A consortium led by Ameren Transmission Company of Illinois and GridLiance Heartland, LLC has been selected by Midcontinent Independent System Operator (MISO) to develop two large-scale transmission projects that would establish a new 765-kilovolt power corridor across parts of Iowa, Illinois and Indiana.

 

The selection advances more than $1.6 billion in planned transmission infrastructure investment tied to MISO’s long-range grid expansion strategy as utilities prepare for rising electricity demand, new generation interconnections and broader grid reliability requirements across the Midwest.

 

The projects — known as STIW and WIIL — are part of MISO’s Long Range Transmission Planning Tranche 2.1 portfolio approved in late 2024. Both are targeted for service by 2034, subject to permitting and regulatory approvals.

Sub T – Iowa/Illinois State Line

The larger of the two developments, the Sub T – Iowa/Illinois State Line – Woodford County project, is expected to cost about $940 million. Plans call for approximately 149 miles of new 765-kV transmission infrastructure stretching from the Woodford County Substation in central Illinois westward to the Iowa border and onward to the Sub T Substation in Iowa.

Illinois/Indiana State Line

The second project, Woodford County – Illinois/Indiana State Line, carries an estimated cost of $718 million. And includes about 88 miles of additional 765-kV transmission lines as well as a new 765/345-kV substation in Illinois. One segment would extend east toward the Illinois-Indiana border, while another shorter segment would connect toward the existing Collins Substation area.

 

Together, the projects will form the backbone of a new extra-high-voltage transmission network designed to improve regional power transfer capability and support long-term load growth across the Midcontinent grid.

Ownership

Under the ownership structure announced for the projects, ATXI and GridLiance will each hold 43% stakes. While Dairyland Power Cooperative will own 11% and Illinois Municipal Electric Agency will hold the remaining 3% once the assets enter service.

 

The projects represent another major phase in the Midwest’s broader transmission buildout. Where grid operators and utilities are pursuing higher-capacity transmission systems to accommodate growing industrial electricity demand, renewable generation expansion and increasing reliability pressures tied to aging infrastructure and extreme weather events.

 

Executives from the participating utilities and transmission developers said the projects would require extensive coordination with regulators, local communities and contractors throughout the multi-year development and construction process.

Additionally, this latest Midwest transmission expansion builds on a broader wave of high-voltage grid development across the United States, including the ongoing land acquisition for major grid link in Texas by Grid United.

Fact Sheet: MISO Midwest 765-kV Transmission Projects

Project Overview

• Acreage & Authority: MISO selected a four-company consortium to develop, build, operate, and maintain two massive transmission infrastructure projects.
• Core Infrastructure: The projects establish a foundational 765-kilovolt (kV) backbone to deliver reliable, cost-competitive energy across Midwest communities.
• Framework: Part of MISO’s competitive Long Range Transmission Planning (LRTP) Tranche 2.1 Portfolio.
• Total Joint Cost: Approximately $1.658 billion based on MISO estimates.
• Projected Timeline: Expected in-service dates of 2034, following regulatory reviews and stakeholder engagement.

Consortium Breakdown & Ownership

• Ameren Transmission Company of Illinois (ATXI): Co-development lead with a 43% ownership stake.
• GridLiance Heartland, LLC: Co-development lead with a 43% ownership stake.
• Dairyland Power Cooperative: Consortium partner with an 11% ownership stake.
• Illinois Municipal Electric Agency (IMEA): Consortium partner with a 3% ownership stake.

Project Breakdown

Sub T – Iowa/Illinois State Line – Woodford County (STIW)

• MISO Estimated Cost: $940 million.
• Total Distance: Approximately 149 miles of new transmission lines.
• Line Segment A: A 765-kV line running from the Woodford County Substation west to the Iowa/Illinois state line.
• Line Segment B: A 765-kV line extending further west from the Iowa/Illinois state line to the Sub T Substation in Iowa.

Woodford County – Illinois/Indiana State Line (WIIL)

• MISO Estimated Cost: $718 million.
• Total Distance: Spans a total of 88 miles.
• New Infrastructure: Construction of a brand-new 765/345-kV substation.
• Line Segment A: 765-kV line from Woodford County Substation to the Illinois–Indiana state line.
• Line Segment B: 765-kV line ~1.5 miles from Woodford County Substation toward Collins Substation, linking to an existing 765-kV line.