The fire broke out just before 2 a.m. on Fairside Ave., in the Coxwell Ave. and Mortimer Ave. area.
Although the fire escalated quickly, crews had it extinguished in about 30 minutes, said Toronto police Acting Staff Sgt. Bruce Newman.
The flames shot above local rooftops, but didn't spread to other structures in the area. Streets in the block around the substation were closed while about 50 firefighters doused the flames.
Fire crews, police and Toronto Hydro investigated the blaze. Fire officials have not yet determined the cause of the blaze, but "they'll take a look and see if it was a transformer that set fire," Newman said.
"With this kind of weather transformers don't like it when it goes from hot to cold and there is a condensation build up."
Something connected to the transformer caught fire said Toronto District Fire Chief Stefan Powell. "Cooling oil from the transformer was burning and made the fire very hot."
The fire caused a power outage that spanned several blocks, from Linsmore Cres. to Woodbine Ave. and O'Connor Dr. to Mortimer.
Toronto Hydro had power restored to a majority of the homes by 6 a.m., but about 300 customers are still without power, said Thelma Hatzis, spokesperson for Toronto Hydro.
There were no injuries reported. The Toronto East General Hospital on Coxwell did not lose power, Hatzis said.