The transformer that caught fire and triggered a widespread power outage in the San Fernando Valley over the weekend was part of an aging unit that had been set to be replaced, the Los Angeles Department of Water and Power said Monday.

The Saturday evening fire was at a receiving station in Northridge, where high voltage is run through a transformer to reduce it for distribution to homes and businesses.

"We fully believe that this was somewhat influenced by the heat and the heat storm when we're in very hot weather conditions," said DWP General Manager David Wright. "The system is not infallible and once in awhile something like this happens."

The transformer that caught fire was at least 40-years-old, he said. Transformers have a lifespan of about 50 years. In all, the DWP operates 70 larger transformers in 21 receiving stations across LA. Fifty of those transformers have been replaced in recent years. The remaining 20 are scheduled to be swapped out in the coming years at a rate of three to five per year, said DWP spokesman Joseph Ramallo.

The transformers are about the size of a school bus and cost about $5 million apiece, said Andy Kendall, DWP's assistant general manager.

The unit that burned still needs to be dismantled and analyzed to determine the cause of its failure. Similar units at other receiving stations are also due to be inspected.

Power use has soared during the current heat wave from people running their air conditioners. At peak demand, distribution equipment like transformers can become stressed, Ramallo said. DWP's transformers use mineral oil for cooling. The oil has a very high ignition temperature, but once ignited is quite flammable.

The fire put a black plume of smoke into the air that could be seen for miles. It started about 6:43 p.m. and by 6:55 p.m., the decision was made to shut down the receiving station, which cut power to about 140,000 customers. About 50,000 of them had power restored by midnight Saturday, and the rest of the accounts were back online by about 10 a.m. Sunday.

"This was more than an inconvenience, it was an emergency situation that posed a danger to communities throughout the San Fernando Valley," said Los Angeles City Councilman Mitchell Englander in an email statement. The outage "showed the importance of maintaining critical infrastructure as thousands of ratepayers were left without power during a record heat wave."