The project, which spans four states, is being purchased by the major wind developer Invenergy, and would deliver wind energy from Kansas to Missouri, Illinois and Indiana.
The Missouri Public Service Commission had previously rejected the link, saying approval must be gained first from individual counties on the route, despite commissioners admitting the Grain Belt would benefit the public and save energy costs.
An appeals court, and then the state supreme court, rejected the commission’s decision and ordered it to re-assess the project.
"There can be no debate that our energy future will require more diversity in energy resources, particularly renewable resources," the commissioners said in a joint statement on 20 March announcing their approval.
"Missouri’s Public Service Commission just handed consumers a big win by approving the Grain Belt Express project," said Amy Farrell, senior vice president of government and public affairs at trade body the American Wind Energy Association (AWEA).
"Upgraded and new transmission lines open up America’s low-cost clean energy resources for consumers and make our grid even more resilient."
"America’s energy infrastructure needs a reboot to efficiently serve the 21st Century economy.
"But we can’t get there without transmission planning and permitting reforms at the regional and federal level. Regulatory decisions shouldn’t take years for projects so clearly in the public interest," Farrell added.
Opponents — mostly landowners who dislike compulsory land purchases — said they would take their objections to the Missouri Court of Appeals.
Grain Belt must still surmount several other hurdles before it can proceed.
It needs re-approval in Illinois, where an appeals court in 2018 overturned the previous regulatory approval.
The 4GW line could be completed by 2023, according to a representative for local utilities in Missouri.