South African state utility Eskom plans to correct design defects at all units of its mammoth Medupi and Kusile coal-fired power stations, it said on Monday, after completing modifications on one of the plants’ units.

Medupi and Kusile have been beset by faults and delays and are a major reason why Eskom is struggling with around 450 billion rand ($24.5 billion) of debt.

Eskom said in a statement that modifications at Medupi Unit 3 had gone smoothly, with the unit reaching its full generation capacity of roughly 800 megawatts (MW) after returning to service.

It said it would start making similar design changes, which are partly aimed at solving boiler problems, on Medupi’s other units over the remainder of the year.

Once the work on Medupi is complete, Eskom will fix flaws at sister plant Kusile, Eskom spokesman Sikonathi Mantshantsha said.

With a capacity of 4 800 MW each, Medupi and Kusile will be among the largest coal power stations in the world when complete.