The fire was reported about 7 p.m. in the Westwing substation near Loop 303 and Happy Valley Road. Five of the substation's 13 transformers have been damaged, said Arizona Public Service spokesman Damon Gross.
Gross said no customers had lost power, but the public was being asked to curtail their power usage as much as possible on Monday.
Firefighters let the fire burn for hours Sunday night because the heat was too intense, said Glendale Assistant Fire Chief Tom Shannon.
``Our tactical plan now is to put water on everything that's not burning,'' he said. However, because the area is sparsely developed, firefighters were forced haul water and fire-retardant foam in from surrounding areas.
``It's pretty much wet stuff on red stuff,'' Shannon said. ``We just need enough wet stuff.''
A mineral oil that is used to cool the transformers continued to burn early Monday, creating a thick black smoke around the plant.
The Arizona Department of Environmental Quality, Department of Health Services and an APS environmental team were on scene late Sunday.
Gross did not know what caused the fire. No injuries were reported.
``It was to our advantage that this happened on a holiday weekend evening,'' Gross said, explaining that's when people traditionally use less power.
The substation is one of four key points of entry for electricity in the Phoenix area, Gross said. It will be down on Monday until the damage can be accessed.
