Engie has completed the first phase of refurbishment work at the Ffestiniog power station, part of the 2GW First Hydro pumped storage operation in Wales.
The £50m revamp involves replacement of two 90MW generating units, extending their operational life for at least 20 years.
Initial works have involved a series of procedures to prepare for the installation of the new units.
Engie said that by June both original units had been disassembled, providing the first access to the lower bearings and turbine.
In addition, one of the two 200-metre vertical hydraulic shafts is being refurbished, a new 2.7km power supply to the upper reservoir has been installed and safety systems have been upgraded.
The 60-year old control desks are also being replaced in readiness for installation of new controllers, the company said.
Future planned works for mid-2019 include testing of main inlet valves and pressure relief valves, installation of further power cables and refurbishment of additional areas of the hydraulic shaft.
Voith Hydro is carrying out the refurbishment of the two units, with an option to carry out the similar work at two more at a later date.