Power utility Eskom has confirmed that an explosion occurred at unit 4 of Medupi power station on Sunday night.
While no injuries have been reported, emergency services attended to seven employees for shock. All employees and contractors have been accounted for, said Eskom.
The incident is thought to have caused unit 5 to trip. The coal-fired power station's other four generating units are in operation.
In a statement on Monday morning, Eskom said the explosion took place at about 22:50 on Sunday.
It said that the area had been cordoned off following the blast. Once it has been declared safe, inspections will start to try and ascertain the cause of the explosion.
Unit was not operational
Eskom said that unit 4, where the blast occurred, had been on a "short-term outage" since 6 August when the blast occurred.
The utility said it was in the process of returning unit 5 to service.
"Eskom will update the public on developments, as well as to what extent will this unfortunate incident impact the national electricity grid," it added.
* This article has been corrected to make clear that the explosion occurred on Sunday evening, not Saturday evening.