The unit was synchronized with the grid at 1730 Belgrade time on Monday, the power plant said in a statement
The $110 million (90 million euro) upgrade, which commenced on March 1, has allowed to extend the lifespan of the B1 unit by 20 years.
The TE-KO Kostolac complex, comprising the Kostolac A and B power stations, has a total installed power generation capacity of around 1,000 MW.
($=0.8213 euro)