Advanced Power Electronics Enables Renewable Development Across NERC Regions
The Tres Amigas HVDC Project is a new concept developed by a nonutility transmission marketer. Its Tres Amigas SuperStation (TASS) will provide transmission access in an open and flexible arrangement by allowing the interconnection of the three intersecting southwestern U.S. transmission networks: the Electric Reliability Council of Texas (ERCOT), the Southwest Power Pool (SPP), and the Western Electricity Coordinating Council (WECC). This will be accomplished by the application of state-of-the-art insulated gate bipolar transistor (IGBT)–based voltage source converters (VSCs) in a three-way asynchronous switchable arrangement. The location of the project near Clovis, New Mexico, is shown in Figure 1.
Besides the fact that this area has significant opportunities for the development of transmission corridors, the site is also in a location central to potentially large wind generation (Figure 2) and solar generation (Figure 3) resources. Figure 4 shows how the Tres Amigas HVdc project is centered close to some of the highest-capacity potential renewable resources in the southwestern United States. Many of these facilities will be able to produce energy at capacity factors that top 30%.
Significant differences in power prices exist in each of the NERC regions. Prices have been examined by ZGlobal in detail for calendar years 2008–2010, and -market analysis has continued to the present time. Detailed studies have shown economic opportunities for providing transmission services among all three NERC regions. The results of the initial 2008 studies showed a significant benefit could be obtained if there was a transmission system that could change direction and provide absolute power level control regardless of network topologies. Additional studies showed that a 15-min schedule interval would provide even greater market opportunities for potential power sales. Figure 5shows the pricing differences between SPP and ERCOT that could be exploited with near-term resource mixes. These market analysis studies continue to be refined, and the current work indicates even more favorable economic opportunities based on the differential pricing of energy among regions.
The second directional comparison was made between the WECC and ERCOT regions. The early study results are shown in Figure 6. Bidirectional opportunities exist between these two NERC regions, just as with ERCOT and SPP; the price differentials strongly suggest that even greater transmission benefits can be realized, however.
More detailed analysis was also made as to the pure transmission benefits, starting from an initial 750-MW ERCOT-to-WECC connection and continuing to a complete three-way, 5,000-MW, back-to-back (BtB) HVdc tie. Several assumptions were made, including the development of appropriate transmission line connections beyond the existing 2012 transmission networks. Figure 7 shows the initial results of that body of work and demonstrates a significant benefit from the transmission capacity provided by the Tres Amigas project.
These transmission (network) benefits are somewhat difficult to determine, in that they are based on a host of assumptions about the completion of new transmission lines and various network upgrades in the Texas (ERCOT), SPP, and WECC regions. Figures 7 and 8 are simply based on the benefits available to ERCOT transmission providers (TPs) and their customers. As transmission network strength is increased in ERCOT, the values shown in Figure 7 should improve. The project’s financial benefits are derived from the power sales enabled through the new set of transmission paths among the three systems. Figure 8 shows the initial results of studies of the ERCOT and SPP transactions.
The initial project concept called for the development of a three-way HVdc bus with the potential use of systems making use of high-temperature superconductor (HTSC) cable and HVdc power circuit breaker (HVdc-PCB) technology. After significant research and market and reliability investigations, it was determined that this concept could not be executed with the current technology offerings in HVdc and without a significant technological risk for these items of hardware, including the proper protection of the IGBT-based HVdc valves. A new concept was formed in late 2010, and it was determined that the HVac side of the station could initially be configured to quickly change configuration (in as little as 1 min) and still be consistent with existing 15-min power and transmission scheduling timing windows. Figure 9 shows the proposed layout of the fully expanded Tres Amigas project. Both VSC and conventional line-commutated converters (LCCs) are proposed in the full system build-out. The TASS concept will start with the development of a single 750-MW VSC to allow the connection of the converter to one area of relatively limited short-circuit (SC) strength in New Mexico and to permit the use of simplified HVac filtering and reactive deployment strategies.
The concept will be expanded to the full 5,000-MW conversion potential by adding three 750-MW VSCs and three 920-MW LCCs. Each of the converters will be configured in a new concept called folded back-to-back. In this configuration, the HVdc bus (~300-kV dc) faces south to allow for the future use of HVdc-PCB switching and HTSC cables, which could extend the TASS system to further extended nodes. The use of other technologies is also made possible by the arrangement of the HVdc station bus, which is configured outdoors.
The center of complexity was moved to the 345-kV HVac side of the station and will be implemented using current 345-kV gas-insulated substation (GIS) and gas-insulated line (GIL) technology from the HVdc original equipment (OEM) systems supplier. The 345-kV bus is to be configured to allow flexible additions of new bay positions while maintaining a high capacity (a main bus rated up to 10,000 A). Each line position, PCB position, and expansion point will be configured with a “dead-front termination.” This installation concept will allow the addition and expansion of the system without an outage of the main bus. These GIS arrangements allow compact and modular design; the use of sets of fast motor-operated disconnect switches within the GIS for rapid configuration changes will permit reliable change-of-power-direction, regional schedule, and maintenance switching in a fully automated and interlocked system that assures the isolation of each of the NERC regional networks.
Network Issues and Challenges
At the present time, interconnection studies are being performed for the first two interconnected utilities, Xcel (SPP) and PNM (WECC). New 345-kV transmission lines are planned to Tolk Substation in Texas and Blackwater Substation near Clovis, New Mexico. Although these studies have not been fully completed, there are “weak network” challenges on the PNM side of the TASS project that favor the application of new VSC technology, capable of operating to lower equivalent short-circuit ratio (ESCR) levels. Line routing and environmental permitting and related work are in progress for these first two interconnections. Current systems studies are reviewing a wide range of reactive support options on the western side of the station. The use of VSC technology allows real-time control of reactive balance, with a smooth vernier control and the possibility of more complex control and coordination of external devices such as SVCs and synchronous condensers and static compensators (STATCOMs) as the dynamic reactive requirements change over a wide range of ac network contingencies. Early study results continue to show that significant reactive reinforcement on the western side of TASS will be required; moderate or no additional reactive support appears to be needed on the eastern side of the TASS station (subject to continued studies and changing network conditions).
Project Details and Staging
The first stage of the project will install the first VSC and the base GIS/GIL system. The second stage will add a second VSC along with additional 345-kV GIS/GIL bays and transmission line positions. It will also extend the 14.4/24.9Y-kV medium-voltage (MV) station service distribution system. Transmission is expected to be expanded to SPP at this level, and there will be transmission improvements to WECC as well. The third through sixth stages will be fully market-driven and subject to interconnection studies, transmission line siting processes, and local renewable energy development near the TASS project, as well as the availability of project funding.
Battery energy storage (BES) systems will augment the medium-voltage station service distribution system. These will initially be sized at 100 MWh (two 50-MWh units). Other wind, solar, and biomass projects may be developed locally and will be able to be interconnected at the 345-, 115-, or 14.4/24.9Y-kV levels.
Figure 10 provides a simulated view of the first 750-MW VSC, showing the folded design with the HVdc air-insulated switchyard (AIS) at the bottom of the figure and the HVac lines from the GIS/GIL system entering (see the top of Figure 9).
Station Single Line
The TASS project is to be built in stages, with each stage taking advantage of substation infrastructure developed in -earlier stages. The first stage of development is shown inFigure 11, which illustrates the incoming 345-kV transmission from Xcel’s Tolk Substation (and its coal-fired thermal generation station) in western Texas (on the SPP grid) and the 345-kV WECC connections to the Blackwater Substation of Public Service of New Mexico (PNM). The PNM 345-kV transmission line extends to the Albuquerque metropolitan area and is connected to significant wind generation. The BESs are connected via the 14.4/24.9Y–to–345-kV transformation, as well as by the possible addition of synchronous condensers. The Blackwater Substation is located approximately 20 mi south of the TASS site. Systems studies show reactive reinforcement may be required for proper operation of the Blackwater HVdc -terminal under certain network conditions. Extensive modeling and simulation work to assure minimal control interactions will be carried out during the TASS design process. A high-speed fiber-optic control link is planned between Blackwater and TASS. The control system will be implemented and be complementary with the existing (and recently replaced) Blackwater HVdc control system.
Description of the HVdc VSC/LCC
A multilevel IGBT converter will be utilized as the basis for the first three converters on the TASS project. The basic single line for the VSC is illustrated in Figure 12, which shows the initial concept for the main circuit connections. The converter is connected to the incoming and outgoing HVac (345-kV) mains through a GIS switching bay. The ac side of the station will be configured as a typical AIS so the line reactors and inrush resistor switching arrangement can be easily constructed and fast equipment replacement is possible, if necessary. This also allows for a less complex converter transformer replacement sequence. The converter transformers will be single-phase, two-winding units to reduce transportation and spare parts costs.
The converter valve modules are built up from separate valve modules and placed within floor-mounted frames. Figure 13 shows a simulated view of the valve hall (midpoint) with the modules stacked and connected to wall bushings on the south wall of the converter building.
The initial development of the project will bring the first three VSCs online and provide additional capacity for the TASS to accept additional transmission circuits. The later stages of development, following the expected adjacent network improvements and improved system strength (better short-circuit ratios), will eventually permit the addition of the three LCCs. The expected steady-state rating for each of the LCCs will be 950 MW, and the LCCs will be equipped with overload capability to provide additional network emergency system benefits. Because these later stages of development are quite dependent on the improvement plans and schedules of the adjacent utility networks, only space and main bus capabilities (at the 345-kV level) will be provided during the first stage of overall 345-kV substation project development.
The valves within the LCCs and VSCs will be cooled using one circuit cooling loop for each of the BtB converters. Cooling of the IGBT and thyristor valves will use a mixture of deionized water and ethylene glycol (fully spill-contained). The outer heat exchangers will be of the dry type, with provision made for additional refrigerated system chillers (precoolers) that may be required during some high-temperature -conditions (subject to final cooling system design requirements). The control and protection systems will include extensive use of IEC 61850–based relays, controls, and SCADA systems. The bulk of the protection and control systems will be provided by the OEM supplier of the HVdc converters.
The overall HVdc and GIS/GIL systems will operate in a semiattended mode, with the main operations staff located at a primary control center away from the Clovis, New Mexico, site. A backup control center will be established at the Clovis TASS site to fully conform to FERC’s reliability and security guidelines and will allow full operational capability if the primary control center is damaged or becomes unavailable due to loss of telecommunications. The final location of the primary control center facility has not been determined, but it will be located in Texas or New Mexico.
The application of large-scale voltage-source, BtB HVdc converters to the three North American (i.e., NERC regional) grids presents unique challenges and significant power- and transmission-scheduling opportunities. The TASS project is a uniquely staged and planned, multistep development of physical facilities that will profoundly affect the marketing of renewable energy in these three regions. The TASS project is expected to support the significant growth of renewable energy projects through significant and flexible improvements in transmission access. The potential for these new renewable resources to exploit time-of-day market diversity should provide additional benefits to the ERCOT, SPP, and WECC network regions of the United States.
While VSC technologies have been installed in other HVdc system arrangements, this project uniquely applies up to six BtB HVdc converters in a configuration that uses proven GIS/GIL technology and leading-edge IGBT and conventional thyristor-based converter valves in an arrangement offering maximum flexibility. The project will expand and interconnect up to 5,000 MW of transmission capacity in a flexible, real-time-controlled environment. The station will be located in central east New Mexico, allowing access to ERCOT, SPP, and WECC networks while continuing isolated interconnection between all three NERC regions. This will take place in the middle of a large concentration of proposed renewable energy areas located in the southwestern and midwestern United States.
For Further Reading
Z. Alaywan, “Early experience in centralized real time energy market,” in Proc. European Engineering Conf, June 2005.
Z. Alaywan, “A perspective on renewable energy development in the western U.S.,” in Proc. Int. Renewable Conf., Mar. 2005.
Z. Alaywan, “A market view from the operating room,” in Proc. 2004 IEEE-PES Conf., June 2004.
Z. Alaywan, “Transitioning the California market from zonal to a nodal framework—An operational perspective” in Proc. 2004 IEEE-PES Conf., Oct. 2004.
Z. Alaywan, “How renewable energy policy benefit consumers and the environment,” California PUC, June 2003.
Z. Alaywan, “Coordinating energy dispatch and planning between regions,” in Proc. EUCI Conf., Apr. 2004.
Z. Alaywan, “Transmission pricing and auction,” European Delegation on Restructuring, University of Toulouse, France, June 2003.
Z. Alaywan, “Capacity payment for local reliability resources,” in Proc. EUCI Conf., May 2003.
Mark Reynolds is with POWER Engineers Inc., Portland, Oregon.
David Stidham is with Tres Amigas LLC, Santa Fe, New Mexico.
Ziad Alaywan is with ZGlobal, Folsom, California.