What exactly caused the blackout was unclear, Brazilian newspaper Folha de Sao Paulo reported.
Marcio Zimmermann, executive secretary of the Ministry of Mines and Energy, said the problem was not caused by an overload in the system.
Mauricio Tolmasquim, president of energy research company EPE, said the system "worked as it should" and the grid's operators "managed to avoid a domino" effect.
ONS, the grid operator, said in a statement the problem, which it did not explain, had been corrected but some areas had yet to be reconnected.
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