The 1,890MW Salto Grande binational plant has 14 generation units, the first of which came online in 1979.
President Andrés Manuel López Obrador criticized previous administration as well as Standard and Poor's after the outlooks of over 80 local firms, including Pemex, CFE and América Móvil, were cut.
New equipment is of the latest technology and will be installed from June.
Construction work on the multibillion-dollar hydropower dam project was halted last April when the collapse of a water diversion tunnel prompted fears of "catastrophic flooding".
Photovoltaic capacity accounts for almost half of the country's NCRE generation park.