When New Century Energies and Northern States Power merged in 2000, the new company, Xcel Energy (Minneapolis), made a commitment to connect the two systems — the 345-kV Kansas system and the 230-kV Colorado system. In February 2003, after an extensive evaluation of technical, economic and commercial considerations, Xcel Energy awarded a turnkey contract to the consortium of Siemens Power Transmission & Distribution and Beta Engineering for the design, construction and commissioning of the 210-MW back-to-back dc tie located near Lamar, Colorado. The Lamar project will connect Xcel Energy's Southwestern Public Service Co. system in the east with its Public Service Co. of Colorado system in the west, and allow 210 MW of power transfer capability in each direction. Once completed, the Lamar project will be the seventh dc tie to connect the eastern grid to the western grid in the United States.

Design Completed

After the merger, Xcel Energy, the fourth-largest combination natural gas and electric company in the United States, was faced with the technical challenge of integrating the company's two systems and confidently delivering power between the nation's east and west grids. Engineering specifications, procurement and project management for the Lamar project have been performed from Xcel Energy's Denver office. Xcel Energy engineers conducted a system-performance study using EMTDC/PSCAD, a software-based design that simulates voltage and transient flows in normal transfer conditions and contingency conditions from both sides of the grid. Transient Network Analysis (TNA) was conducted in the electrical laboratory to test the controls under various operational scenarios to confirm their performance and robustness prior to the field testing, energizing and commissioning.

Outsourcing Draws on Technical Expertise

A key component to the success in meeting the technical challenges of this important dc tie has been the decision to outsource for expertise and experience.

The partnership between Xcel Energy and the turnkey consortium of Siemens High Voltage Systems (HVS) division (Wendell, North Carolina), Siemens Reactive Power (Erlangen, Germany) and Beta Engineering (Pineville, Louisiana) has brought together the necessary experience and know-how. Xcel Energy has regulated operations in 11 western and Midwestern states. Xcel Energy provides a comprehensive portfolio of energy-related products and services to 3.3 million electricity customers and 1.7 million natural gas customers through its regulated operating companies.

The highly technical nature of the project calls for state-of-the-art power electronics to ensure a smooth and reliable interconnection. Siemens Power Transmission & Distribution (Siemens PTD; Raleigh, North Carolina) is providing the components and deploying Grid Power Flow Controller technology, which meets all of the protection and control requirements for the project design. Siemens has provided customized turnkey solutions and projects for high-voltage systems around the world. Siemens' expertise includes solutions for Series Capacitors, Static Var Systems and Grid Power Flow Controllers. The Grid Power Flow Controller design for the Lamar project is being managed under the Siemens HVS.

Beta Engineering, the construction partner, is a turnkey firm specializing in engineering, procurement and construction of high-voltage electrical substations, transmission lines, distribution systems and FACTS projects. Beta performs turnkey high-voltage projects throughout the United States and works with most of America's major utilities and general design/build firms. Siemens and Beta have jointly completed numerous electrical systems projects in the United States. Because of close communications between the turnkey consortium and Xcel Energy engineers, the project has remained on schedule with an on-time completion date of January 2005.

Reliability Control Strategy

State-of-the-art architecture has been design into the new tie to maximize reliability. Components provided by Siemens include a current source six-pulse converter, which uses the Grid Power Flow Controller design and three single-phase transformers on each HV side; light triggered thyristors converter valve modules in combination with outdoor filters, mechanically switched reactors and capacitors and other outdoors equipment; and Siemens' SIMADYN-D controls with dual redundancy. The ac and dc protection is also provided by Siemens' conventional solid-state and custom relays. The converter valves use a closed-loop cooling system with de-ionized water and glycol. All functions for the new dc tie are automated, and the subsystems also include redundancy for reliable operation. Furthermore, the dc tie is designed for the local ambient weather conditions.

Site Construction Criteria

Site construction is well underway for an on-time January 2005 completion date. The consortium staff are coordinating and supervising the day-to-day work of the construction contractors. Weekly conference calls and monthly meetings ensure timely progress and quality of the work. Siemens PTD is coordinating construction with respective procurement departments, project management and engineering from Beta and Siemens, Germany.

During field-testing, additional system models and capabilities studies will be completed using PSSE and PSLF. The operational performance is designed for a 210-MW power flow while maintaining a voltage of 0.95 p.u.-1.05 p.u. at both HV busses. Designed into the new dc tie is the capability of providing voltage support to the west (230 kV, CO) side when the east side is blocked. The tie is designed for unmanned operation using SCADA systems. The Xcel Energy EMS system, also provided by Siemens, interfaces with the dc tie SCADA system. The power sources and loads on both sides will realize benefits from the dc tie's primary features of power transfer and voltage control and secondary features in system stability. Both east and west sides will have power scheduling and controlling capabilities under a hierarchy via their SCADA master stations, which also allows for coordinated redundant capabilities. In addition, local control is available using a Human Machine Interface system.

Conclusion

The dc tie will enable economic exchange of power between Xcel Energy's two control areas and an enhancement in system stability to the 200-mile transmission system connecting Finney, Kansas, to Boone, Colorado.

Rick Chapel is a project manager at Xcel Energy in Denver. He has been in the electric utility industry for 34 years.
rich.chapel@xcelenergy.com

Ernie Poggi, PE, has been with Xcel Energy Energy for 15 years. He has 28 years of experience in protection/substation engineering and in his present position as substation maintenance engineer for four years.
ernie.poggi@xcelenergy.com

Moe Aslam, PE, is a program/project manager at Siemens PT&D. He has been with Siemens for eight years and in the T&D industry, in project management and engineering, for 25 years. His responsibilities on the project include overall project management for the Siemens-BETA Consortium and chief customer interface for Siemens.
moe.aslam@siemens.com

George Brashear, PE, is vice president/project manager at Beta Engineering. He has 16 years of experience in the design, management and construction of engineering projects.
George.Brashear@BetaEngineering.com