Tres Amigas Superstation: A landmark HVDC project

The pioneering Tres Amigas Superstation project in New Mexico, USA, will utilize superconducting HVDC ‘pipelines’ and voltage converter stations to connect three independent US power grids: the Western, Eastern, and Texas Interconnections. Phillip G. Harris, chairman and CEO of Tres Amigas LLC, details perhaps the most exciting T&D project in the world.

Phillip G. Harris, Tres Amigas LLC, USA

While abundant renewable power sources are available in the United States, this clean energy cannot be fully utilized unless it can be economically transmitted to load centres where it is needed.

The vast wind and solar opportunities that are available in Texas and the western deserts of America are only two key examples of the bountiful resources to be found in the US. However, the Tres Amigas Superstation, planned for construction in Clovis, New Mexico, will facilitate the greater adoption and distribution of renewable energy across North America.

The Superstation will interconnect America’s three currently isolated power grids creating the nation’s first energy market hub. As a result, wind, solar, hydro and geothermal power sources that currently do not have access to transmission lines and/or customers will, for the first time, be able to tap into multiple markets.

The Superstation will utilize ultra-efficient, high capacity direct current (DC) superconductor cables, coupled with voltage source converters (VSC), to enable multi-terminal transmission of many gigawatts of electricity. This landmark project has successfully achieved numerous milestones and is on track to commence construction in 2011. The SuperStation is targeted for commercial operation in 2014.

Contracts and Land-lease Agreement Propel Project

A technological tour de force, the Tres Amigas Superstation has passed regulatory muster ” namely, that of the Federal Energy Regulatory Commission (FERC) ” and has countless interested stakeholders.

Maintaining a steady pace for this multi-billion dollar grid intertie project of epic engineering advances, Tres Amigas LLC has lately appointed a chief commercial officer, signed an engineering services contract with Power Engineers, and signed a 99-year lease agreement with the New Mexico State Land Office. The 99-year lease for 14 400 acres of state trust land will eventually, according to a release from the Land Office, generate $9.4 million in revenues for the state annually when the Superstation is fully operational.

The contract with Power Engineers covers a wide range of engineering support to design, specify and manage the engineering activities for Tres Amigas. Power Engineers staff, who are currently involved with several HVDC projects under development in the United States, have begun engineering design work for the Superstation, which will employ the latest HVDC technology in VSCs from Alstom Grid and superconductor cables from Nexans and LS Cable. The superconductor cables will be fabricated using American Superconductor Corporation’s (AMSC) proprietary Amperium second generation high-temperature superconductor (HTS) wire.

Power Engineers is also providing the design services that will lead to construction of the Viridity Energy commercial operations centre for Tres Amigas. By closely integrating these resources through CH2M Hill’s programme management services and with the engineering analytics of Burns & McDonnell, the planning insights of ZGlobal and the battery capability of Xtreme Power, Tres Amigas has assembled an all-American team of companies to construct this much needed grid intertie for North America.

Strategic Grid Balancing Resources in the Right Spot

The Superstation will interconnect America’s three power grids known as the Eastern Interconnection, the Western Interconnection and the Texas Interconnection, also known as the Electric Reliability Council of Texas (ERCOT), enabling the buying and selling of electricity between them. A key Superstation design qualifier is that the potential total power transfer capability of the station far exceeds the sum of all the existing AC/DC interconnections between ERCOT, the Western Electricity Coordinating Council (WECC) and the Eastern Interconnection.

Tres Amigas will be able to operate as a large, approved Balancing Authority Area within the WECC. Tres Amigas will permit power sellers in ERCOT to schedule power to either the Eastern Interconnection or the WECC; power sellers in the Eastern Interconnection to schedule power to either ERCOT or the WECC; and power sellers in WECC to schedule power to either ERCOT or the Eastern Interconnection.

The strategic location of the Superstation near Clovis, New Mexico ” around 1.5 km from the Texas border ” is in close proximity to the transmission systems in each of the three interconnections and adjacent to geographic areas that have among the highest potential in the nation for the development of renewable energy generation. The Superstation’s engineering is guided by the objective of providing substantial expansion capacity on a component basis, so that the station capacity can grow along with the growth of renewables generation in the region.

Reducing Costs Via the Power Hub Concept

The Superstation has been referred to as a renewable power hub because of its large potential capacity for renewable power transfer. Studies have already established that the quality of intermittent and variable renewable energy can be enhanced by aggregating sources from geographic locations that may experience high winds or sunshine at different times.

Technically, this means that the Superstation will make it practical and economic to firm up these intermittent and variable renewable energy resources by taking advantage of geographical diversity and energy storage, such as the on-site batteries at Tres Amigas or systems such as compressed air energy storage (CAES). This capability greatly enhances the value of new generation, creating additional economic incentives for the development of such generation in the region.

Tres Amigas will expand the geographic reach of markets, offering new opportunities to take advantage of load and resource diversity. The effect of this is to reduce costs. At present, marginal prices for energy in the three US interconnections typically diverge because these markets operate in isolation. By integrating the grid, prices will converge as lower cost producers get a chance to compete.

Studies that Tres Amigas submitted during FERC proceedings in 2009″2010 showed that marginal energy prices vary significantly between the Southwest Power Pool (SPP) in the Eastern Interconnection, ERCOT and the WECC. During sample time intervals, the data showed that for over 2000 hours per year, energy prices varied by more than $50/MWh between the California Independent System Operator (CAISO) and ERCOT, over 1600 hours per year between ERCOT and the Palo Verde hub, over 1500 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.

Furthermore, the Tres Amigas Superstation will enhance the value of transmission investments made in the region by allowing power to freely move between the various interconnections. In this way, Tres Amigas will permit power to move to and from the different markets, expanding the potential use of the existing transmission grid and expansions thereto. Tres Amigas should also provide system planners new opportunities to improve the efficiency and reliability of the electric system at a lower overall cost.

High Tech, Precisely Controllable ‘Traffic Rotary’

The basic capability and configuration of Tres Amigas is shown schematically in Figure 1. The design places multiple high capacity AC/DC VSCs at the interconnection points (or terminals) of the Eastern Interconnection, WECC and ERCOT.

Figure 1: Basic configuration of the Tres Amigas Superstation

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 VSCs 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. The ultimate number and size of the AC/DC converters constructed at each terminal will depend on the demand for transmission service and the number and size of the interconnecting lines built to Tres Amigas.

Figure 2: DC superconductor power cables will create paths to connect renewable power sources Source: NREL

Based on discussions with the vendors of VSC technology, it appears that the optimal design for Tres Amigas will be based on utilizing 750 MW sized converters that can be installed on a component basis to provide whatever level of AC/DC conversion is consistent with the demand for service at each terminal.

Although specific engineering analyses 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.

These inherent capabilities of the Tres Amigas SuperStation are particularly important because it will be located in a remote area, where a strong source of reactive power is necessary to support both new renewable energy generation and new transmission. 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.

DC Superconductor ‘PowerPipelines’ Also Key

The interconnection points, or terminals, will be tied together with several kilometres of underground superconductor DC transmission cable called PowerPipelines. This development will be spearheaded by Tres Amigas participant AMSC. Phase one of the Tres Amigas SuperStation will comprise a 750 MW two-way connection to tie together the Eastern and Western Connections.

The use of underground superconductor DC transmission cable will permit Tres Amigas to reliably move large quantities of power between the three terminals, with a manageable footprint, minimal environmental impact and virtually no losses or heat generation.

This project will become a showcase for high temperature superconductors, a technology that has been spearheaded by many laboratories around the world, including Los Alamos National Laboratory, a New Mexico neighbour of Tres Amigas.

PowerPipelines have a broad scope of application because 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. For example, a 5000 MW superconductor DC cable may have 250 MW injected at 20 locations as it passes through the wind energy rich upper Midwest portion of the US, and deliver 500 MW to each of the ten cities that it passes on the way to the east and west coasts.

As an open access facility, allowing the benefits to be shared across the industry, Tres Amigas will advance the public interest in several important ways. Tres Amigas represents a cost-effective and efficient way for large amounts of existing and potential renewable energy generation to get to market.

The developers of this new generation will enjoy more favourable economics, which will enhance the value of new generation and create additional incentives for its development. This benefit is particularly notable because Tres Amigas is located adjacent to areas of the country that have the best potential to develop renewable power.

Prices for energy in markets that are currently electrically separated will tend to converge as the current lack of facilities has prevented trading across the three interconnections. By enabling these price differences to be reduced, Tres Amigas will allow energy to be supplied more efficiently, producing substantial savings for electric consumers. Opportunities will be created to strengthen intermittent and variable renewable energy by taking advantage of geographical diversity and by using the on-site batteries at Tres Amigas.

The value of transmission investments in the affected regions will be enhanced by allowing power to move more freely between the three interconnections, and reliability will be improved. The use of VSCs will permit others to construct AC lines to Tres Amigas without having to invest in series compensation or other costly mechanisms to provide voltage support to the AC system in a remote area. The technology employed at Tres Amigas should also reduce losses on the interconnecting AC lines, lowering the cost of delivering power.

Finally, electric system reliability in the area around Tres Amigas will be improved because Tres Amigas will connect the three asynchronous grids at a robust station with back-up power and VSC technology that will provide substantial, controllable reactive power to the transmission system, particularly important because of Tres Amigas remote location

This article was co-authored by Jack McCall, director of HTS T&D Systems for American Superconductor. The author would like to thank him for his invaluable contribution to the article.

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