JOHANNESBURG (Reuters) - South African electricity utility Eskom said on Friday it would not cut power this winter when it only plans minor maintenance on its stations and that one of its units at the partially built Medupi plant was now generating at full capacity. The first unit at Medupi, whose construction has been bogged down by strikes, technical delays and cost overruns, is generating 800 MW of power, Eskom said. Its other five units are expected to come online by 2021. The coal-fired Medupi is one of three large-scale power plants Eskom is building to reduce chronic electricity shortages in Africa's most advanced economy. It has had to enforced months of rolling blackouts to ensure the grid does not collapse, but on Friday said it was not planning on cutting power this winter season as it would only conduct limited maintenance. Maintenance is usually done during the summer when demand is lower, but the utility is attempting to catch up with a maintenance backlog following years of postponements. Eskom is also owed billions of rand by municipalities, and has threatened to switch the lights off in areas that have not paid up. On Friday it said all but three of the top 20 defaulting districts had settled their outstanding dues. The three municipalities in default, which it did not identify, faced electricity cuts next week, it said.