Hawaiian Electric Co. sent in its engineers Thursday to investigate why Oahu’s largest generating plant lost power for the second day in a row and the third time this year, leaving thousands without electricity.

The coal-fired generator, independently owned and operated by AES Hawaii at Campbell Industrial Park, crashed at 11:23 a.m. Wednesday, touching off outages of roughly 30 minutes. The plant was returned to service at 8 a.m. Thursday and failed again at 11:22 that morning.

Power was restored to all circuits by 12:40 p.m. Thursday, HECO spokesman Darren Pai said.

An AES spokeswoman said the plant was expected to come back online by midnight Thursday. She did not have a cause. Pai said HECO’s engineers are assisting AES in its investigation.

The loss of the generating unit forced HECO to shut off power Thursday to 57,000 customers in various sections of the island, including Waialua, Kahuku, Waihee, Kaneohe, parts of Kailua, the Kapiolani area, downtown Honolulu, Waiakamilo, Moanalua, Halawa, Moanalua and Waipahu, a HECO news release said.

On Wednesday the AES plant unexpectedly shut down, touching off outages that lasted about 30 minutes, affecting the same customers, HECO said.

The 23-year-old, 180-megawatt AES power plant also went down on June 9, 2014, and again Jan. 12, prompting HECO to implement rolling outages.

The targeted emergency outages were necessary to avoid a more widespread power failure or damage to the electrical system from an imbalance of too much demand versus too little supply, according to a news release.

There is no time to warn neighborhoods of these potential outages, Pai said.

The AES plant shutdown was completely unexpected, he said, automatically triggering outages that are programmed into the system.

“We need to react instantaneously (or) we could potentially have an islandwide outage,” he said.

HECO’s other generators, which are on standby, need time to ramp up, Pai said.

The company tries to make sure the outages don’t affect “areas or customers critical to public health and safety,” he said, including hospitals, airports and police stations.

HECO also tries to schedule outages so they don’t “impact the same customers over and over again,” which is part of the way the system is programmed, he added.

Pai said the last two days of outages had nothing to do with increased power use.

“We’re very concerned that this happened again,” he said. “AES has a contract to sell power to us, to meet our customers’ energy needs.”

In a written statement, Pai said: “We understand these outages are disruptive to our customers, so we thank them for their patience and understanding. We apologize and want our customers to know that we are working with AES to thoroughly investigate this issue and take whatever steps are necessary to ensure this source of generation will continue to provide reliable power.”