Jakarta. State utility PLN has restored electricity to more than 8.3 million customers across Sumatra by Saturday morning after a major transmission disruption triggered massive blackouts across the island.

The blackout began on Friday evening after a disturbance hit a 275-kilovolt extra-high-voltage transmission line linking Muara Bungo and Sungai Rumbai in Jambi province, according to PLN.

The outage affected around 13.1 million customers across several provinces in Sumatra, including Jambi, West Sumatra, Riau, North Sumatra, and Aceh.

PLN President Director Darmawan Prasodjo said the disruption occurred at around 6:44 p.m. local time on Friday and was initially linked to severe weather conditions that affected part of Sumatra’s power system.

“The transmission disturbance spread across parts of the Sumatra transmission network, causing a drop in frequency due to heavy generator loads and triggering a cascading effect in several regions,” Darmawan told a press conference on Saturday.

He said the damaged transmission network was restored within about two hours, allowing PLN to begin reconnecting power plants to the grid.

According to Darmawan, hydroelectric and gas-fired plants were able to quickly support the recovery process because they can respond rapidly to system disruptions. Coal-fired thermal plants, however, require a longer restart process of between 15 and 20 hours before reaching full operation.

PLN deployed hundreds of personnel to work around the clock on restoring the island-wide power system, including transmission lines, substations, and generating units.

By 10 a.m. on Saturday, PLN had restored more than 3,192 megawatts of electricity supply out of the 5,334 MW affected by the outage. A total of 157 out of 176 impacted substations were also back in operation.

The utility said electricity service was being restored gradually to 8.3 million customers as recovery efforts continued.