The connection of the second unit brings India's installed nuclear generating capacity to 6,780 MWe.
The second unit of Kudankulam Nuclear Power Plant, which was connected to the southern grid last week, is scheduled to begin commercial operation in December this year.
The unit with the Russian-designed VVER-1000 reactor was synchronised with the southern grid on August 29 and has been generating about 245 MWe.
The connection of the second unit brings India's installed nuclear generating capacity to 6,780 MWe.
Excavation for Kudankulam units 3 and 4 is in progress and first concrete for the new units will be poured in March next year, Rosatom, the Russian counterpart of India's Department of Atomic Energy (DAE) said.
The units 3 and 4 will begin power production by 2022-2023.
Construction of the unit 2 was completed in July, 2015 and was loaded with fuel in May, 2016. Following safety tests, it attained criticality on 10 July.
The Kudankulam first unit achieved first criticality in mid-2013, was connected to the grid in October, 2013 and began commercial operation in December, 2014.
Kudankulam units 1 and 2 have been built by Nuclear Power Corporation of India Ltd. (NPCIL). These are operated by NPCIL under IAEA safeguards, with supervision from Russian specialists. Enriched uranium fuel for the entire life of the plant is to be supplied by Russia.