Ameren Illinois must still issue refunds over transmission rate errors uncovered by a central Illinois co-op, FERC ruled last week.

The commission defended its prior ruling that Ameren Illinois must correct its annual transmission revenue requirement (ER20-1237).

FERC in March said Ameren overcharged transmission customers by millions for construction-related materials and supplies by misplacing them in its books and likely misclassified about $20,000 worth of transmission operations and maintenance costs under an account meant for regulatory costs. (See FERC Finds Few Errors in Co-op’s Challenge of Ameren Illinois Rates.)

The commission was reacting to a challenge from Southwestern Electric Cooperative over Ameren’s rates. Southwestern has lodged formal rate disputes against Ameren Illinois every year since 2016, often unsuccessfully. (See Challenge to Ameren Illinois Rate Rejected Again.)

Ameren sought rehearing of FERC’s decision, arguing that it hadn’t made an error and the commission violated its rule against retroactive ratemaking when it ordered the company to correct inputs to its formula rate.

FERC disagreed that Ameren’s “incorrect reporting was minor or ministerial.” It said the misclassified materials and supplies costs led to a more than $11.5 million in rate overcharges over multiple years and said it had a duty to order Ameren to recalculate and issue refunds.

The commission said Ameren’s formula rate didn’t permit the recovery of construction-related materials and supplies but Ameren “nonetheless recovered” them “by incorrectly reporting them.”