Power Transmission

Western Area Power Administration is in the business of both transmitting power, and power marketing, or the selling and purchasing of additional power transmission capacity on the open market.

After reserving capacity to deliver Federal hydropower to WAPA’s long-standing, firm power contracts, any remaining transmission capacity may be purchased on the open market. Available capacity is sold on the Open Access Same-Time Information System.

The Power Grid

The United States does not have one national electric transmission power grid, it has three separate grids—a Western grid, an Eastern grid, and one in Texas managed by the Electric Reliability Council of Texas.

The electric transmission system (grid) in the western United States is called the Western Interconnection, and it operates independently from the electric transmission systems in the eastern United States and Texas.

WAPA operates primarily in the Western Interconnection where it owns and maintains over 17,000 miles, or more than 10 percent, of transmission lines in the Western Electricity Coordinating Council area.

WAPA’s transmission lines originate at federally owned dams and hydropower plants, and begin the transmission journey of federally produced hydropower to wholesale customers and ultimately 40,000,000 Americans.

Transmission Control Areas

Within the three power grids in the United States and Canada, WAPA operates 4 of 150 control areas.  Of 30 control areas within the Western Interconnection, WAPA operates three control areas.

The three Western Interconnection control areas are called:

• Western Area Upper Missouri-West (WAUM-West), operated by our Upper Great Plains Region;
• Western Area Lower Colorado (WALC), which our Desert Southwest Region operates;
• Western Area Colorado Missouri (WACM), which our Rocky Mountain Region operates.

In the west, WAPA’s Sierra Nevada region operates as a contract-based, sub-control area within the Sacramento Municipal Utility District control area.

Within the Eastern Interconnection, WAPA’s Upper Great Plains region also operates a control area called the Western Area Upper Missouri-East (WAUM-East).

WAPA owns and maintains more than 30 percent of the transmission line mileage in the Mid-Continent Area Power Pool area within the Eastern Interconnection.

Transmission Services

WAPA offers three types of transmission services:

• Point-to-point service is transmission between points of receipt and delivery.
• Network integration transmission service delivers capacity and energy over WAPA’s system from a network customer’s designated resources to its designated load.
• Control area ancillary services support the transmission of capacity and energy from resources to loads while maintaining the reliable operation of the system.

WAPA’s Control Area Ancillary Services

The Federal Energy Regulatory Commission defines ancillary services as:

• Scheduling, System Control and Dispatch—scheduling the amount of energy to be delivered, assigning load and ensuring operational security.
• Reactive Supply and Voltage Control—maintaining correct voltage through adjustments to generator output.
• Regulation and Frequency Response—following the moment-to-moment variations in the demand or supply in the control area.
• Energy Imbalance—providing energy correction for any hourly mismatch between a transmission customer’s energy supply and demand served.
• Spinning Reserves—providing immediate backup service from a reserve unit to serve load in case of a system contingency. (When the reserve unit is operating, it is spinning, thus the name spinning reserve.)
• Supplemental Reserves—serving loads when a contingency exists; not available immediately to serve load but can be available within a short time.

Power Dispatchers and Balancing Supply and Demand

Grid stability relies on power dispatchers balancing the supply of energy generation, to demand delivery schedules, by monitoring and maintaining system voltage. On an hourly and daily basis, real-time dispatchers validate delivery schedules with other scheduling entities, and then validate them again at the end of the day.

Power dispatchers also verify balanced delivery schedules for customers. They manage real-time e-tags—which document scheduling energy production from an energy supplier, with reservations to transmit that energy over the transmission lines.  They also assist Automatic Generation Control dispatchers with energy purchases and sales for managing Area Control Error requirements.

Power dispatchers follow the reliability procedures and protocols of the North American Electric Reliability Council and the Western Electricity Coordinating Council.

Preventing Service Interruptions

Supply and demand must be perfectly in balance. Even small imbalances can cause voltage levels to swing significantly.

To prevent damage, the grid is designed so that whenever unacceptable fluctuations in voltage levels or frequencies begin to occur at any point in the system (due to lines de-energizing), the generation in the area will immediately shut down.

When shutdowns occur at the power plants, the connections to adjoining areas are designed to automatically isolate the area where the imbalance occurs from the rest of the grid. So power that had been flowing over the line that failed automatically shifts to other, adjacent lines, reducing the risk of interrupting service to end-use customers or overloading lines.

Direct Current (DC) Ties

The power grid Interconnections run on 3-phase alternating current (AC).  DC ties between Interconnections provide a means to exchange or buy and sell power between the nation’s interconnections, without requiring them to be in sync or operating in phase as a single grid.

DC ties increase grid stability by preventing failures from cascading between Interconnections, while enabling the exchange of power resources to balance systems, or in times of emergency.

There are six DC ties connecting the Western Interconnection and the Eastern Interconnection in the United States; and an additional DC tie to Canada.
WAPA is associated with four of them.

WAPA owns and operates the Virginia Smith (Sidney, Nebraska) DC tie; owns 60 percent of the Miles City DC (Montana) tie and operates it; and operates the David A. Hamil DC Tie (Stegall, Nebraska) owned by Tri-State Generation and Transmission Cooperative.

WAPA also operates the back-to-back DC Converter Station project in Rapid City, S.D., owned by Basin Electric Power Cooperative and Black Hills Power and Light. The station can transfer up to 200 MW of power between the Western Electricity Coordinating Council in the Western Interconnection and the Mid-Continent Area Power Pool in the Eastern Interconnection.

The other two U.S. ties are Public Service Company of New Mexico ‘s Blackwater N.M., DC tie and the El Paso Electric and Texas-New Mexico Power Company’s Artesia, N.M., DC tie.

In May 2005, Xcel Energy commercialized the latest High-voltage DC, or HVDC, tie in Lamar, Colo. This provided up to 210 megawatts of capacity between its operating companies of Southwestern Public Service in the Southwest Power Pool and Public Service Company of Colorado.