Long view of wind turbine towers in a vast field in Illinois.

Illinois Wind Farm Design


With 139 wind turbines operating on 24,000 acres near Decatur, the largest single-phase wind farm in Illinois has 306 megawatts (MW) of installed capacity. Its output can provide enough renewable energy to power 90,000 households and displaces approximately 500,000 metric tons of carbon dioxide emissions each year. The facility’s owner operates some of the largest wind farms in the world. The Illinois plant is part of its growing renewable energy investment in the U.S.

The wind farm’s power system includes three basic components: a collection system connected to the turbine generators, a substation to step up the collected power’s voltage for transmission, and a high-voltage power line connecting the substation to the grid. The client turned to Gannett Fleming to provide engineering design for all three.

What We Did

The power generated by the plant’s individual turbines feeds into eight underground collector circuits. The group designed the collection system, providing equipment and construction specifications, as well as plans for excavating the trenches that accommodate the power cables. To increase the collected power voltage to 345 kilovolts (kV), the group designed a substation with three main power transformers. Safety features include a motor-operated line disconnect switch, primary side breakers, secondary side disconnects, and grounding-type feeder circuit breakers. Three reactive compensation banks at the substation improve voltage regulation in the network. The group also provided specifications and drawings for the substation’s steel structure, foundation, and control house.

Transmission design included selecting the conductor and shield wire and determining the best route for the high-voltage line to connect to the local utility switchyard, 7.5 miles away. The group conducted an arc flash hazard analysis to place more than 1,000 warning labels throughout the network, ensuring worker safety. As construction was ending, the group provided on-site support to make certain the wind farm was up and running as planned.

Key Features

  • 2 MW wind turbine generators connected to 34.5 kV collection system.
  • Eight underground collector circuits connect to a substation.
  • Three main power transformers increase voltage to 345 kV.
  • 7.5-mile 345 kV line transmits power to the local utility switchyard.


  • Renewable energy production to power 90,000 households.
  • Annual 500,000 metric ton reduction in greenhouse gas emissions.
  • Emissions displacement equal to taking 85,000 cars off the road.


Warrensburg, Ill.



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