TECHNOLOGY:

Devices emerge to handle the quirks of adding more renewable energy to the grid

On Feb. 26, 2008, a short circuit in a Miami electric power substation and an operator's error gave managers of the nation's electrical grids a glimpse of an uneasy future. The events triggered a chain reaction of power plant and transmission line outages in the state, unleashing sharp swings in voltages and power frequency that blacked out power for nearly 1 million customers in southern and central Florida for up to four hours.

A stunning consequence of the outage was the lightning-like spread of destabilizing power frequencies throughout the eastern United States and southern Canada in the space of six seconds. Then, fortunately, the grid managed to settle itself without a much bigger blackout.

This and similar threats to the stability of the nation's power supply might go undetected until too late but for the development of monitoring systems that can provide snapshots of the grid's changing conditions in each fraction of a second. Experts say this emerging capability will play a vital role in the next big evolution of the nation's power transmission system: equipping it to handle a rising flow of wind and solar power that is central to the nation's climate policy.

A video depicting the Florida incident's rippling spread has been created by Virginia Polytechnic Institute and State University's electrical and computer engineering department, which caught the disturbance on its first-generation grid frequency monitoring network. Some grid executives have downloaded the video on their laptops as a kind of horror flick for engineers of what could happen.

Power companies and grid managers are developing an advanced monitoring system across the United States and Canada, using what are called synchrophasor units to gather, analyze and distribute data on grid conditions. About 150 of the units have been deployed, and the industry has created the North American SynchroPhasor Initiative -- supported by the Department of Energy and the North American Electric Reliability Corp. (NERC) -- to expand the technology. DOE is offering grants totaling $615 million for smart grid demonstration projects, including synchrophasors, as part of the American Recovery and Reinvestment Act.

Trouble can happen in milliseconds

"The wonderful aspect of synchrophasor monitoring is that you can see things happening much sooner," says Bob Cummings, director of events analysis and information exchange at NERC, the Princeton, N.J.-based organization that oversees reliability of North America's transmission systems. Today's technology looks at the grid's condition once every two or four seconds. In a world where problems can happen in milliseconds, that's not good enough, Cummings said.

"It's as if you're driving a car at 60 miles an hour and you open your eyes every four or five seconds to see where you're going. That's kind of what we're doing now," said Chantal Hendrzak, general manager of applied solutions at the PJM Interconnection. "Where that really bites you is when you have these dynamic stability conditions that have the potential to quickly cascade and spiral out of control undetected." With synchrophasors, "you're blinking your eyes 30 to 60 times a second." And that should greatly improve understanding of the grid's condition, she said.

The impact of synchrophasors comes from their ability to measure electric power flows at the same instant at hundreds of critical points across a 1,500-mile-wide grid, using precise time signals from global positioning system, or GPS, satellites, engineers say.

Such real-time understanding of grid conditions will be essential as the nation adds increasing amounts of wind and solar power to the mix, said Yilu Liu, a Virginia Tech professor whose students employ the university's FNet monitoring system. "If you have a large amount of wind and solar, you don't just need to know information at the plug-in point, you have to know if the system is able to take it," she said.

"You have to put them together in an intelligent, flexible way," Liu said, speaking of the new renewable units. "If you simply connect them without having the intelligence, it will make the system weaker."

The far-reaching, unexpected disturbance from the Florida blackout was a warning, she said. "That's showing us there are things we don't understand, that we take for granted."

The road to early warning and a 'self healing' grid

"Because renewables are intermittent by nature -- and they are new to our system -- we don't have a lot of experience with them," said Hendrzak. Synchrophasor measurements will permit detailed modeling of the variable generation so their integration into the grid will be better understood. Then the models can be checked against actual operations to improve them and allow greater confidence in their predictive capabilities, she said. This could lead eventually to "self-healing" technologies that provide automatic detection and response for problems, reducing the need for human intervention. "This technology could enable that," she said.

"Verification of the systems models is the key to this," agreed Cummings.

The early warning capabilities alone justify the technology, he said. Had synchrophasors been in place in 2003, they could have prevented the Aug. 14 blackout that year that knocked out electricity service for 50 million people, he said.

Generators create the voltage or pressure that delivers electric energy to customers, with the voltage continuously rising and ebbing in a wave-like path. If power is flowing, say, from a generating plant in Ohio to a utility in New Jersey, the timing of the voltage wave will be different, or out of phase, in the two places. The differences are measured in phase angles, and the greater the difference, the more power will flow -- unless the difference gets too big, and then the system becomes unstable, Cummings explained. At that point, the interconnected transmission system virtually breaks apart, and power flow ceases. Grid operators are left to restore the system to operation piece by piece.

Cummings has a laptop chart showing voltage phase differences in Cleveland and Detroit in the hours prior to the 2003 blackout. "Normally, these [phase] angles should both be around the 25 degree mark, but they were already up at a 40 degree angle at 3 o'clock," he said. That was an hour before the series of power line outages suddenly escalated and the blackout moved too quickly for human intervention, according to after-the-fact investigations.

An unseen warning in Ohio, an hour before the 2003 blackout

The grid managers at Ohio's FirstEnergy Corp. knew they had a problem, Cummings said. They were beset that day by systems breakdowns and human error. But the precise warnings from synchrophasor data would have made the threat unmistakable, he said. "An hour before the event happened, had we had [synchrophasor] monitoring, what we would have seen is Cleveland walking away from the rest of the system," shown clearly by the difference in voltage angles, he said. And that could have provided enough time to prevent the blackout, he added.

The price of the hardware is not a key issue, officials said. Roy Moxley, marketing manager for Schweitzer Engineering Laboratories Inc. in Pullman, Wash., a manufacturer of the units, said the least expensive units are built into transmission relay devices, which cost $1,500. More elaborate units go for $15,000. Installing and linking them into a collector, to store and process massive amounts of data, is far more expensive.

A debate is under way about whether expanding synchrophasor use could make the grid more vulnerable to cyber-attack, and thus whether the devices should be subject to NERC's Critical Infrastructure Protection standards. That could add costs and delay to some synchrophasor installations, as close security monitoring and background checks for network personnel could be required.