The commission will hear final arguments January 13 and is to start its deliberations January 15 in St. Paul, Minnesota.
A ruling on the permits could be made that day.
Electric power providers, including Otter Tail Power, Montana-Dakota Utilities and Missouri River Energy Services, want to build the $1.5 billion, 500-megawatt plant next to an existing plant near Big Stone City.
Transmission line upgrades into Minnesota would be needed to handle the additional electric power as well as create more capacity for wind-generated power from South Dakota.
Environmentalists have urged rejection of the permits.