Superstation in New Mexico would unite fragmented US electrical grid

The Tres Amigas site lies about 2 km from the Texas state line and less than 150 km from existing or proposed transmission lines for all three interconnections. The site location allows the substations to be placed close to each of the regional grids, which lowers the cost to build new grid-to-substation transmission lines, if they’re needed. The HTS cables between any two substations could carry 5 GW, enough to power 3.5 million homes; the capacity could be increased to 30 GW with additional HTS cables. Even the 5-GW design is ambitious, says Steven Eckroad, underground transmission program manager for the California-based Electric Power Research Institute (EPRI). “There are a few small [links] between the Eastern and Western interconnections, but their total combined capacity is around 1 GW.”

To carry 5-30 GW of power, transmission lines could be placed underground to avoid weather-related outages, says Eckroad. HTS cables transmit the same power load in less than one-fifth the space required by copper cables, a particular advantage underground where installation costs are higher. At its core, an HTS cable is a bundling of razor-thin, flexible ceramic wires that conduct electricity below 77 K with no resistive losses. American Superconductor Corp, a part owner of Tres Amigas, will supply the HTS wire. The US Department of Energy has partially funded several in-grid demonstrations of AMSC’s technology (see Physics Today, January 2008, page 30).

Instead of the AC transmission lines used in those demonstrations, AMSC has designed a DC cable for the Tres Amigas hub. That’s because the three grids are out of phase with each other, even though they all operate on the same 60-Hz frequency. Therefore, incoming AC power from one grid must be converted to DC before it can be synchronized with the phase of the receiving grid. At the hub’s substations, that process will be done by voltage source converters. “The largest VSCs right now can handle around 600 MW, but VSCs allow us to grow in capacity because we can splice in as many of them as we want,” says Harris. And Tres Amigas plans to store excess energy in 150-MW-capacity batteries to be installed at each substation.

The project has attracted the interest of energy companies in the Southwest, including PNM Resources Inc, New Mexico’s largest energy provider, which has submitted a letter expressing its desire to connect to Tres Amigas’s proposed hub. Director of engineering and operations Greg Miller says that up to 20% of the company’s power capacity comes from wind energy. In addition to exporting excess wind and solar energy by plugging into the hub, he says, “PNM could also see some potential reliability benefits.”

Tres Amigas has received the blessing of New Mexico politicians, including Governor Bill Richardson and US Senator Jeff Bingaman (D), chairman of the Senate Committee on Energy and Natural Resources. In a statement, Bingaman said the station “will catalyze the adoption of renewable energy while … increasing the reliability of [the nation’s] electricity network.” Political support notwithstanding, “we’re not asking for stimulus funds, or guarantees, or grants,” says Harris, who maintains that Tres Amigas will be a privately funded venture.

Harris would not say how much it would cost to build the hub, but EPRI estimates that its HTS DC cable will cost $5 million per kilometer once it becomes commercially mature, says Eckroad. Whatever the costs end up being, Harris says that his experience and research convinces him that HTS cables are the only way, technologically and economically, to pull off the Tres Amigas concept. And funding it privately, he says, will “allow American Superconductor to show that their HTS technology is commercially viable and ready.”

For now, though, plans are on hold as FERC considers whether to grant Tres Amigas permission to negotiate and set prices with utility companies and whether to exempt ERCOT from FERC’s authority once the now-independent Texas interconnection plugs into the hub. At press time, FERC had not yet issued its decisions, and declined Physics Today’s request for an interview. But if FERC approves the first request, denies the second, and ERCOT refuses to join, Harris says that the hub will still go ahead with “dos amigas” and immediately begin to link the Eastern and Western grids.