New device aims to optimize transmission line capacity

Smart Wire Grid, an Oakland, Calif.-based company, has developed a device to help utilities maximize the capacity of their power lines.

The company will begin putting the devices, called distributed series reactors (DSRs), in service by September.

”There’s a lot of unused capacity in the transmission system,” Stewart Ramsay, CEO of Smart Wire Grid, told TransmissionHub on June 6. “The system tends to be constrained by a relatively small percentage of the lines.”

“When we’re at maximum loading on the grid, we’re in the 60 to 70% [range] of total nameplate capacity utilization on the transmission system,” he said. “We’ve done some analysis that has shown us that optimum placement of DSRs could take a constrained system from a 60% utilization up into the 90 to 93% utilization so we’d be able to use existing capacity that the physics of the system doesn’t allow right now.”

According to the developer, DSRs work by increasing the overall impedance of the line, which in turn limits the amount of energy that can flow down the line.

“Think of it as an electrical valve,” Ramsay said. “By changing the impedance of a specific line, we can effectively close it down to some degree as a path for the electrons, which then forces the current flow to go to other parts of the system that are in parallel,” literally seeking the path of least resistance.

Jerry Melcher, the company’s director of program management, said, “Adjacent lines that are parallel in the system that may be unloaded will actually end up carrying additional load and the overall system capacity throughput will increase in greater proportion than just increasing the impedance.”

At present, utilities control the flow on transmission lines by adjusting the output of generation units under their control, ramping down those generators feeding into lines that are reaching their limits and ramping up others feeding lines that have additional capacity. They can also make adjustments to tap changers on transformers or use phase-shifting transformers.

“All of those remedies are fairly broad in scope,” resulting in the curtailing of multiple lines “whether all of the lines need to be curtailed or not,” Ramsay said. DSRs, he said, “tend to be more discrete. If you have [DSRs] on multiple lines you could, in effect, redispatch the transmission system rather than [redispatch] generation.”

The developers see the devices, pictured at right, as a way to more smoothly incorporate the large amounts of renewable generation being built around the country. Ramsay said that theoretically, DSRs could have helped the Bonneville Power Administration (BPA) avoid turning off wind generators this spring and last.

“Being able to redirect the power flow in transmission lines, it’s possible that BPA wouldn’t have had to spill the water or curtail the wind because they wouldn’t have hit the same limits,” he said.

DSRs can function automatically, can be controlled by utilities from their control rooms, or can be preprogrammed by the utility, which would determine appropriate parameters. They could also be operated in a “set-it-and-forget-it” mode.

“You could give each of the sets of devices an individual set point – say, 455 amps – so when they see 455 amps, they activate, and the next ones activate at a slightly higher level, so you’d have an autonomous cascade of impedance to slowly valve the line down,” Ramsay said.

The devices, like renewable energy itself, could introduce an additional element of variability that would complicate control room operations.

“It’s a possibility, yes,” Ramsay admitted, but said his company is working with software developers to create programs that simplify the integration of DSRs into utilities’ systems.

They could also complicate things for those who buy and sell in energy markets.

Currently, generators take line ratings into account when determining how, where, and what to bid into the market based on their calculations of what the market looks like. DSRs could make those calculations more complicated.

Ramsay said several generators told him the devices would, “change the whole dynamic of the market because now, transmission line parameters are no longer static, they’re a variable.”

Ramsay’s company developed the devices in partnership with the National Electric Energy Testing, Research & Applications Center (NEETRAC) at Georgia Tech’s School of Electrical and Computer Engineering. NEETRAC’s membership includes approximately 20 major utilities, including the Tennessee Valley Authority (TVA).

Several units have been manufactured, tested through NEETRAC, and “passed with flying colors,” according to Ramsay. Units will initially be manufactured at a company facility in St. Louis.

The first devices to be placed into commercial service will be installed on TVA’s transmission system before the end of September.

The company hasn’t determined the cost of the devices yet but they will not be inexpensive.

“The cost of adding DSRs to solve constraint problems compared to the cost of reconductoring overloaded lines are fairly comparable, slightly favoring the DSRs,” Ramsay said. “In terms of building new capacity, it’s much cheaper to install DSRs.”

“In the current mode where it’s very difficult to get new transmission lines built, we can use DSRs to unlock some of the existing capacity,” Ramsay said.