Researchers demonstrate system that addresses inter-area oscillation issue

Sandia National Laboratories and Montana Tech University recently demonstrated a control system that smooths out inter-area oscillations, which can be a problem when demand for power is high.

When electricity travels long distances, the standard frequency of 60 cycles per second on the utility side of the transmission line increases while the frequency on the customer side decreases, switching back and forth, or oscillating, every second or two. This is what is known as inter-area oscillation.

As more power is transmitted, the amplitudes of the oscillations increase and become more disruptive. They can even cause power outages. Previously, the only safe and effective method of preventing disruptive oscillations was to decrease the amount of power sent through a transmission line.

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

The new system, which was applied on the western power grid, uses new smart grid technology, to decrease these oscillations, enabling utilities to move more electricity through transmission lines, decreasing costs for utilities and consumers and improving grid stability.

The new control system smooths the oscillations on the AC corridor by modulating power flow on the Pacific DC Intertie, an 850-mile high voltage DC transmission line that stretches from northern Oregon to Los Angeles. It uses real-time measurements from sensors placed throughout the western power grid that gauge how the frequency of the electricity is behaving at their location to determine the amount of power to add or subtract to the power flow.

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