Maj. Gen. George Stoneman was in charge of several raids bearing his name. One such was in april-May, 1863 during the Chancellorsville Campaign while he was with the Army of the Potomac. Another was in December of 1864, in Southwest Virginia and East Tennessee. He also made a raid in the last weeks of the war, March-May, 1865 through Western North Carolina and Southwest Virginia. But I believe the raid you are asking about is the one in July-August, 1864 during the Atlanta Campaign. Stoneman's force engaged confederate forces in the Battle of Macon, Georgia on July 29, 1864. Stoneman, himself, and 700 Union troops were captured by the rebels during this operation. This is probably the operation in which your ancestor was captured.
This capture made Stoneman the highest ranking Union soldier to become a POW. He only spent 3 months in captivity, being exchanged on a personal request of none other than Gen. William T. Sherman.