A ROTARY ELECTRIC MACHINE

1. A rotary electric machine of alternating current type designed to be connected directly to a distribution or transmission network and comprising at least one electric winding, characterized in that the winding comprises at least one electric conductor, a first layer with semiconducting properties surrounding the conductor, a solid insulating layer surrounding the first layer and a second layer with semiconducting properties surrounding the insulating layer, and also in that a brushless excitation system, switchable between positive and negative excitation, is arranged for excitation of the machine. 2. A machine as claimed in claim 1, characterized in that the first insulating layer is designed to be substantially connected to the predetermined potential source. 3. A machine as claimed in claim 2, characterized in that the first insulating layer is designed to be substantially an earth potential. 4. A machine as claimed in any of the preceding claims characterized in that at least two adjacent layers of the machine's winding have substantially equally large coefficients of thermal expansion. 5. A machine as claimed in any of the preceding claims characterized in that at least some conductor strands are in electric contact with each other. 6. A machine as claimed in any of the preceding claims, characterized in that each of said three layers is firmly joined to adjacent layers along substantially its entire contact surface. 7. A machine as claimed in any of the preceding claims, characterized in that said layers are arranged to adhere to each other even when the insulated conductor is bent. 8. A machine comprising at least one main electric machine of alternating current type designed to be connected directly to a distribution or transmission network and comprising at least one magnetic core and at least one electric winding, characterized in that the winding is formed from a cable comprising one or more current-carrying conductors, each conductor having a number of strands, an inner semiconducting layer arranged around each conductor, an insulating layer of solid insulating material arranged around said inner semiconducting layer, and an outer semiconducting layer arranged around the insulating layer, and in that a brushless excitation system, switchable between positive and negative excitation, is arranged for excitation of the machine. 9. A machine as claimed in claim 8, characterized in that said cable comprises a metal screen or sheath. 10. A machine