Hawaiian Electric officials are still trying to determine what caused three power generating units to shut down this morning, resulting in the loss of electricity for about 45,000 customers around Oahu.

The outage affected Waipahu, portions of town, Leeward and Windward Oahu, Aina Haina and Hawaii Kai.

The company said it isn’t clear which of the system’s three generating units tripped offline first: A 180-megawatt plant in Campbell Industrial Park; its 92-megawatt unit at Kahe; or and Waiau’s 92-megawatt facility.

“We are conducting an investigation to determine the cause,” said Peter Rosegg, Hawaiian Electric spokesman. 

Electricity was turned off to some customers briefly as a protective measure to restore the balance of power supply and demand on the Oahu grid and prevent a possibly more widespread or longer outage, HECO said in a written statement.

Hawaiian Electric said it quickly brought additional generators on-line to restore the balance, and system operators were able to begin restoring customers within 25 minutes. Almost all customers had service restored by 10 a.m., with small remaining pockets of customers restored within the hour

The shut down knocked out traffic lights throughout the island, slowing traffic at the end of the morning commute.

Jan Miller, Hawaiian Electric spokeswoman, said one of its plants stopped generating power at 8:50 a.m., triggering the shutdown in the other two plants as a safety mechanism tried to balance the load on the electrical distribution grid.

Hawaiian Electric officials began restoring power at 9:15 a.m. to Mililani and Waipahu first, followed by the other parts of the system. Full electrical power was restored to Hawaii Kai by 9:45 a.m..

“We sincerely apologize for the disruption to our customers,” said Lynne Unemori, Hawaiian Electric vice president for corporate relations. “Our employees worked to safely restore power as quickly as possible and we appreciate our customers’ patience and understanding.”