A breather device (1) for dehumidifying air to be supplied into an expansion vessel (103) of a power electrical device (100), said breather device comprising an air demoisturizer unit (2) including one or more tanks (21), each tank having an internal volume (20) accommodating a regenerable moisture absorbing substance and a heater (23) for regenerating said moisture absorbing substance, at least a first opening (25) configured to put the internal volume of said tank in fluid-dynamic communication with the external environment and at least a second opening (22) configured to allow a flow of air to exit from the internal volume of said tank towards another section of said breather device. Each tank (21) comprises at least a third opening (24) in fluid-dynamic communication with the external environment and valve means (26) operatively associated to said third opening, said valve means (26) being configured to automatically allow or prevent a flow of air to pass through said third opening, depending on the internal temperature of said tank.
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