Johannesburg - A group of between 500 and 1000 workers protested at the Medupi power station on Wednesday morning, Eskom said.
"The protest turned violent with stone-throwing and damage to two vehicles, but no one was injured as far as we are aware," the power utility said in a statement.
"The situation is under control and calm has been restored on site."
The Medupi power station is a new, dry-cooled, coal-fired power station being built by Eskom near Lephalale in Limpopo.
Eskom said most Medupi workers had been sent off site, but the utility was working to get operations back to normal as soon as possible.
"The protest apparently related to dissatisfaction with one of the allowances which workers receive, which is currently the subject of negotiation."
Two weeks ago, Eskom said in a statement the new power station would probably begin contributing to the national grid only in the second half of next year because of construction problems. The previous target was December.
Public Enterprises Minister Malusi Gigaba insisted earlier this year that the December deadline would not change.