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National Lab Solves Grid Inter-area Oscillation Problem

Grid controls make power less expensive and more stable.

Electricity is traditionally sent along power lines and into homes at 60 Hz, but this isn’t quite true. After traveling long distances, the standard frequency of 60 Hz increases on one side of transmission line (utility or consumer), while it decreases on the other side (consumer or utility, conversely), and it switches back and forth every second or two. This phenomenon—called inter-area oscillations—can be a problem on hot summer days when demand for power is high. As more power is transmitted, the amplitudes of the oscillations build and can become disruptive to the point of causing power outages. Until now, the only safe and effective way to prevent disruptive oscillations has been to reduce the amount of power sent through transmission lines.

Engineers at Sandia National Laboratories and Montana Tech University have demonstrated a controller that smooths out these oscillations using smart grid technology in the western power grid. The controller lets utilities push more electricity through transmission lines, leading to lower costs for utilities and consumers and greater overall grid stability.

“Most of the time these oscillations are well-behaved and not a problem, and they are always there,” says Sandia engineer David Schoenwald. “But when you are trying to push a large amount of power, such as on a very hot day in the summer, the oscillations become less well behaved and can start to swing wildly.”

Sandia National Laboratories’ control system is the first successful grid demonstration of feedback control, making it a game-changer for the smart grid. (Photo courtesy of Sandia National Laboratories)

In August 1996, such oscillations became so strong they effectively split the entire western electric power grid, isolating the Southwest from the Northwest. As a result, there were large-scale power outages affecting millions of people in Arizona, California, Colorado, Idaho, Oregon, Nevada, New Mexico, and Washington.

“The economic damage and the new policies and standards that were instituted because of this catastrophe cost the utility companies several billion dollars,” Schoenwald says. “For the last 21 years, utilities have handled these oscillations by not pushing as much power through that corridor as they did before. Basically, they leave a lot of potential revenue on the table, which is not ideal for anyone because customers have needed to find additional power from other sources at higher prices.”

Scientists and utility companies have known about inter-area oscillations for more than 40 years, but developing a safe and effective way of damping, or controlling, the oscillations has been elusive because of the lack of real-time measurement data from throughout the grid. Developing a way to control the oscillations is especially enticing, because the alternative approach for sending more power is to build more transmission lines, which cost about $10 million per mile and take more than 10 years to build and deploy.

During the last four years, the Department of Energy’s Office of Electricity Delivery & Energy Reliability and the Bonneville Power Administration have funded a research team at Sandia National Laboratories and Montana Tech University. Its goals are to build, test, and demonstrate a controller that can smooth out inter-area oscillations in the western power grid by using new smart grid technology.

“When oscillations start to grow, our controller actively counters them,” says Schoenwald. It’s essentially like a force that pushes down on a teeter-totter going too high on end, then pushes down on the other end as the oscillations continue and the other side goes too high.

Sandia’s new controller smooths the inter-area oscillations on the AC corridor by modulating power flow on the Pacific DC Intertie, an 850-mile high-voltage DC transmission line running from northern Oregon to Los Angeles and can carry 3,220 megawatts of power, enough to run the entire city of Los Angeles during peak demand.

“We developed a controller that adds a modulation signal on top of the scheduled power transfer on the PDCI, which simply means we can add or subtract up to 125 megawatts from the scheduled power flow through that line to counter oscillations as needed,” Schoenwald explains.

The controller determines the amount of power to add or subtract to the power flow based on real-time measurements from sensors throughout the western power grid that determine how the frequency of the electricity is behaving at their locations.

These sensors (phasor measurement units) are the game changers that have made this controller realizable and effective. The idea of modulating power flow though the Pacific DC Intertie has been around for a long time, but what made it ineffective and even dangerous to use was the fact that you couldn’t get a wide-area real-time picture of what was happening on the grid, so the controller would be somewhat blind to how things were changing from moment to moment.

The Department of Energy has been encouraging and funding the installation and deployment of phasor measurement units throughout the western grid, and this has let the research team to design, develop, and demonstrate a controller that does exactly what has been dreamed about for the better part of half a century.” {Is this quotation mark here erroneously?}

“We have been able to successfully damp oscillations in real time so that the power flow through the corridor can be closer to the thermal limits of the transmission line,” Schoenwald says. “It saves utilities from building new transmission lines, it greatly reduces the chance of an outage, and it helps stabilize the grid.”

Because accurate real-time data about how the grid behaves is critical to ensuring the controller’s ability to safely counter strong oscillations, the research team has built in a supervisory system that guards against data-quality concerns.

Sandia’s controller and sensors throughout the grid use GPS time stamping, so every piece of data has an age associated with it. If the time delay between when the sensor sent the data and when the controller receives it is too long (greater than 150 milliseconds), the controller ignores that data.

“When the data is too old, there’s just too much that could have happened, and it’s not a real-time measurement for us,” Schoenwald says. “To keep from disarming all the time due to minor things, we have a basket of sensors that we query every 16 milliseconds in the North and South we can switch between. We switch from one sensor to another when delays are too long or the data is nonsensical or just doesn’t match what other locations are saying is happening.”

Sandia demonstrated the controller on the Western grid during three recent trials. During the trials the team used controlled disruptions—events that excite the inter-area oscillations—and compared grid performance with Sandia’s controller working to counter the oscillations versus no controller being used. The demonstrations verified that the controller successfully damps oscillations and operates as designed.

“This is the first successful demonstration of wide-area damping control of a power system in the United States,” Sandia manager Ray Byrne says. “This project addresses one north-south mode in the Western North America power system. Our next step is to design controllers that can simultaneously damp several inter-area oscillations on various modes throughout a large power system.”

“A lot of time R&D efforts don’t make it to the prototype and actual demonstration phase, so it was exciting to achieve a successful demonstration on the grid,” Sandia engineer Brian Pierre said.

Sandia’s control system could be replicated for use on other high-voltage DC lines in the future, and components of this system, including the supervisory system, will be used for future grid applications.

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