

The combination of factors caused system frequency to drop suddenly at about 6:30 p.m. ERCOT responded by first calling upon its balancing energy supplies, and then by declaring its step-two emergency protocols—most notably cutting power to about 200 MW of interruptible industrial load. This succeeded in restoring frequency, and when non-wind generation started coming back online between 6:50 and 7:00 p.m., the interruptible load was restored.

While the wind discrepancy was the single biggest factor in the episode—representing a shortfall of nearly 1,000 MW continuously for about 30 minutes—it was nothing ERCOT hadn’t seen before, even in recent months. “On December 22 of last year, there was an even quicker and more significant decline in wind generation,” Saathoff says. “But that time we had more generation available than we did on February 26, so we could fill in the blank.”

ERCOT also had access to additional generation on February 26, in the form of non-spinning reserves—800 MW of which started generating power at around 7 o’clock. But because these were non-spinning reserves, they weren’t immediately available to support voltage—mainly because ERCOT’s forecasts didn’t indicate they’d be needed.

“The primary factor is the need to accurately forecast wind,” Saathoff says. “Wind is certainly a good resource and it has a lot of benefits. We just have to take its variability into account and develop forecasting tools that we can reliably incorporate into our system.”

Reading the Wind

Today ERCOT’s daily resource plans include forecasts from wind generators who sell power into the grid. Those forecasts went awry

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