PJM Interconnection plans to delay its next annual capacity auction by about six months while the largest U.S. grid operator addresses a complaint brought by environmental groups, the biggest U.S. grid operator said in a letter on Thursday.

Talen Energy (NASDAQ:TLN) finished -5.2% on Friday after grid operator PJM Interconnection said it will delay its next annual base residual capacity auction by around six months to give it time to address a complaint brought by environmental groups.

The delay will give the PJM Interconnection time to craft potential market reforms sought by state utility regulators and others, the grid operator said.