Nationalizing the Grid

Feb 06, 2011 - Phillip G. Harris and Jack McCall - Mechanical Engineering Magazine

The U.S. is blessed with abundant renewable energy, from wind in west Texas to sunlight in the western deserts. And the nation is rapidly putting that clean energy to work. Although most forecasts project annual load growth of only 1 percent for the foreseeable future, renewable power’s share of net generation is projected to double by 2035.

But this power cannot be put to work unless it can be economically transmitted to load centers. At present, a key deficiency inhibiting the development of our renewable energy sector is America’s balkanized power grid: the largely isolated Eastern, Western, and Texas interconnections. Only relatively small, bilateral dc links currently exist between any two interconnections—a mere 2,000 megawatts of combined power transfer. And the three interconnections have never been integrated.

Uniting these interconnections has been a goal for some in the power industry for decades. But the Federal Energy Regulatory Commission recently approved a project designed to break this longstanding transmission bottleneck and create a power-marketing hub that will provide developers of renewable—and conventional—generation with expanded regional and national markets in which to sell their power. This project, called the Tres Amigas SuperStation, will be located in Clovis, N.M., close to large amounts of established and potential renewable generation, and in its first phase, slated to begin construction in 2012, will support the 5,000 megawatts of power transfer capacity.

What’s more, the Tres Amigas project will be a technological wonder. The project will employ ultra-efficient, high-capacity dc superconductor cables coupled with voltage-source converters in what will be, in essence, a superconductive electricity pipeline.

High temperature superconductors used in power transmission are perfect direct current conductors. Wires made from superconductor materials are over 100 times more powerful than copper or aluminum wires of the same size and they can transmit power with zero energy loss when carrying direct current. The lack of resistance makes it possible, indeed practical, to construct dc superconductor cables with virtually any desired power transmission capability.

The power density and efficiency advantages drive system economics, and they are fundamental to the reason that underground superconductor cables can achieve cost parity with overhead alternating current power lines over long distances while also delivering superior returns on investment. In addition to eliminating energy losses in transmission, superconductor cables are compact, lightweight, and emit no heat or electromagnetic fields, and they are particularly easy to install, even in close proximity to other underground infrastructure. The right of way needed to move 5,000 megawatts using a superconducting electricity pipeline is significantly smaller than that of conventional 765 kV transmission lines.

Power cables employing superconductor wires are available from several commercial producers and have demonstrated their reliability and performance in in-grid sites around the world. Such cables have been installed in New York City, Long Island, and Albany, N.Y.; Columbus, Ohio; Detroit; Tokyo, and South Korea. While all installations to date have been for ac applications, applying this established technology to dc transmission is straightforward.

In dc applications in particular, modern power electronics have paved the way for superconducting electricity pipelines to become a significant operational solution for transmission operators around the world. The technology can be used for such diverse applications as collecting wind turbine output from onshore and offshore wind farms; collecting renewable energy from solar and geothermal rich areas; enabling the delivery of renewable power to distant major population centers, including regions that have less productive renewable resources; and transferring power from region to region to take advantage of seasonal and daily power generation and load profiles.

Nationalizing the Grid - Superconducting cables

Compared to traditional high voltage power lines, superconducting cables are
much more compact.

Direct current power transmission itself is not new: it has been used for decades around the world to move large amounts of power from a single source of power generation to one load center. While a few multi-terminal systems using conventional cables have been built, they were very difficult to implement. But recently, multi-terminal technology based on new power electronic designs incorporating voltage-source converters has become available. VSC technology provides greater control and flexibility and, most important, enables dc lines to connect to multiple generation sources and multiple areas of electrical demand.

Direct current terminals employing VSC technology, however, are available only at mid-level voltages in the range of 100 to 300 kV. By comparison, ultra-high voltages (around 800 kV) are used for conventional point-to-point dc transmission. Thus, to be used in high power transmission, the lower voltage levels require the use of very high currents. But transmitting high currents long distances through conventional aluminum or copper conductors results in considerable resistive losses.

Superconductor power cables bypass that limitation by providing the ability to carry very high levels of current with zero electrical loss. The combination of a VSC-based multi-terminal dc system and superconductor cables makes for a compelling new transmission option, uniquely suited to transmitting renewable energy over long distances with multiple collection and distribution points. In the target applications, superconductor dc cables have higher lifecycle returns on investment because of their efficiency and operational advantages.

The Tres Amigas SuperStation will employ this technology toward an exciting goal. The SuperStation will make it practical, and economical, to “firm up” intermittent and variable renewable energy by taking advantage of geographical diversity and energy storage, such as the onsite batteries at Tres Amigas or systems such as compressed air energy storage. This capability greatly enhances the value of new generation, creating additional economic incentives for its development.

Tres Amigas also will expand the geographic reach of markets, offering new opportunities to take advantage of load and resource diversity, which will reduce costs. For example, at present, marginal prices for energy in the three U.S. interconnections typically diverge because these three markets operate in isolation. Studies that Tres Amigas submitted during FERC proceedings showed that marginal energy prices do vary significantly between the Southwest Power Pool (in the Eastern Interconnection), ERCOT and the WECC. Our studies, during sample time intervals, showed that energy prices vary by more than $50 per MWh for over 2,000 hours per year between CAISO and ERCOT, over 1,600 hours per year between ERCOT and the Palo Verde hub, over 1,500 hours per year between SPP and the CAISO, and over approximately 800 hours per year between ERCOT and the SPP. The key conclusion is that significant opportunities exist to bring lower cost power to market by allowing more efficient producers access to the market.

Nationalizing the Grid - The Tres Amigas SuperStation

Superconducting cables will enable the Tres Amigas SuperStation to move electricity
among the three interconnected grids.

Just as important is that the Tres Amigas SuperStation will enhance the value of transmission investments made in the region by allowing power to move freely between the interconnections. Tres Amigas will permit power to move to and from different markets, expanding the potential use of the existing transmission grid and future additions. Tres Amigas should also provide system planners new opportunities to improve the efficiency and reliability of the electric system at a lower overall cost.

Also, the reliability of the electric system in the area around Tres Amigas will be improved. Tres Amigas will connect the three asynchronous grids at a robust station with backup power and voltage-source converter technology that will provide substantial reactive power to the transmission system in each of the interconnections. VSCs can rapidly control both active and reactive power independently. The reactive power will be controlled separately at each synchronous interconnection independent of the voltage levels on the other synchronized systems.

By using VSCs, Tres Amigas will not place restrictions on each ac network’s minimum short-circuit capacity.
The self-commutation feature of VSC technology will permit the set of three different phase voltages to be synthesized. Because VSC converters themselves have no reactive demand, Tres Amigas will be able to control reactive power for regulation in each separate ac system. The real-time dynamic support of each ac system will be managed based on each system’s separate and individual needs, thus improving stability and transfer capability, and most likely reducing losses on each connecting ac system.

Finally, the VSC will provide black start capability to each interconnection separately. Tres Amigas will appear, electrically, to each interconnection as a large generator. The value of this to the long-distance ac transmission lines planned for the region surrounding Tres Amigas is significant. Although specific engineering analysis of each line interconnecting to Tres Amigas will need to be carried out, intuitively we can predict that the technology that Tres Amigas is deploying will solve many voltage, reactive support, stability, and dynamic control problems that long-distance ac lines connected to large intermittent generation resources create.

It may sound futuristic, but the groundbreaking for this project is actually close at hand. With the recent signing of vendor contracts with CH2M Hill, Xtreme Power, Burns & McDonnell, ZGlobal Inc., and Viridity Energy Inc., the design and build-out of the Tres Amigas SuperStation is near fruition. The vendor agreements that Tres Amigas has completed in recent months cover equipment, proprietary trading platforms, and transmission system planning. Tres Amigas is in the final stages of selecting the vendors for the detailed engineering design and construction services, as well as the supplier for the SuperStation’s high-voltage dc technology.

Nationalizing the Grid - Thick copper cables and slender ribbons of superconducting material

When chilled by liquid nitrogen, slender ribbons of superconducting material
can carry as much electricity as thick copper cables.

When completed, the power conversion technology at the Tres Amigas SuperStation will be a vast improvement over the equipment used at most of the existing ac/dc ties in the United States. It will apply advanced and proven power grid technologies such as high-voltage direct current superconductor power cables, voltage-source converters, and energy storage systems. Tres Amigas also will be the first high-capacity system to connect all three of America’s power grids, transmitting gigawatts of power from region to region through the nation’s first renewable energy market hub.

It will be the first step toward a green—and continent-spanning—grid.

PICK UP AND DROP OFF

Superconductor electricity pipeline systems have a broad scope of application. The superconductor dc cable functions as a virtual bus-bar that can carry a set amount of power across its length. The VSC terminals are then able to inject power onto the line, or pull power off the line in precisely controlled amounts. This operation would be akin to valves on a gas pipeline or on- and off-ramps on a highway.

One possible use of that capability is long-distance power transmission. For instance, a superconductor electricity pipeline system could consist of several linked loops of long-distance superconducting cable, each with discrete points of connection to the ac grid. (Connecting such a pipeline in loops increases reliability, since maintenance work or unavailability in one section would not prevent power from flowing from one location to another.) Traditional ac transmission would be utilized to collect power from geographically adjacent wind farms and provide a common point of connection to the pipeline at converter stations. For example, 250 MW would be injected into a 5,000 MW superconductor dc cable at 20 locations as it passes through the wind-energy-rich upper Midwest, and 500 MW would be delivered to each of ten cities on the East or West Coast.

IN FOR THE LONG HAUL

Superconductor electricity pipelines can be the most efficient option for long-haul transmission, whether in ac or dc grids. In all applications, high-temperature superconductor cabling cuts power losses by a factor of two or three when compared with conventional transmission options. This results in improved return on investment, and reduced pollution and carbon emissions.

These advantages underpin the growing market for HTS cabling. For example, in the United States, the world’s first transmission-voltage cable system has been operating successfully near New York City since April 2008. This 138 kV system is now a permanent part of Long Island Power Authority’s primary transmission corridor. At full capacity, LIPA’s power cable system is capable of transmitting up to 574 MW of electricity and powering 300,000 homes. The cabling for the LIPA installation was designed, manufactured, and installed by Nexans. National Grid and American Electric Power energized distribution-voltage superconductor cable systems in their commercial power grids in Albany, N.Y., and Columbus, Ohio, respectively, in 2006.

In the summer of 2010, Nexans completed the first-ever successful demonstration of an HVDC high temperature superconducting cable operating at 200 kV. This prototype cable, together with a newly designed termination, passed a series of tests based on CIGRE (International Council on Large Electric Systems) recommendations.

In October LS Cable Ltd. of South Korea announced the purchase of 3 million meters of American Superconductor’s Amperium high-temperature superconducting wire for fabrication into transmission cables.
Just last year, AMSC received its first commercial order from LS Cable for approximately 80,000 meters of its Amperium wire to manufacture a 22.9 kV (distribution voltage) ac cable system that will be installed in a Korea Electric Power Corp. substation near Seoul. Rated at 50 megawatts, that cable system will be 0.5 kilometer long, making it the world’s longest distribution-voltage superconductor cable system.

LS Cable also is actively developing a 154 kV (transmission voltage) ac superconductor cable system. KEPCO, which is Korea’s only power grid operator, has forecasted the wide deployment of superconductor power cables in the Korean grid starting in the 2012-2013 time frame.

Phillip G. Harris is chairman and chief executive officer of Tres Amigas, LLC, in Sante Fe, N.M. Jack McCall is director of high temperature superconductor transmission and distribution systems for American Superconductor in Devens, Mass.