The second 1,000 MW nuclear power unit at Kudankulam is expected to restart power generation on October 7, the Power System Operation Corporation Ltd (Posoco) said.
The mega nuclear power plant was shut down on August 4, due to hydrogen concentration in the stator.
The unit was earlier expected to restart generation on September 4.
India's atomic power plant operator Nuclear Power Corporation of India Ltd (NPCIL) has two 1,000 MW nuclear power plants at Kudankulam nuclear power plant (KNPP) built with Russian equipment.
Kudankulam is in Tirunelveli district around 650 km from Chennai.
However, officials of NPCIL at Kudankulam were not available for comment as to the reason for the delay in the plant's restart.
The first 1,000 MW atomic power unit at the Kudankulam Nuclear Power Project (KNPP) restarted generation after completion of refuelling on August 29, said an official.
"The first 1,000 MW atomic power unit was synchronised with the grid between 6.30 am and 7am today (Tuesday). The power generation is around 300 MW," an official had told IANS.
The first unit was shut down on April 13 for annual maintenance and refuelling.
Every year, one third of the reactor's 163 fuel assemblies, or 54 assemblies, are replaced.