Last month, Energy Secretary Rick Perry signed an emergency order allowing the utility to operate the units, which don't meet federal pollution standards and shut down in April.
The order came at the request of PJM Interconnection, which manages the high-voltage electricity grid in Virginia. PJM says due to summer weather and high demand, the units will likely be needed to prevent "uncontrolled power disruptions."
The Sierra Club argued the department didn't show an emergency exists or set standards to limit pollution.
Dominion says the group's request would jeopardize health, safety and national security at a time when heat indexes are over 100 degrees.