The Western Area Power Administration, Heartland Consumers District, and Basin Electric, owners of the transmission system formerly known as the Integrated System (IS), voluntarily elected to join Southwest Power Pool (SPP) effective Oct. 1, 2015. SPP is a regional transmission organization with a footprint that spans nearly 575,000 square miles in all or parts of 14 states in the central U.S. and includes more than 800 generating plants, nearly 5,000 substations and about 56,000 miles of high voltage transmission lines. The integration of the former IS adds about 5,000 megawatts of peak demand and 7,600 megawatts of generating capacity.

The former IS is referred to as the Upper Missouri Zone because the transmission system, along with Western's hydro generation and Basin Electric's generation, is located in the northern, or "upper," portion of the Missouri River and tributaries.

The facilities of Basin Electric, Western and Heartland comprise the backbone of the high-voltage transmission grid in the upper Great Plains region of eastern Montana, North Dakota and South Dakota.

This jointly developed transmission system evolved from the 1962 Joint Transmission System (JTS) agreement of the Bureau of Reclamation with Basin Electric and 103 cooperative and municipal preference customers in this region.

This 1962 agreement was a crucial component in making Basin Electric a reality. It enabled Basin Electric to lease capacity on the federal transmission system and deliver output from its first power plant to members throughout the region by building only 12 miles of high-voltage transmission line. Under the JTS agreement, the parties planned, constructed and operated transmission facilities needed to serve the customers of the region on an integrated single-system basis. In 1977, the U.S. Congress transferred the power marketing functions of the Bureau to Western, and Western assumed the responsibility for the operation of the JTS.

In response to the Federal Energy Regulatory Commission (FERC) open-access transmission orders 888 and 889, which set forth billing methodologies inconsistent with the JTS methodology, the JTS agreements were terminated. The IS agreement between Western, Basin Electric, and Heartland, signed in 1998, replaced the JTS agreements and provided open-access transmission service to customers in the region until Oct. 1, 2015.

On Oct. 1, 2015, Western, Basin Electric and Heartland joined SPP. Additionally, other transmission owners in the region have also joined SPP and now include their transmission facilities in the Upper Missouri Zone of SPP, including East River Electric Power Cooperative, Northwest Iowa Power Cooperative, Corn Belt Power Cooperative and Central Power Electric Cooperative.

The transmission facilities of the SPP - Upper Missouri Zone are available for use by any eligible transmission customer. The availability is posted for potential customers on the Internet-based OASIS (Open Access Same-time Information Systems). Third-party users pay the same transmission charges as the owners of the system in accordance with the comparability provisions of the FERC-approved tariff rate.

Open access transmission service is provided under SPP's Open Access Transmission Tariff on file with the Federal Energy Regulatory Commission.