The explosion of a transformer caused the engine room to flood at the Sayano-Shushinskaya power station in southern Siberia, the Investigative Committee of the federal prosecutors office said.
The plant's dam was not damaged, and the accident posed no threat to towns further south along the Yenisei River, Emergency Situations Minister Sergei Shoigu said.
The accident caused an oil spill, however, and the slick was floating downriver, the ministry said.
Two of the plant's 10 turbines in the engine room were destroyed, and a third was seriously damaged, said Vasily Zubakin, acting chief executive of the plant's owner, RusHydrop.
Shoigu said the repairs would be difficult. We're probably talking about years rather than months to restore three of the 10 turbines, he said on state-run television.
Electricity supplies from other plants were being routed to areas normally serviced by the Sayano-Shushinskaya station, Shoigu said.
The world's largest aluminum producer, Rusal, was operating as usual, with its smelters being powered from other plants, company spokeswoman Yelena Shuliveistrova said.
Power shortages were reported, however, in the city of Tomsk and the mining area of Kuzbass, state RIA Novosti news agency said.
Aging infrastructure has long been regarded as a key obstacle to Russia's development.
Analysts have warned that Russia needs to boost its power production significantly to meet the growing demand of industrial producers or it would face regular power shortages as soon as several years from now. Monday's accident put these plans in jeopardy.
RusHydro's stocks were down more than 7 per cent on the MICEX when the stock exchange suspended trading for its shares.